diff --git a/src/compute.cpp b/src/compute.cpp
index 5f41cbf837..f3b24f7bbc 100644
--- a/src/compute.cpp
+++ b/src/compute.cpp
@@ -93,6 +93,11 @@ Compute::Compute(LAMMPS *lmp, int narg, char **arg) : Pointers(lmp)
 
   datamask = ALL_MASK;
   datamask_ext = ALL_MASK;
+
+  // force init to zero in case these are used as logicals
+
+  vector = vector_atom = vector_local = NULL;
+  array = array_atom = array_local = NULL;
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/compute_atom_molecule.cpp b/src/compute_atom_molecule.cpp
index 4442a72255..8b0d413c23 100644
--- a/src/compute_atom_molecule.cpp
+++ b/src/compute_atom_molecule.cpp
@@ -106,7 +106,7 @@ ComputeAtomMolecule(LAMMPS *lmp, int narg, char **arg) :
       int ifix = modify->find_fix(ids[i]);
       if (ifix < 0)
         error->all(FLERR,"Fix ID for compute atom/molecule does not exist");
-      if (modify->fix[ifix]->peratom_flag)
+      if (modify->fix[ifix]->peratom_flag == 0)
         error->all(FLERR,"Compute atom/molecule fix does not "
                    "calculate per-atom values");
       if (argindex[i] == 0 &&
@@ -299,9 +299,8 @@ void ComputeAtomMolecule::compute_one(int m)
       peratom = compute->vector_atom;
       nstride = 1;
     } else {
-      if (compute->array_atom) peratom = &compute->array_atom[0][aidx-1];
-      else peratom = NULL;
-      nstride = compute->size_array_cols;
+      peratom = &compute->array_atom[0][aidx-1];
+      nstride = compute->size_peratom_cols;
     }
 
   // access fix fields, check if fix frequency is a match
@@ -317,7 +316,7 @@ void ComputeAtomMolecule::compute_one(int m)
       nstride = 1;
     } else {
       peratom = &fix->array_atom[0][aidx-1];
-      nstride = fix->size_array_cols;
+      nstride = fix->size_peratom_cols;
     }
 
   // evaluate atom-style variable
diff --git a/src/fix_gravity.cpp b/src/fix_gravity.cpp
index 24bc7b4b3c..2e9d6e40f4 100644
--- a/src/fix_gravity.cpp
+++ b/src/fix_gravity.cpp
@@ -41,6 +41,7 @@ FixGravity::FixGravity(LAMMPS *lmp, int narg, char **arg) :
 {
   if (narg < 5) error->all(FLERR,"Illegal fix gravity command");
 
+  dynamic_group_allow = 1;
   scalar_flag = 1;
   global_freq = 1;
   extscalar = 1;