diff --git a/src/variable.cpp b/src/variable.cpp
index 6766a67a9a..eb1deaae15 100644
--- a/src/variable.cpp
+++ b/src/variable.cpp
@@ -39,6 +39,7 @@
 #include <cmath>
 #include <cstring>
 #include <unistd.h>
+#include <unordered_map>
 #include <vector>
 
 using namespace LAMMPS_NS;
@@ -76,6 +77,20 @@ enum{SUM,XMIN,XMAX,AVE,TRAP,SLOPE};
 
 #define BIG 1.0e20
 
+// constants for variable expressions. customize by adding new items.
+// if needed (cf. 'version') initialize in Variable class constructor.
+
+static std::unordered_map<std::string, double> constants = {
+  {"PI", MY_PI },
+  {"version", -1 },
+  {"yes", 1 },
+  {"no", 0 },
+  {"on", 1 },
+  {"off", 0 },
+  {"true", 1 },
+  {"false", 0 }
+};
+
 /* ---------------------------------------------------------------------- */
 
 Variable::Variable(LAMMPS *lmp) : Pointers(lmp)
@@ -98,6 +113,10 @@ Variable::Variable(LAMMPS *lmp) : Pointers(lmp)
   randomequal = nullptr;
   randomatom = nullptr;
 
+  // override initializer since LAMMPS class needs to be instantiated
+
+  constants["version"] = lmp->num_ver;
+
   // customize by assigning a precedence level
 
   precedence[DONE] = 0;
@@ -2093,8 +2112,8 @@ double Variable::evaluate(char *str, Tree **tree, int ivar)
         // constant
         // ----------------
 
-        } else if (is_constant(word)) {
-          value1 = constant(word);
+        } else if (constants.find(word) != constants.end()) {
+          value1 = constants[word];
           if (tree) {
             Tree *newtree = new Tree();
             newtree->type = VALUE;
@@ -4663,43 +4682,6 @@ void Variable::atom_vector(char *word, Tree **tree,
   }
 }
 
-/* ----------------------------------------------------------------------
-   check if word matches a constant
-   return 1 if yes, else 0
-   customize by adding a constant: PI, version
-------------------------------------------------------------------------- */
-
-int Variable::is_constant(char *word)
-{
-  if (strcmp(word,"PI") == 0) return 1;
-  if (strcmp(word,"version") == 0) return 1;
-  if (strcmp(word,"yes") == 0) return 1;
-  if (strcmp(word,"no") == 0) return 1;
-  if (strcmp(word,"on") == 0) return 1;
-  if (strcmp(word,"off") == 0) return 1;
-  if (strcmp(word,"true") == 0) return 1;
-  if (strcmp(word,"false") == 0) return 1;
-  return 0;
-}
-
-/* ----------------------------------------------------------------------
-   process a constant in formula
-   customize by adding a constant: PI, version
-------------------------------------------------------------------------- */
-
-double Variable::constant(char *word)
-{
-  if (strcmp(word,"PI") == 0) return MY_PI;
-  if (strcmp(word,"version") == 0) return lmp->num_ver;
-  if (strcmp(word,"yes") == 0) return 1.0;
-  if (strcmp(word,"no") == 0) return 0.0;
-  if (strcmp(word,"on") == 0) return 1.0;
-  if (strcmp(word,"off") == 0) return 0.0;
-  if (strcmp(word,"true") == 0) return 1.0;
-  if (strcmp(word,"false") == 0) return 0.0;
-  return 0.0;
-}
-
 /* ----------------------------------------------------------------------
    parse string for comma-separated args
    store copy of each arg in args array
diff --git a/src/variable.h b/src/variable.h
index cb74e71b51..5dc8748936 100644
--- a/src/variable.h
+++ b/src/variable.h
@@ -122,8 +122,6 @@ class Variable : protected Pointers {
                       Tree **, Tree **, int &, double *, int &);
   int is_atom_vector(char *);
   void atom_vector(char *, Tree **, Tree **, int &);
-  int is_constant(char *);
-  double constant(char *);
   int parse_args(char *, char **);
   void print_var_error(const std::string &, int, const std::string &,
                        int, int global=1);
diff --git a/unittest/commands/test_variables.cpp b/unittest/commands/test_variables.cpp
index 8c0a24bc0d..cc20ea2b96 100644
--- a/unittest/commands/test_variables.cpp
+++ b/unittest/commands/test_variables.cpp
@@ -211,14 +211,17 @@ TEST_F(VariableTest, AtomicSystem)
 
 TEST_F(VariableTest, Expressions)
 {
+    atomic_system();
     ASSERT_EQ(variable->nvar, 0);
     if (!verbose) ::testing::internal::CaptureStdout();
     command("variable one    index     1");
     command("variable two    equal     2");
     command("variable three  equal     v_one+v_two");
     command("variable four   equal     PI");
+    command("variable five   equal     version");
+    command("variable six    equal     XXX");
     if (!verbose) ::testing::internal::GetCapturedStdout();
-    ASSERT_EQ(variable->nvar, 4);
+    ASSERT_EQ(variable->nvar, 6);
 
     int ivar = variable->find("one");
     ASSERT_FALSE(variable->equalstyle(ivar));
@@ -229,11 +232,18 @@ TEST_F(VariableTest, Expressions)
     ASSERT_DOUBLE_EQ(variable->compute_equal(ivar),3.0);
     ivar = variable->find("four");
     ASSERT_DOUBLE_EQ(variable->compute_equal(ivar),MY_PI);
+    ivar = variable->find("five");
+    ASSERT_GE(variable->compute_equal(ivar),20210310);
+
+    TEST_FAILURE(".*ERROR: Variable six: Invalid thermo keyword 'XXX' in variable formula.*",
+                 command("print \"${six}\""););
 }
 
 TEST_F(VariableTest, Functions)
 {
+    atomic_system();
     file_vars();
+
     ASSERT_EQ(variable->nvar, 0);
     if (!verbose) ::testing::internal::CaptureStdout();
     command("variable one    index     1");