diff --git a/lib/colvars/colvar.cpp b/lib/colvars/colvar.cpp
index 61e83f44ab..1002dc35d8 100644
--- a/lib/colvars/colvar.cpp
+++ b/lib/colvars/colvar.cpp
@@ -22,7 +22,7 @@
 
 colvar::colvar()
 {
-  runave_os = nullptr;
+  runave_os = NULL;
 
   prev_timestep = -1L;
   after_restart = false;
@@ -62,7 +62,7 @@ int colvar::init(std::string const &conf)
   get_keyval(conf, "name", this->name,
              (std::string("colvar")+cvm::to_str(cv->variables()->size())));
 
-  if ((cvm::colvar_by_name(this->name) != nullptr) &&
+  if ((cvm::colvar_by_name(this->name) != NULL) &&
       (cvm::colvar_by_name(this->name) != this)) {
     cvm::error("Error: this colvar cannot have the same name, \""+this->name+
                       "\", as another colvar.\n",
@@ -744,7 +744,7 @@ template<typename def_class_name> int colvar::init_components_type(std::string c
               : ".\n"));
     cvm::increase_depth();
     cvc *cvcp = new def_class_name(def_conf);
-    if (cvcp != nullptr) {
+    if (cvcp != NULL) {
       cvcs.push_back(cvcp);
       cvcp->check_keywords(def_conf, def_config_key);
       cvcp->config_key = def_config_key;
@@ -994,7 +994,7 @@ int colvar::parse_analysis(std::string const &conf)
       acf_type = acf_vel;
       enable(f_cv_fdiff_velocity);
       colvar *cv2 = cvm::colvar_by_name(acf_colvar_name);
-      if (cv2 == nullptr) {
+      if (cv2 == NULL) {
         return cvm::error("Error: collective variable \""+acf_colvar_name+
                           "\" is not defined at this time.\n", INPUT_ERROR);
       }
@@ -1215,14 +1215,14 @@ colvar::~colvar()
   for (std::vector<Lepton::CompiledExpression *>::iterator cei = value_evaluators.begin();
        cei != value_evaluators.end();
        ++cei) {
-    if (*cei != nullptr) delete (*cei);
+    if (*cei != NULL) delete (*cei);
   }
   value_evaluators.clear();
 
   for (std::vector<Lepton::CompiledExpression *>::iterator gei = gradient_evaluators.begin();
        gei != gradient_evaluators.end();
        ++gei) {
-    if (*gei != nullptr) delete (*gei);
+    if (*gei != NULL) delete (*gei);
   }
   gradient_evaluators.clear();
 #endif
@@ -1972,7 +1972,7 @@ void const *colvar::get_cvc_param_ptr(std::string const &param_name)
   }
   cvm::error("Error: calling colvar::get_cvc_param() for a variable "
              "with more than one component.\n", COLVARS_NOT_IMPLEMENTED);
-  return nullptr;
+  return NULL;
 }
 
 
@@ -1983,7 +1983,7 @@ colvarvalue const *colvar::get_cvc_param_grad(std::string const &param_name)
   }
   cvm::error("Error: calling colvar::get_cvc_param_grad() for a variable "
              "with more than one component.\n", COLVARS_NOT_IMPLEMENTED);
-  return nullptr;
+  return NULL;
 }
 
 
@@ -2422,7 +2422,7 @@ int colvar::calc_acf()
   // the pointer to each vector is changed at every step
 
   colvar const *cfcv = cvm::colvar_by_name(acf_colvar_name);
-  if (cfcv == nullptr) {
+  if (cfcv == NULL) {
     return cvm::error("Error: collective variable \""+acf_colvar_name+
                       "\" is not defined at this time.\n", INPUT_ERROR);
   }
@@ -2660,7 +2660,7 @@ int colvar::calc_runave()
 
       if ((*x_history_p).size() >= runave_length-1) {
 
-        if (runave_os == nullptr) {
+        if (runave_os == NULL) {
           if (runave_outfile.size() == 0) {
             runave_outfile = std::string(cvm::output_prefix()+"."+
                                          this->name+".runave.traj");
diff --git a/lib/colvars/colvaratoms.cpp b/lib/colvars/colvaratoms.cpp
index 2796f3d28b..180fc69a85 100644
--- a/lib/colvars/colvaratoms.cpp
+++ b/lib/colvars/colvaratoms.cpp
@@ -105,7 +105,7 @@ cvm::atom_group::~atom_group()
 
   if (fitting_group) {
     delete fitting_group;
-    fitting_group = nullptr;
+    fitting_group = NULL;
   }
 
   cvm::main()->unregister_named_atom_group(this);
@@ -217,7 +217,7 @@ int cvm::atom_group::init()
   b_center = false;
   b_rotate = false;
   b_user_defined_fit = false;
-  fitting_group = nullptr;
+  fitting_group = NULL;
 
   noforce = false;
 
@@ -368,7 +368,7 @@ int cvm::atom_group::parse(std::string const &group_conf)
 
   // Optional group name will let other groups reuse atom definition
   if (get_keyval(group_conf, "name", name)) {
-    if ((cvm::atom_group_by_name(this->name) != nullptr) &&
+    if ((cvm::atom_group_by_name(this->name) != NULL) &&
         (cvm::atom_group_by_name(this->name) != this)) {
       cvm::error("Error: this atom group cannot have the same name, \""+this->name+
                         "\", as another atom group.\n",
@@ -395,7 +395,7 @@ int cvm::atom_group::parse(std::string const &group_conf)
     std::string atoms_of = "";
     if (get_keyval(group_conf, "atomsOfGroup", atoms_of)) {
       atom_group * ag = atom_group_by_name(atoms_of);
-      if (ag == nullptr) {
+      if (ag == NULL) {
         cvm::error("Error: cannot find atom group with name " + atoms_of + ".\n");
         return COLVARS_ERROR;
       }
diff --git a/lib/colvars/colvarbias.cpp b/lib/colvars/colvarbias.cpp
index cdbc13699b..09bcca01b5 100644
--- a/lib/colvars/colvarbias.cpp
+++ b/lib/colvars/colvarbias.cpp
@@ -47,7 +47,7 @@ int colvarbias::init(std::string const &conf)
 
     {
       colvarbias *bias_with_name = cvm::bias_by_name(this->name);
-      if (bias_with_name != nullptr) {
+      if (bias_with_name != NULL) {
         if ((bias_with_name->rank != this->rank) ||
             (bias_with_name->bias_type != this->bias_type)) {
           cvm::error("Error: this bias cannot have the same name, \""+this->name+
@@ -295,8 +295,8 @@ int colvarbias::update()
     colvar_values[i] = colvars[i]->value();
   }
 
-  error_code |= calc_energy(nullptr);
-  error_code |= calc_forces(nullptr);
+  error_code |= calc_energy(NULL);
+  error_code |= calc_forces(NULL);
 
   return error_code;
 }
@@ -547,8 +547,8 @@ colvarbias_ti::colvarbias_ti(char const *key)
   : colvarbias(key)
 {
   provide(f_cvb_calc_ti_samples);
-  ti_avg_forces = nullptr;
-  ti_count = nullptr;
+  ti_avg_forces = NULL;
+  ti_count = NULL;
 }
 
 
@@ -560,13 +560,13 @@ colvarbias_ti::~colvarbias_ti()
 
 int colvarbias_ti::clear_state_data()
 {
-  if (ti_avg_forces != nullptr) {
+  if (ti_avg_forces != NULL) {
     delete ti_avg_forces;
-    ti_avg_forces = nullptr;
+    ti_avg_forces = NULL;
   }
-  if (ti_count != nullptr) {
+  if (ti_count != NULL) {
     delete ti_count;
-    ti_count = nullptr;
+    ti_count = NULL;
   }
   return COLVARS_OK;
 }
@@ -622,7 +622,7 @@ int colvarbias_ti::init(std::string const &conf)
 int colvarbias_ti::init_grids()
 {
   if (is_enabled(f_cvb_calc_ti_samples)) {
-    if (ti_avg_forces == nullptr) {
+    if (ti_avg_forces == NULL) {
       ti_bin.resize(num_variables());
       ti_system_forces.resize(num_variables());
       for (size_t icv = 0; icv < num_variables(); icv++) {
@@ -643,7 +643,7 @@ int colvarbias_ti::init_grids()
 
 int colvarbias_ti::update()
 {
-  return update_system_forces(nullptr);
+  return update_system_forces(NULL);
 }
 
 
@@ -679,7 +679,7 @@ int colvarbias_ti::update_system_forces(std::vector<colvarvalue> const
           ti_system_forces[i] = variables(i)->total_force();
         } else {
           ti_system_forces[i] = variables(i)->total_force() -
-            ((subtract_forces != nullptr) ?
+            ((subtract_forces != NULL) ?
              (*subtract_forces)[i] : previous_colvar_forces[i]);
         }
       }
@@ -759,7 +759,7 @@ int colvarbias_ti::write_output_files()
 
   std::string const ti_output_prefix = cvm::output_prefix()+"."+this->name;
 
-  std::ostream *os = nullptr;
+  std::ostream *os = NULL;
 
   if (is_enabled(f_cvb_write_ti_samples)) {
     std::string const ti_count_file_name(ti_output_prefix+".ti.count");
diff --git a/lib/colvars/colvarbias.h b/lib/colvars/colvarbias.h
index 20c346f81c..5179c42853 100644
--- a/lib/colvars/colvarbias.h
+++ b/lib/colvars/colvarbias.h
@@ -58,12 +58,12 @@ public:
   virtual int update();
 
   /// Compute the energy of the bias
-  /// Uses the vector of colvar values provided if not nullptr, and the values
+  /// Uses the vector of colvar values provided if not NULL, and the values
   /// currently cached in the bias instance otherwise
   virtual int calc_energy(std::vector<colvarvalue> const *values);
 
   /// Compute the forces due to the bias
-  /// Uses the vector of colvar values provided if not nullptr, and the values
+  /// Uses the vector of colvar values provided if not NULL, and the values
   /// currently cached in the bias instance otherwise
   virtual int calc_forces(std::vector<colvarvalue> const *values);
 
diff --git a/lib/colvars/colvarbias_abf.cpp b/lib/colvars/colvarbias_abf.cpp
index 9926579a8d..5382a828f7 100644
--- a/lib/colvars/colvarbias_abf.cpp
+++ b/lib/colvars/colvarbias_abf.cpp
@@ -18,16 +18,16 @@ colvarbias_abf::colvarbias_abf(char const *key)
     b_UI_estimator(false),
     b_CZAR_estimator(false),
     pabf_freq(0),
-    system_force(nullptr),
-    gradients(nullptr),
-    samples(nullptr),
-    pmf(nullptr),
-    z_gradients(nullptr),
-    z_samples(nullptr),
-    czar_gradients(nullptr),
-    czar_pmf(nullptr),
-    last_gradients(nullptr),
-    last_samples(nullptr)
+    system_force(NULL),
+    gradients(NULL),
+    samples(NULL),
+    pmf(NULL),
+    z_gradients(NULL),
+    z_samples(NULL),
+    czar_gradients(NULL),
+    czar_pmf(NULL),
+    last_gradients(NULL),
+    last_samples(NULL)
 {
 }
 
@@ -261,37 +261,37 @@ colvarbias_abf::~colvarbias_abf()
 {
   if (samples) {
     delete samples;
-    samples = nullptr;
+    samples = NULL;
   }
 
   if (gradients) {
     delete gradients;
-    gradients = nullptr;
+    gradients = NULL;
   }
 
   if (pmf) {
     delete pmf;
-    pmf = nullptr;
+    pmf = NULL;
   }
 
   if (z_samples) {
     delete z_samples;
-    z_samples = nullptr;
+    z_samples = NULL;
   }
 
   if (z_gradients) {
     delete z_gradients;
-    z_gradients = nullptr;
+    z_gradients = NULL;
   }
 
   if (czar_gradients) {
     delete czar_gradients;
-    czar_gradients = nullptr;
+    czar_gradients = NULL;
   }
 
   if (czar_pmf) {
     delete czar_pmf;
-    czar_pmf = nullptr;
+    czar_pmf = NULL;
   }
 
   // shared ABF
@@ -299,17 +299,17 @@ colvarbias_abf::~colvarbias_abf()
   // but now we can call shared externally
   if (last_samples) {
     delete last_samples;
-    last_samples = nullptr;
+    last_samples = NULL;
   }
 
   if (last_gradients) {
     delete last_gradients;
-    last_gradients = nullptr;
+    last_gradients = NULL;
   }
 
   if (system_force) {
     delete [] system_force;
-    system_force = nullptr;
+    system_force = NULL;
   }
 }
 
@@ -488,7 +488,7 @@ int colvarbias_abf::update()
   }
 
   /// Add the bias energy for 1D ABF
-  bias_energy = calc_energy(nullptr);
+  bias_energy = calc_energy(NULL);
 
   return COLVARS_OK;
 }
diff --git a/lib/colvars/colvarbias_histogram.cpp b/lib/colvars/colvarbias_histogram.cpp
index 44862ba256..92985b0f61 100644
--- a/lib/colvars/colvarbias_histogram.cpp
+++ b/lib/colvars/colvarbias_histogram.cpp
@@ -15,7 +15,7 @@
 
 colvarbias_histogram::colvarbias_histogram(char const *key)
   : colvarbias(key),
-    grid(nullptr), out_name("")
+    grid(NULL), out_name("")
 {
   provide(f_cvb_bypass_ext_lagrangian); // Allow histograms of actual cv for extended-Lagrangian
 }
@@ -105,7 +105,7 @@ colvarbias_histogram::~colvarbias_histogram()
 {
   if (grid) {
     delete grid;
-    grid = nullptr;
+    grid = NULL;
   }
 }
 
diff --git a/lib/colvars/colvarbias_meta.cpp b/lib/colvars/colvarbias_meta.cpp
index 03c1886a8f..05f293d872 100644
--- a/lib/colvars/colvarbias_meta.cpp
+++ b/lib/colvars/colvarbias_meta.cpp
@@ -36,7 +36,7 @@ colvarbias_meta::colvarbias_meta(char const *key)
   : colvarbias(key), colvarbias_ti(key)
 {
   new_hills_begin = hills.end();
-  hills_traj_os = nullptr;
+  hills_traj_os = NULL;
 
   hill_width = 0.0;
 
@@ -45,8 +45,8 @@ colvarbias_meta::colvarbias_meta(char const *key)
   use_grids = true;
   grids_freq = 0;
   rebin_grids = false;
-  hills_energy = nullptr;
-  hills_energy_gradients = nullptr;
+  hills_energy = NULL;
+  hills_energy_gradients = NULL;
 
   dump_fes = true;
   keep_hills = false;
@@ -153,7 +153,7 @@ int colvarbias_meta::init(std::string const &conf)
     get_keyval(conf, "keepHills", keep_hills, keep_hills);
     get_keyval(conf, "keepFreeEnergyFiles", dump_fes_save, dump_fes_save);
 
-    if (hills_energy == nullptr) {
+    if (hills_energy == NULL) {
       hills_energy           = new colvar_grid_scalar(colvars);
       hills_energy_gradients = new colvar_grid_gradient(colvars);
     }
@@ -262,7 +262,7 @@ int colvarbias_meta::init_well_tempered_params(std::string const &conf)
 int colvarbias_meta::init_ebmeta_params(std::string const &conf)
 {
   // for ebmeta
-  target_dist = nullptr;
+  target_dist = NULL;
   get_keyval(conf, "ebMeta", ebmeta, false);
   if(ebmeta){
     if (use_grids && expand_grids) {
@@ -328,12 +328,12 @@ colvarbias_meta::~colvarbias_meta()
 
   if (hills_traj_os) {
     proxy->close_output_stream(hills_traj_file_name());
-    hills_traj_os = nullptr;
+    hills_traj_os = NULL;
   }
 
   if (target_dist) {
     delete target_dist;
-    target_dist = nullptr;
+    target_dist = NULL;
   }
 }
 
@@ -342,12 +342,12 @@ int colvarbias_meta::clear_state_data()
 {
   if (hills_energy) {
     delete hills_energy;
-    hills_energy = nullptr;
+    hills_energy = NULL;
   }
 
   if (hills_energy_gradients) {
     delete hills_energy_gradients;
-    hills_energy_gradients = nullptr;
+    hills_energy_gradients = NULL;
   }
 
   hills.clear();
@@ -451,8 +451,8 @@ int colvarbias_meta::update()
     error_code |= replica_share();
   }
 
-  error_code |= calc_energy(nullptr);
-  error_code |= calc_forces(nullptr);
+  error_code |= calc_energy(NULL);
+  error_code |= calc_forces(NULL);
 
   return error_code;
 }
@@ -593,7 +593,7 @@ int colvarbias_meta::update_bias()
         std::vector<int> curr_bin = hills_energy->get_colvars_index();
         hills_energy_sum_here = hills_energy->value(curr_bin);
       } else {
-        calc_hills(new_hills_begin, hills.end(), hills_energy_sum_here, nullptr);
+        calc_hills(new_hills_begin, hills.end(), hills_energy_sum_here, NULL);
       }
       hills_scale *= cvm::exp(-1.0*hills_energy_sum_here/(bias_temperature*cvm::boltzmann()));
     }
@@ -887,14 +887,14 @@ void colvarbias_meta::project_hills(colvarbias_meta::hill_iter  h_first,
   std::vector<cvm::real> colvar_forces_scalar(num_variables());
 
   std::vector<int> he_ix = he->new_index();
-  std::vector<int> hg_ix = (hg != nullptr) ? hg->new_index() : std::vector<int> (0);
+  std::vector<int> hg_ix = (hg != NULL) ? hg->new_index() : std::vector<int> (0);
   cvm::real hills_energy_here = 0.0;
   std::vector<colvarvalue> hills_forces_here(num_variables(), 0.0);
 
   size_t count = 0;
   size_t const print_frequency = ((hills.size() >= 1000000) ? 1 : (1000000/(hills.size()+1)));
 
-  if (hg != nullptr) {
+  if (hg != NULL) {
 
     // loop over the points of the grid
     for ( ;
@@ -1310,8 +1310,8 @@ std::istream & colvarbias_meta::read_state_data(std::istream& is)
       hills_energy_gradients = new colvar_grid_gradient(colvars);
     }
 
-    colvar_grid_scalar   *hills_energy_backup = nullptr;
-    colvar_grid_gradient *hills_energy_gradients_backup = nullptr;
+    colvar_grid_scalar   *hills_energy_backup = NULL;
+    colvar_grid_gradient *hills_energy_gradients_backup = NULL;
 
     if (has_data) {
       if (cvm::debug())
@@ -1327,7 +1327,7 @@ std::istream & colvarbias_meta::read_state_data(std::istream& is)
     size_t const hills_energy_pos = is.tellg();
     std::string key;
     if (!(is >> key)) {
-      if (hills_energy_backup != nullptr) {
+      if (hills_energy_backup != NULL) {
         delete hills_energy;
         delete hills_energy_gradients;
         hills_energy           = hills_energy_backup;
@@ -1343,7 +1343,7 @@ std::istream & colvarbias_meta::read_state_data(std::istream& is)
       is.seekg(hills_energy_pos, std::ios::beg);
       grids_from_restart_file = false;
       if (!rebin_grids) {
-        if (hills_energy_backup == nullptr)
+        if (hills_energy_backup == NULL)
           cvm::fatal_error("Error: couldn't read the free energy grid for metadynamics bias \""+
                            this->name+"\""+
                            ((comm != single_replica) ? ", replica \""+replica_id+"\"" : "")+
@@ -1365,7 +1365,7 @@ std::istream & colvarbias_meta::read_state_data(std::istream& is)
 
     size_t const hills_energy_gradients_pos = is.tellg();
     if (!(is >> key)) {
-      if (hills_energy_backup != nullptr)  {
+      if (hills_energy_backup != NULL)  {
         delete hills_energy;
         delete hills_energy_gradients;
         hills_energy           = hills_energy_backup;
@@ -1381,7 +1381,7 @@ std::istream & colvarbias_meta::read_state_data(std::istream& is)
       is.seekg(hills_energy_gradients_pos, std::ios::beg);
       grids_from_restart_file = false;
       if (!rebin_grids) {
-        if (hills_energy_backup == nullptr)
+        if (hills_energy_backup == NULL)
           cvm::fatal_error("Error: couldn't read the free energy gradients grid for metadynamics bias \""+
                            this->name+"\""+
                            ((comm != single_replica) ? ", replica \""+replica_id+"\"" : "")+
@@ -1406,7 +1406,7 @@ std::istream & colvarbias_meta::read_state_data(std::istream& is)
                this->name+"\""+
                ((comm != single_replica) ? ", replica \""+replica_id+"\"" : "")+"\n");
 
-    if (hills_energy_backup != nullptr) {
+    if (hills_energy_backup != NULL) {
       // now that we have successfully updated the grids, delete the
       // backup copies
       if (cvm::debug())
@@ -1594,7 +1594,7 @@ int colvarbias_meta::setup_output()
 
     // TODO: one may want to specify the path manually for intricated filesystems?
     char *pwd = new char[3001];
-    if (GETCWD(pwd, 3000) == nullptr)
+    if (GETCWD(pwd, 3000) == NULL)
       cvm::fatal_error("Error: cannot get the path of the current working directory.\n");
     replica_list_file =
       (std::string(pwd)+std::string(PATHSEP)+
@@ -1885,7 +1885,7 @@ int colvarbias_meta::reopen_replica_buffer_file()
 {
   int error_code = COLVARS_OK;
   colvarproxy *proxy = cvm::proxy;
-  if (proxy->get_output_stream(replica_hills_file) != nullptr) {
+  if (proxy->get_output_stream(replica_hills_file) != NULL) {
     error_code |= proxy->close_output_stream(replica_hills_file);
   }
   error_code |= proxy->remove_file(replica_hills_file);
diff --git a/lib/colvars/colvarcomp.cpp b/lib/colvars/colvarcomp.cpp
index d45d85111a..05d1195d58 100644
--- a/lib/colvars/colvarcomp.cpp
+++ b/lib/colvars/colvarcomp.cpp
@@ -135,7 +135,7 @@ cvm::atom_group *colvar::cvc::parse_group(std::string const &conf,
                                           char const *group_key,
                                           bool optional)
 {
-  cvm::atom_group *group = nullptr;
+  cvm::atom_group *group = NULL;
   std::string group_conf;
 
   if (key_lookup(conf, group_key, &group_conf)) {
@@ -298,7 +298,7 @@ colvar::cvc::~cvc()
   free_children_deps();
   remove_all_children();
   for (size_t i = 0; i < atom_groups.size(); i++) {
-    if (atom_groups[i] != nullptr) delete atom_groups[i];
+    if (atom_groups[i] != NULL) delete atom_groups[i];
   }
 }
 
@@ -344,7 +344,7 @@ colvarvalue const *colvar::cvc::get_param_grad(std::string const &param_name)
 {
   colvarvalue const *ptr =
     reinterpret_cast<colvarvalue const *>(get_param_grad_ptr(param_name));
-  return ptr != nullptr ? ptr : nullptr;
+  return ptr != NULL ? ptr : NULL;
 }
 
 
@@ -553,7 +553,7 @@ void colvar::cvc::debug_gradients()
       }
     }
 
-    if ((group->is_enabled(f_ag_fit_gradients)) && (group->fitting_group != nullptr)) {
+    if ((group->is_enabled(f_ag_fit_gradients)) && (group->fitting_group != NULL)) {
       cvm::atom_group *ref_group = group->fitting_group;
       group->read_positions();
       group->calc_required_properties();
diff --git a/lib/colvars/colvarcomp_coordnums.cpp b/lib/colvars/colvarcomp_coordnums.cpp
index f36d055528..f0fbabfc4f 100644
--- a/lib/colvars/colvarcomp_coordnums.cpp
+++ b/lib/colvars/colvarcomp_coordnums.cpp
@@ -91,7 +91,7 @@ cvm::real colvar::coordnum::switching_function(cvm::real const &r0,
 
 
 colvar::coordnum::coordnum(std::string const &conf)
-  : cvc(conf), b_anisotropic(false), pairlist(nullptr)
+  : cvc(conf), b_anisotropic(false), pairlist(NULL)
 
 {
   function_type = "coordnum";
@@ -102,7 +102,7 @@ colvar::coordnum::coordnum(std::string const &conf)
   group1 = parse_group(conf, "group1");
   group2 = parse_group(conf, "group2");
 
-  if (group1 == nullptr || group2 == nullptr) {
+  if (group1 == NULL || group2 == NULL) {
     cvm::error("Error: failed to initialize atom groups.\n",
                 INPUT_ERROR);
     return;
@@ -183,7 +183,7 @@ colvar::coordnum::coordnum(std::string const &conf)
 
 colvar::coordnum::~coordnum()
 {
-  if (pairlist != nullptr) {
+  if (pairlist != NULL) {
     delete [] pairlist;
   }
 }
@@ -218,11 +218,11 @@ template<int flags> void colvar::coordnum::main_loop(bool **pairlist_elem)
 
 template<int compute_flags> int colvar::coordnum::compute_coordnum()
 {
-  bool const use_pairlist = (pairlist != nullptr);
-  bool const rebuild_pairlist = (pairlist != nullptr) &&
+  bool const use_pairlist = (pairlist != NULL);
+  bool const rebuild_pairlist = (pairlist != NULL) &&
     (cvm::step_relative() % pairlist_freq == 0);
 
-  bool *pairlist_elem = use_pairlist ? pairlist : nullptr;
+  bool *pairlist_elem = use_pairlist ? pairlist : NULL;
 
   if (b_anisotropic) {
 
@@ -239,7 +239,7 @@ template<int compute_flags> int colvar::coordnum::compute_coordnum()
     } else {
 
       int const flags = compute_flags | ef_anisotropic;
-      main_loop<flags>(nullptr);
+      main_loop<flags>(NULL);
     }
 
   } else {
@@ -257,7 +257,7 @@ template<int compute_flags> int colvar::coordnum::compute_coordnum()
     } else {
 
       int const flags = compute_flags;
-      main_loop<flags>(nullptr);
+      main_loop<flags>(NULL);
     }
   }
 
@@ -367,7 +367,7 @@ void colvar::h_bond::calc_value()
     coordnum::switching_function<flags>(r0, r0_vec, en, ed,
                                         (*atom_groups[0])[0],
                                         (*atom_groups[0])[1],
-                                        nullptr, 0.0);
+                                        NULL, 0.0);
 }
 
 
@@ -378,7 +378,7 @@ void colvar::h_bond::calc_gradients()
   coordnum::switching_function<flags>(r0, r0_vec, en, ed,
                                       (*atom_groups[0])[0],
                                       (*atom_groups[0])[1],
-                                      nullptr, 0.0);
+                                      NULL, 0.0);
 }
 
 
@@ -393,7 +393,7 @@ simple_scalar_dist_functions(h_bond)
 
 
 colvar::selfcoordnum::selfcoordnum(std::string const &conf)
-  : cvc(conf), pairlist(nullptr)
+  : cvc(conf), pairlist(NULL)
 {
   function_type = "selfcoordnum";
   x.type(colvarvalue::type_scalar);
@@ -438,7 +438,7 @@ colvar::selfcoordnum::selfcoordnum(std::string const &conf)
 
 colvar::selfcoordnum::~selfcoordnum()
 {
-  if (pairlist != nullptr) {
+  if (pairlist != NULL) {
     delete [] pairlist;
   }
 }
@@ -448,11 +448,11 @@ template<int compute_flags> int colvar::selfcoordnum::compute_selfcoordnum()
 {
   cvm::rvector const r0_vec(0.0); // TODO enable the flag?
 
-  bool const use_pairlist = (pairlist != nullptr);
-  bool const rebuild_pairlist = (pairlist != nullptr) &&
+  bool const use_pairlist = (pairlist != NULL);
+  bool const rebuild_pairlist = (pairlist != NULL) &&
     (cvm::step_relative() % pairlist_freq == 0);
 
-  bool *pairlist_elem = use_pairlist ? pairlist : nullptr;
+  bool *pairlist_elem = use_pairlist ? pairlist : NULL;
   size_t i = 0, j = 0;
   size_t const n = group1->size();
 
@@ -601,13 +601,13 @@ void colvar::groupcoordnum::calc_value()
     x.real_value = coordnum::switching_function<flags>(r0, r0_vec, en, ed,
                                                        group1_com_atom,
                                                        group2_com_atom,
-                                                       nullptr, 0.0);
+                                                       NULL, 0.0);
   } else {
     int const flags = coordnum::ef_null;
     x.real_value = coordnum::switching_function<flags>(r0, r0_vec, en, ed,
                                                        group1_com_atom,
                                                        group2_com_atom,
-                                                       nullptr, 0.0);
+                                                       NULL, 0.0);
   }
 }
 
@@ -624,13 +624,13 @@ void colvar::groupcoordnum::calc_gradients()
     coordnum::switching_function<flags>(r0, r0_vec, en, ed,
                                         group1_com_atom,
                                         group2_com_atom,
-                                        nullptr, 0.0);
+                                        NULL, 0.0);
   } else {
     int const flags = coordnum::ef_gradients;
     coordnum::switching_function<flags>(r0, r0_vec, en, ed,
                                         group1_com_atom,
                                         group2_com_atom,
-                                        nullptr, 0.0);
+                                        NULL, 0.0);
   }
 
   group1->set_weighted_gradient(group1_com_atom.grad);
diff --git a/lib/colvars/colvarcomp_distances.cpp b/lib/colvars/colvarcomp_distances.cpp
index 88a5088aaf..c3c1ee55d5 100644
--- a/lib/colvars/colvarcomp_distances.cpp
+++ b/lib/colvars/colvarcomp_distances.cpp
@@ -964,7 +964,7 @@ colvar::rmsd::rmsd(std::string const &conf)
   }
 
   bool b_Jacobian_derivative = true;
-  if (atoms->fitting_group != nullptr && b_Jacobian_derivative) {
+  if (atoms->fitting_group != NULL && b_Jacobian_derivative) {
     cvm::log("The option \"fittingGroup\" (alternative group for fitting) was enabled: "
               "Jacobian derivatives of the RMSD will not be calculated.\n");
     b_Jacobian_derivative = false;
diff --git a/lib/colvars/colvardeps.cpp b/lib/colvars/colvardeps.cpp
index f647211f6e..f83458b51c 100644
--- a/lib/colvars/colvardeps.cpp
+++ b/lib/colvars/colvardeps.cpp
@@ -33,7 +33,7 @@ colvardeps::~colvardeps() {
   // Do not delete features if it's a static object
   // may change in the future though
 //     for (i=0; i<features.size(); i++) {
-//       if (features[i] != nullptr) delete features[i];
+//       if (features[i] != NULL) delete features[i];
 //     }
 
   remove_all_children();
diff --git a/lib/colvars/colvargrid.cpp b/lib/colvars/colvargrid.cpp
index 27afb884eb..7c2fe3b33c 100644
--- a/lib/colvars/colvargrid.cpp
+++ b/lib/colvars/colvargrid.cpp
@@ -34,21 +34,21 @@ colvar_grid_count::colvar_grid_count(std::vector<colvar *>  &colvars,
 {}
 
 colvar_grid_scalar::colvar_grid_scalar()
-  : colvar_grid<cvm::real>(), samples(nullptr)
+  : colvar_grid<cvm::real>(), samples(NULL)
 {}
 
 colvar_grid_scalar::colvar_grid_scalar(colvar_grid_scalar const &g)
-  : colvar_grid<cvm::real>(g), samples(nullptr)
+  : colvar_grid<cvm::real>(g), samples(NULL)
 {
 }
 
 colvar_grid_scalar::colvar_grid_scalar(std::vector<int> const &nx_i)
-  : colvar_grid<cvm::real>(nx_i, 0.0, 1), samples(nullptr)
+  : colvar_grid<cvm::real>(nx_i, 0.0, 1), samples(NULL)
 {
 }
 
 colvar_grid_scalar::colvar_grid_scalar(std::vector<colvar *> &colvars, bool margin)
-  : colvar_grid<cvm::real>(colvars, 0.0, 1, margin), samples(nullptr)
+  : colvar_grid<cvm::real>(colvars, 0.0, 1, margin), samples(NULL)
 {
 }
 
@@ -122,15 +122,15 @@ cvm::real colvar_grid_scalar::entropy() const
 
 
 colvar_grid_gradient::colvar_grid_gradient()
-  : colvar_grid<cvm::real>(), samples(nullptr)
+  : colvar_grid<cvm::real>(), samples(NULL)
 {}
 
 colvar_grid_gradient::colvar_grid_gradient(std::vector<int> const &nx_i)
-  : colvar_grid<cvm::real>(nx_i, 0.0, nx_i.size()), samples(nullptr)
+  : colvar_grid<cvm::real>(nx_i, 0.0, nx_i.size()), samples(NULL)
 {}
 
 colvar_grid_gradient::colvar_grid_gradient(std::vector<colvar *> &colvars)
-  : colvar_grid<cvm::real>(colvars, 0.0, colvars.size()), samples(nullptr)
+  : colvar_grid<cvm::real>(colvars, 0.0, colvars.size()), samples(NULL)
 {}
 
 void colvar_grid_gradient::write_1D_integral(std::ostream &os)
diff --git a/lib/colvars/colvarmodule.cpp b/lib/colvars/colvarmodule.cpp
index 48a38a6076..1d5f75bbca 100644
--- a/lib/colvars/colvarmodule.cpp
+++ b/lib/colvars/colvarmodule.cpp
@@ -30,11 +30,11 @@ colvarmodule::colvarmodule(colvarproxy *proxy_in)
   depth_s = 0;
   log_level_ = 10;
 
-  cv_traj_os = nullptr;
+  cv_traj_os = NULL;
 
   xyz_reader_use_count = 0;
 
-  if (proxy == nullptr) {
+  if (proxy == NULL) {
     proxy = proxy_in; // Pointer to the proxy object
     parse = new colvarparse(); // Parsing object for global options
     version_int = proxy->get_version_from_string(COLVARS_VERSION);
@@ -573,7 +573,7 @@ colvarbias * colvarmodule::bias_by_name(std::string const &name)
       return (*bi);
     }
   }
-  return nullptr;
+  return NULL;
 }
 
 
@@ -587,7 +587,7 @@ colvar *colvarmodule::colvar_by_name(std::string const &name)
       return (*cvi);
     }
   }
-  return nullptr;
+  return NULL;
 }
 
 
@@ -601,7 +601,7 @@ cvm::atom_group *colvarmodule::atom_group_by_name(std::string const &name)
       return (*agi);
     }
   }
-  return nullptr;
+  return NULL;
 }
 
 
@@ -631,7 +631,7 @@ int colvarmodule::change_configuration(std::string const &bias_name,
   cvm::increase_depth();
   colvarbias *b;
   b = bias_by_name(bias_name);
-  if (b == nullptr) {
+  if (b == NULL) {
     cvm::error("Error: bias not found: " + bias_name);
     return COLVARS_ERROR;
   }
@@ -647,7 +647,7 @@ std::string colvarmodule::read_colvar(std::string const &name)
   colvar *c;
   std::stringstream ss;
   c = colvar_by_name(name);
-  if (c == nullptr) {
+  if (c == NULL) {
     cvm::error("Error: colvar not found: " + name);
     return std::string();
   }
@@ -663,7 +663,7 @@ cvm::real colvarmodule::energy_difference(std::string const &bias_name,
   colvarbias *b;
   cvm::real energy_diff = 0.;
   b = bias_by_name(bias_name);
-  if (b == nullptr) {
+  if (b == NULL) {
     cvm::error("Error: bias not found: " + bias_name);
     return 0.;
   }
@@ -678,7 +678,7 @@ int colvarmodule::bias_current_bin(std::string const &bias_name)
   int ret;
   colvarbias *b = bias_by_name(bias_name);
 
-  if (b != nullptr) {
+  if (b != NULL) {
     ret = b->current_bin();
   } else {
     cvm::error("Error: bias not found.\n");
@@ -695,7 +695,7 @@ int colvarmodule::bias_bin_num(std::string const &bias_name)
   int ret;
   colvarbias *b = bias_by_name(bias_name);
 
-  if (b != nullptr) {
+  if (b != NULL) {
     ret = b->bin_num();
   } else {
     cvm::error("Error: bias not found.\n");
@@ -712,7 +712,7 @@ int colvarmodule::bias_bin_count(std::string const &bias_name, size_t bin_index)
   int ret;
   colvarbias *b = bias_by_name(bias_name);
 
-  if (b != nullptr) {
+  if (b != NULL) {
     ret = b->bin_count(bin_index);
   } else {
     cvm::error("Error: bias not found.\n");
@@ -729,7 +729,7 @@ int colvarmodule::bias_share(std::string const &bias_name)
   int ret;
   colvarbias *b = bias_by_name(bias_name);
 
-  if (b != nullptr) {
+  if (b != NULL) {
     b->replica_share();
     ret = COLVARS_OK;
   } else {
@@ -1037,7 +1037,7 @@ int colvarmodule::write_restart_file(std::string const &out_name)
 
 int colvarmodule::write_traj_files()
 {
-  if (cv_traj_os == nullptr) {
+  if (cv_traj_os == NULL) {
     if (open_traj_file(cv_traj_name) != COLVARS_OK) {
       return cvm::get_error();
     } else {
@@ -1057,7 +1057,7 @@ int colvarmodule::write_traj_files()
     write_traj(*cv_traj_os);
   }
 
-  if (restart_out_freq && (cv_traj_os != nullptr)) {
+  if (restart_out_freq && (cv_traj_os != NULL)) {
     // flush the trajectory file if we are at the restart frequency
     if ( (cvm::step_relative() > 0) &&
          ((cvm::step_absolute() % restart_out_freq) == 0) ) {
@@ -1151,8 +1151,8 @@ colvarmodule::~colvarmodule()
     atom_group::delete_features();
 
     delete parse;
-    parse = nullptr;
-    proxy = nullptr;
+    parse = NULL;
+    proxy = NULL;
   }
 }
 
@@ -1184,7 +1184,7 @@ int colvarmodule::reset()
 
   proxy->reset();
 
-  if (cv_traj_os != nullptr) {
+  if (cv_traj_os != NULL) {
     // Do not close traj file here, as we might not be done with it yet.
     proxy->flush_output_stream(cv_traj_os);
   }
@@ -1395,7 +1395,7 @@ std::istream & colvarmodule::read_objects_state(std::istream &is)
     if (static_cast<size_t>(is.tellg()) == pos) {
       // This block has not been read by any object: discard it and move on
       // to the next one
-      is >> colvarparse::read_block(word, nullptr);
+      is >> colvarparse::read_block(word, NULL);
     }
 
     if (!is) break;
@@ -1456,7 +1456,7 @@ int colvarmodule::write_output_files()
   }
   cvm::decrease_depth();
 
-  if (cv_traj_os != nullptr) {
+  if (cv_traj_os != NULL) {
     // do not close, there may be another run command
     proxy->flush_output_stream(cv_traj_os);
   }
@@ -1575,7 +1575,7 @@ std::ostream & colvarmodule::write_restart(std::ostream &os)
 
 int colvarmodule::open_traj_file(std::string const &file_name)
 {
-  if (cv_traj_os != nullptr) {
+  if (cv_traj_os != NULL) {
     return COLVARS_OK;
   }
 
@@ -1591,7 +1591,7 @@ int colvarmodule::open_traj_file(std::string const &file_name)
     cv_traj_os = (cvm::proxy)->output_stream(file_name);
   }
 
-  if (cv_traj_os == nullptr) {
+  if (cv_traj_os == NULL) {
     cvm::error("Error: cannot write to file \""+file_name+"\".\n",
                FILE_ERROR);
   }
@@ -1602,9 +1602,9 @@ int colvarmodule::open_traj_file(std::string const &file_name)
 
 int colvarmodule::close_traj_file()
 {
-  if (cv_traj_os != nullptr) {
+  if (cv_traj_os != NULL) {
     proxy->close_output_stream(cv_traj_name);
-    cv_traj_os = nullptr;
+    cv_traj_os = NULL;
   }
   return cvm::get_error();
 }
@@ -1672,7 +1672,7 @@ void cvm::log(std::string const &message, int min_log_level)
 {
   if (cvm::log_level() < min_log_level) return;
   // allow logging when the module is not fully initialized
-  size_t const d = (cvm::main() != nullptr) ? depth() : 0;
+  size_t const d = (cvm::main() != NULL) ? depth() : 0;
   if (d > 0) {
     proxy->log((std::string(2*d, ' '))+message);
   } else {
@@ -1780,7 +1780,7 @@ int cvm::read_index_file(char const *filename)
       }
       if (index_of_group < 0) {
         index_group_names.push_back(group_name);
-        index_groups.push_back(nullptr);
+        index_groups.push_back(NULL);
         index_of_group = index_groups.size()-1;
       }
     } else {
@@ -1799,7 +1799,7 @@ int cvm::read_index_file(char const *filename)
       pos = is.tellg();
     }
 
-    if (old_index_group != nullptr) {
+    if (old_index_group != NULL) {
       bool equal = false;
       if (new_index_group->size() == old_index_group->size()) {
         if (std::equal(new_index_group->begin(), new_index_group->end(),
@@ -1810,13 +1810,13 @@ int cvm::read_index_file(char const *filename)
       if (! equal) {
         new_index_group->clear();
         delete new_index_group;
-        new_index_group = nullptr;
+        new_index_group = NULL;
         return cvm::error("Error: the index group \""+group_name+
                           "\" was redefined.\n", INPUT_ERROR);
       } else {
         old_index_group->clear();
         delete old_index_group;
-        old_index_group = nullptr;
+        old_index_group = NULL;
       }
     }
 
@@ -1850,7 +1850,7 @@ int colvarmodule::reset_index_groups()
   size_t i = 0;
   for ( ; i < index_groups.size(); i++) {
     delete index_groups[i];
-    index_groups[i] = nullptr;
+    index_groups[i] = NULL;
   }
   index_group_names.clear();
   index_groups.clear();
@@ -2179,7 +2179,7 @@ std::string cvm::wrap_string(std::string const &s, size_t nchars)
 
 
 // shared pointer to the proxy object
-colvarproxy *colvarmodule::proxy = nullptr;
+colvarproxy *colvarmodule::proxy = NULL;
 
 // static runtime data
 cvm::real colvarmodule::debug_gradients_step_size = 1.0e-07;
diff --git a/lib/colvars/colvarmodule.h b/lib/colvars/colvarmodule.h
index e06f05aca0..9346e8e1d6 100644
--- a/lib/colvars/colvarmodule.h
+++ b/lib/colvars/colvarmodule.h
@@ -459,13 +459,13 @@ public:
   /// Backup a file before writing it
   static int backup_file(char const *filename);
 
-  /// Look up a bias by name; returns nullptr if not found
+  /// Look up a bias by name; returns NULL if not found
   static colvarbias * bias_by_name(std::string const &name);
 
-  /// Look up a colvar by name; returns nullptr if not found
+  /// Look up a colvar by name; returns NULL if not found
   static colvar * colvar_by_name(std::string const &name);
 
-  /// Look up a named atom group by name; returns nullptr if not found
+  /// Look up a named atom group by name; returns NULL if not found
   static atom_group * atom_group_by_name(std::string const &name);
 
   /// Load new configuration for the given bias -
diff --git a/lib/colvars/colvarparams.cpp b/lib/colvars/colvarparams.cpp
index ea45c01104..e957a1841d 100644
--- a/lib/colvars/colvarparams.cpp
+++ b/lib/colvars/colvarparams.cpp
@@ -76,7 +76,7 @@ void const *colvarparams::get_param_ptr(std::string const &param_name)
     return param_map[param_name];
   }
   cvm::error("Error: parameter \""+param_name+"\" not found.\n", INPUT_ERROR);
-  return nullptr;
+  return NULL;
 }
 
 
@@ -87,7 +87,7 @@ void const *colvarparams::get_param_grad_ptr(std::string const &param_name)
   }
   cvm::error("Error: gradient of parameter \""+param_name+"\" not found.\n",
              INPUT_ERROR);
-  return nullptr;
+  return NULL;
 }
 
 
@@ -95,7 +95,7 @@ cvm::real colvarparams::get_param(std::string const &param_name)
 {
   cvm::real const *ptr =
     reinterpret_cast<cvm::real const *>(get_param_ptr(param_name));
-  return ptr != nullptr ? *ptr : 0.0;
+  return ptr != NULL ? *ptr : 0.0;
 }
 
 
diff --git a/lib/colvars/colvarparse.cpp b/lib/colvars/colvarparse.cpp
index e61ebf355f..a09d6be9ef 100644
--- a/lib/colvars/colvarparse.cpp
+++ b/lib/colvars/colvarparse.cpp
@@ -674,12 +674,12 @@ bool colvarparse::key_lookup(std::string const &conf,
   std::string const conf_lower(to_lower_cppstr(conf));
 
   // by default, there is no value, unless we found one
-  if (data != nullptr) {
+  if (data != NULL) {
     data->clear();
   }
 
   // start from the first occurrence of key
-  size_t pos = conf_lower.find(key, (save_pos != nullptr) ? *save_pos : 0);
+  size_t pos = conf_lower.find(key, (save_pos != NULL) ? *save_pos : 0);
 
   // iterate over all instances of the substring until it finds it as isolated keyword
   while (true) {
@@ -727,7 +727,7 @@ bool colvarparse::key_lookup(std::string const &conf,
     }
   }
 
-  if (save_pos != nullptr) {
+  if (save_pos != NULL) {
   // save the pointer for a future call (when iterating over multiple
   // valid instances of the same keyword)
     *save_pos = pos + key.size();
@@ -820,7 +820,7 @@ bool colvarparse::key_lookup(std::string const &conf,
                                        data_end) + 1;
     }
 
-    if (data != nullptr) {
+    if (data != NULL) {
       data->append(line, data_begin, (data_end-data_begin));
 
       if (cvm::debug()) {
@@ -834,7 +834,7 @@ bool colvarparse::key_lookup(std::string const &conf,
     }
   }
 
-  if (save_pos != nullptr) *save_pos = line_end;
+  if (save_pos != NULL) *save_pos = line_end;
 
   return true;
 }
diff --git a/lib/colvars/colvarparse.h b/lib/colvars/colvarparse.h
index fc1362b2b1..b7d42fdffa 100644
--- a/lib/colvars/colvarparse.h
+++ b/lib/colvars/colvarparse.h
@@ -275,12 +275,12 @@ public:
     /// The keyword that identifies the block
     std::string const key;
 
-    /// Where to keep the data (may be nullptr)
+    /// Where to keep the data (may be NULL)
     std::string * const data;
 
   public:
 
-    read_block(std::string const &key_in, std::string *data_in = nullptr);
+    read_block(std::string const &key_in, std::string *data_in = NULL);
 
     ~read_block();
 
@@ -300,8 +300,8 @@ public:
   /// within "conf", useful when doing multiple calls
   bool key_lookup(std::string const &conf,
                   char const *key,
-                  std::string *data = nullptr,
-                  size_t *save_pos = nullptr);
+                  std::string *data = NULL,
+                  size_t *save_pos = NULL);
 
   /// \brief Reads a configuration line, adds it to config_string, and returns
   /// the stream \param is Input stream \param line String that will hold the
diff --git a/lib/colvars/colvarproxy.cpp b/lib/colvars/colvarproxy.cpp
index 7eb0b22cb5..63ae07a57c 100644
--- a/lib/colvars/colvarproxy.cpp
+++ b/lib/colvars/colvarproxy.cpp
@@ -296,7 +296,7 @@ void colvarproxy_atom_groups::clear_atom_group(int index)
 colvarproxy_smp::colvarproxy_smp()
 {
   b_smp_active = true; // May be disabled by user option
-  omp_lock_state = nullptr;
+  omp_lock_state = NULL;
 #if defined(_OPENMP)
   if (smp_thread_id() == 0) {
     omp_lock_state = reinterpret_cast<void *>(new omp_lock_t);
@@ -459,7 +459,7 @@ int colvarproxy_smp::smp_unlock()
 
 colvarproxy_script::colvarproxy_script()
 {
-  script = nullptr;
+  script = NULL;
 }
 
 
@@ -507,7 +507,7 @@ int colvarproxy_script::run_colvar_gradient_callback(std::string const & /* name
 
 colvarproxy_tcl::colvarproxy_tcl()
 {
-  tcl_interp_ = nullptr;
+  tcl_interp_ = NULL;
 }
 
 
@@ -528,7 +528,7 @@ char const *colvarproxy_tcl::tcl_obj_to_str(unsigned char *obj)
 #if defined(COLVARS_TCL)
   return Tcl_GetString(reinterpret_cast<Tcl_Obj *>(obj));
 #else
-  return nullptr;
+  return NULL;
 #endif
 }
 
@@ -666,7 +666,7 @@ std::ostream * colvarproxy_io::output_stream(std::string const &output_name,
   }
 
   std::ostream *os = get_output_stream(output_name);
-  if (os != nullptr) return os;
+  if (os != NULL) return os;
 
   if (!(mode & (std::ios_base::app | std::ios_base::ate))) {
     backup_file(output_name);
@@ -675,7 +675,7 @@ std::ostream * colvarproxy_io::output_stream(std::string const &output_name,
   if (!osf->is_open()) {
     cvm::error("Error: cannot write to file/channel \""+output_name+"\".\n",
                FILE_ERROR);
-    return nullptr;
+    return NULL;
   }
   output_stream_names.push_back(output_name);
   output_files.push_back(osf);
@@ -692,7 +692,7 @@ std::ostream *colvarproxy_io::get_output_stream(std::string const &output_name)
       return *osi;
     }
   }
-  return nullptr;
+  return NULL;
 }
 
 
@@ -794,7 +794,7 @@ int colvarproxy_io::rename_file(char const *filename, char const *newfilename)
 
 colvarproxy::colvarproxy()
 {
-  colvars = nullptr;
+  colvars = NULL;
   b_simulation_running = true;
   b_delete_requested = false;
 }
diff --git a/lib/colvars/colvarproxy.h b/lib/colvars/colvarproxy.h
index a46589a526..ba91afb40e 100644
--- a/lib/colvars/colvarproxy.h
+++ b/lib/colvars/colvarproxy.h
@@ -607,7 +607,7 @@ public:
                                       std::ios_base::openmode mode =
                                       std::ios_base::out);
 
-  /// Returns a reference to output_name if it exists, nullptr otherwise
+  /// Returns a reference to output_name if it exists, NULL otherwise
   virtual std::ostream *get_output_stream(std::string const &output_name);
 
   /// \brief Flushes the given output channel
diff --git a/lib/colvars/colvarscript.cpp b/lib/colvars/colvarscript.cpp
index a8b5d6ab2f..10b276c89e 100644
--- a/lib/colvars/colvarscript.cpp
+++ b/lib/colvars/colvarscript.cpp
@@ -97,7 +97,7 @@ int colvarscript::run(int objc, unsigned char *const objv[])
     }
     std::string const name(obj_to_str(objv[2]));
     colvar *cv = cvm::colvar_by_name(name);
-    if (cv == nullptr) {
+    if (cv == NULL) {
       result = "Colvar not found: " + name;
       return COLVARSCRIPT_ERROR;
     }
@@ -111,7 +111,7 @@ int colvarscript::run(int objc, unsigned char *const objv[])
     }
     std::string const name(obj_to_str(objv[2]));
     colvarbias *b = cvm::bias_by_name(name);
-    if (b == nullptr) {
+    if (b == NULL) {
       result = "Bias not found: " + name;
       return COLVARSCRIPT_ERROR;
     }
@@ -186,7 +186,7 @@ int colvarscript::run(int objc, unsigned char *const objv[])
 
   /// Parse config from string
   if (cmd == "config") {
-    return exec_command(cv_config, nullptr, objc, objv);
+    return exec_command(cv_config, NULL, objc, objv);
   }
 
   /// Load an input state file
@@ -247,7 +247,7 @@ int colvarscript::run(int objc, unsigned char *const objv[])
     } else if (objc == 3) {
       // Failure of this function does not trigger an error, but
       // returns nonzero, to let scripts detect available frames
-      int error = proxy->set_frame(strtol(obj_to_str(objv[2]), nullptr, 10));
+      int error = proxy->set_frame(strtol(obj_to_str(objv[2]), NULL, 10));
       result = cvm::to_str(error == COLVARS_OK ? 0 : -1);
       return COLVARS_OK;
     } else {
@@ -258,7 +258,7 @@ int colvarscript::run(int objc, unsigned char *const objv[])
 
   if (cmd == "addenergy") {
     if (objc == 3) {
-      colvars->total_bias_energy += strtod(obj_to_str(objv[2]), nullptr);
+      colvars->total_bias_energy += strtod(obj_to_str(objv[2]), NULL);
       return COLVARS_OK;
     } else {
       result = "Wrong arguments to command \"addenergy\"\n" + help_string();
@@ -267,7 +267,7 @@ int colvarscript::run(int objc, unsigned char *const objv[])
   }
 
   if (cmd == "help") {
-    return exec_command(cv_help, nullptr, objc, objv);
+    return exec_command(cv_help, NULL, objc, objv);
   }
 
   result = "Syntax error\n" + help_string();
@@ -320,7 +320,7 @@ int colvarscript::proc_colvar(colvar *cv, int objc, unsigned char *const objv[])
     // colvar destructor is tasked with the cleanup
     delete cv;
     // TODO this could be done by the destructors
-    if (colvars->cv_traj_os != nullptr) {
+    if (colvars->cv_traj_os != NULL) {
       colvars->write_traj_label(*(colvars->cv_traj_os));
     }
     return COLVARS_OK;
@@ -513,7 +513,7 @@ int colvarscript::proc_bias(colvarbias *b, int objc, unsigned char *const objv[]
     // the bias destructor takes care of the cleanup at cvm level
     delete b;
     // TODO this could be done by the destructors
-    if (colvars->cv_traj_os != nullptr) {
+    if (colvars->cv_traj_os != NULL) {
       colvars->write_traj_label(*(colvars->cv_traj_os));
     }
     return COLVARS_OK;
@@ -564,7 +564,7 @@ int colvarscript::proc_features(colvardeps *obj,
   if ((subcmd == "get") || (subcmd == "set")) {
     std::vector<colvardeps::feature *> const &features = obj->features();
     std::string const req_feature(obj_to_str(objv[3]));
-    colvardeps::feature *f = nullptr;
+    colvardeps::feature *f = NULL;
     int fid = 0;
     for (fid = 0; fid < int(features.size()); fid++) {
       if (features[fid]->description ==
@@ -574,7 +574,7 @@ int colvarscript::proc_features(colvardeps *obj,
       }
     }
 
-    if (f == nullptr) {
+    if (f == NULL) {
 
       result = "Error: feature \""+req_feature+"\" does not exist.\n";
       return COLVARSCRIPT_ERROR;
diff --git a/lib/colvars/colvarscript.h b/lib/colvars/colvarscript.h
index 04679b37ad..69d52cbb51 100644
--- a/lib/colvars/colvarscript.h
+++ b/lib/colvars/colvarscript.h
@@ -278,7 +278,7 @@ extern "C" {
            1, 1,
            { "E (float) - Amount of energy to add" },
            cvm::main()->total_bias_energy +=
-             strtod(script->obj_to_str(objv[2]), nullptr);
+             strtod(script->obj_to_str(objv[2]), NULL);
            return COLVARS_OK;
            )
 
diff --git a/lib/colvars/colvartypes.h b/lib/colvars/colvartypes.h
index 55401576b1..e5f12154e6 100644
--- a/lib/colvars/colvartypes.h
+++ b/lib/colvars/colvartypes.h
@@ -59,7 +59,7 @@ public:
     if (data.size() > 0) {
       return &(data[0]);
     } else {
-      return nullptr;
+      return NULL;
     }
   }
 
@@ -520,7 +520,7 @@ public:
     if (rows.size() > 0) {
       return &(pointers[0]);
     } else {
-      return nullptr;
+      return NULL;
     }
   }
 
diff --git a/lib/colvars/lepton/include/lepton/Operation.h b/lib/colvars/lepton/include/lepton/Operation.h
index 68474137f1..1ddde0b8c0 100644
--- a/lib/colvars/lepton/include/lepton/Operation.h
+++ b/lib/colvars/lepton/include/lepton/Operation.h
@@ -186,7 +186,7 @@ public:
     }
     bool operator!=(const Operation& op) const {
         const Constant* o = dynamic_cast<const Constant*>(&op);
-        return (o == nullptr || o->value != value);
+        return (o == NULL || o->value != value);
     }
 private:
     double value;
@@ -217,7 +217,7 @@ public:
     ExpressionTreeNode differentiate(const std::vector<ExpressionTreeNode>& children, const std::vector<ExpressionTreeNode>& childDerivs, const std::string& variable) const;
     bool operator!=(const Operation& op) const {
         const Variable* o = dynamic_cast<const Variable*>(&op);
-        return (o == nullptr || o->name != name);
+        return (o == NULL || o->name != name);
     }
 private:
     std::string name;
@@ -264,7 +264,7 @@ public:
     }
     bool operator!=(const Operation& op) const {
         const Custom* o = dynamic_cast<const Custom*>(&op);
-        return (o == nullptr || o->name != name || o->isDerivative != isDerivative || o->derivOrder != derivOrder);
+        return (o == NULL || o->name != name || o->isDerivative != isDerivative || o->derivOrder != derivOrder);
     }
 private:
     std::string name;
@@ -955,7 +955,7 @@ public:
     }
     bool operator!=(const Operation& op) const {
         const AddConstant* o = dynamic_cast<const AddConstant*>(&op);
-        return (o == nullptr || o->value != value);
+        return (o == NULL || o->value != value);
     }
 private:
     double value;
@@ -988,7 +988,7 @@ public:
     }
     bool operator!=(const Operation& op) const {
         const MultiplyConstant* o = dynamic_cast<const MultiplyConstant*>(&op);
-        return (o == nullptr || o->value != value);
+        return (o == NULL || o->value != value);
     }
 private:
     double value;
@@ -1042,7 +1042,7 @@ public:
     }
     bool operator!=(const Operation& op) const {
         const PowerConstant* o = dynamic_cast<const PowerConstant*>(&op);
-        return (o == nullptr || o->value != value);
+        return (o == NULL || o->value != value);
     }
     bool isInfixOperator() const {
         return true;
diff --git a/lib/colvars/lepton/src/CompiledExpression.cpp b/lib/colvars/lepton/src/CompiledExpression.cpp
index 6abcad2de2..1ad348b47d 100644
--- a/lib/colvars/lepton/src/CompiledExpression.cpp
+++ b/lib/colvars/lepton/src/CompiledExpression.cpp
@@ -40,10 +40,10 @@ using namespace std;
     using namespace asmjit;
 #endif
 
-CompiledExpression::CompiledExpression() : jitCode(nullptr) {
+CompiledExpression::CompiledExpression() : jitCode(NULL) {
 }
 
-CompiledExpression::CompiledExpression(const ParsedExpression& expression) : jitCode(nullptr) {
+CompiledExpression::CompiledExpression(const ParsedExpression& expression) : jitCode(NULL) {
     ParsedExpression expr = expression.optimize(); // Just in case it wasn't already optimized.
     vector<pair<ExpressionTreeNode, int> > temps;
     compileExpression(expr.getRootNode(), temps);
@@ -59,11 +59,11 @@ CompiledExpression::CompiledExpression(const ParsedExpression& expression) : jit
 
 CompiledExpression::~CompiledExpression() {
     for (int i = 0; i < (int) operation.size(); i++)
-        if (operation[i] != nullptr)
+        if (operation[i] != NULL)
             delete operation[i];
 }
 
-CompiledExpression::CompiledExpression(const CompiledExpression& expression) : jitCode(nullptr) {
+CompiledExpression::CompiledExpression(const CompiledExpression& expression) : jitCode(NULL) {
     *this = expression;
 }
 
diff --git a/lib/colvars/lepton/src/ExpressionTreeNode.cpp b/lib/colvars/lepton/src/ExpressionTreeNode.cpp
index 09c5352d20..8519878262 100644
--- a/lib/colvars/lepton/src/ExpressionTreeNode.cpp
+++ b/lib/colvars/lepton/src/ExpressionTreeNode.cpp
@@ -59,14 +59,14 @@ ExpressionTreeNode::ExpressionTreeNode(Operation* operation) : operation(operati
         throw Exception("wrong number of arguments to function: "+operation->getName());
 }
 
-ExpressionTreeNode::ExpressionTreeNode(const ExpressionTreeNode& node) : operation(node.operation == nullptr ? nullptr : node.operation->clone()), children(node.getChildren()) {
+ExpressionTreeNode::ExpressionTreeNode(const ExpressionTreeNode& node) : operation(node.operation == NULL ? NULL : node.operation->clone()), children(node.getChildren()) {
 }
 
-ExpressionTreeNode::ExpressionTreeNode() : operation(nullptr) {
+ExpressionTreeNode::ExpressionTreeNode() : operation(NULL) {
 }
 
 ExpressionTreeNode::~ExpressionTreeNode() {
-    if (operation != nullptr)
+    if (operation != NULL)
         delete operation;
 }
 
@@ -91,7 +91,7 @@ bool ExpressionTreeNode::operator==(const ExpressionTreeNode& node) const {
 }
 
 ExpressionTreeNode& ExpressionTreeNode::operator=(const ExpressionTreeNode& node) {
-    if (operation != nullptr)
+    if (operation != NULL)
         delete operation;
     operation = node.getOperation().clone();
     children = node.getChildren();
diff --git a/lib/colvars/lepton/src/ParsedExpression.cpp b/lib/colvars/lepton/src/ParsedExpression.cpp
index c259756346..fd3b091d3c 100644
--- a/lib/colvars/lepton/src/ParsedExpression.cpp
+++ b/lib/colvars/lepton/src/ParsedExpression.cpp
@@ -46,7 +46,7 @@ ParsedExpression::ParsedExpression(const ExpressionTreeNode& rootNode) : rootNod
 }
 
 const ExpressionTreeNode& ParsedExpression::getRootNode() const {
-    if (&rootNode.getOperation() == nullptr)
+    if (&rootNode.getOperation() == NULL)
         throw Exception("Illegal call to an initialized ParsedExpression");
     return rootNode;
 }
diff --git a/lib/kokkos/cmake/compile_tests/pthread.cpp b/lib/kokkos/cmake/compile_tests/pthread.cpp
index 7fa98611be..92310da029 100644
--- a/lib/kokkos/cmake/compile_tests/pthread.cpp
+++ b/lib/kokkos/cmake/compile_tests/pthread.cpp
@@ -4,11 +4,11 @@ void* kokkos_test(void* args) { return args; }
 
 int main(void) {
   pthread_t thread;
-  /* Use nullptr to avoid C++11. Some compilers
+  /* Use NULL to avoid C++11. Some compilers
      do not have C++11 by default.  Forcing C++11
      in the compile tests can be done, but is unnecessary
   */
-  pthread_create(&thread, nullptr, kokkos_test, nullptr);
-  pthread_join(thread, nullptr);
+  pthread_create(&thread, NULL, kokkos_test, NULL);
+  pthread_join(thread, NULL);
   return 0;
 }
diff --git a/lib/kokkos/core/src/HIP/Kokkos_HIP_Space.cpp b/lib/kokkos/core/src/HIP/Kokkos_HIP_Space.cpp
index d4db2c538d..a97fb2f7cc 100644
--- a/lib/kokkos/core/src/HIP/Kokkos_HIP_Space.cpp
+++ b/lib/kokkos/core/src/HIP/Kokkos_HIP_Space.cpp
@@ -640,7 +640,7 @@ void SharedAllocationRecord<Kokkos::Experimental::HIPSpace, void>::
 /*--------------------------------------------------------------------------*/
 
 void* hip_resize_scratch_space(size_t bytes, bool force_shrink) {
-  static void* ptr           = nullptr;
+  static void* ptr           = NULL;
   static size_t current_size = 0;
   if (current_size == 0) {
     current_size = bytes;
diff --git a/lib/kokkos/core/src/impl/Kokkos_HBWSpace.cpp b/lib/kokkos/core/src/impl/Kokkos_HBWSpace.cpp
index 613793fc60..fad36c02f1 100644
--- a/lib/kokkos/core/src/impl/Kokkos_HBWSpace.cpp
+++ b/lib/kokkos/core/src/impl/Kokkos_HBWSpace.cpp
@@ -139,7 +139,7 @@ void *HBWSpace::allocate(const char *arg_label, const size_t arg_alloc_size,
     }
     msg << " ]( " << arg_alloc_size << " ) FAILED";
     if (ptr == nullptr) {
-      msg << " NULL";
+      msg << " nullptr";
     } else {
       msg << " NOT ALIGNED " << ptr;
     }
diff --git a/lib/kokkos/core/src/impl/Kokkos_SharedAlloc.cpp b/lib/kokkos/core/src/impl/Kokkos_SharedAlloc.cpp
index 2b11adbf6d..a9a2778813 100644
--- a/lib/kokkos/core/src/impl/Kokkos_SharedAlloc.cpp
+++ b/lib/kokkos/core/src/impl/Kokkos_SharedAlloc.cpp
@@ -215,7 +215,7 @@ SharedAllocationRecord<void, void>::SharedAllocationRecord(
 
   } else {
     Kokkos::Impl::throw_runtime_exception(
-        "Kokkos::Impl::SharedAllocationRecord given NULL allocation");
+        "Kokkos::Impl::SharedAllocationRecord given nullptr allocation");
   }
 }
 
diff --git a/lib/kokkos/core/unit_test/TestCTestDevice.cpp b/lib/kokkos/core/unit_test/TestCTestDevice.cpp
index c54c75f94b..b2ee79b856 100644
--- a/lib/kokkos/core/unit_test/TestCTestDevice.cpp
+++ b/lib/kokkos/core/unit_test/TestCTestDevice.cpp
@@ -13,7 +13,7 @@ int setenv(const char *name, const char *value, int overwrite) {
   int errcode = 0;
   if (!overwrite) {
     size_t envsize = 0;
-    errcode        = getenv_s(&envsize, nullptr, 0, name);
+    errcode        = getenv_s(&envsize, NULL, 0, name);
     if (errcode || envsize) return errcode;
   }
   return _putenv_s(name, value);