diff --git a/src/LEPTON/pair_lepton.cpp b/src/LEPTON/pair_lepton.cpp
index 46377a341a..f596a901f0 100644
--- a/src/LEPTON/pair_lepton.cpp
+++ b/src/LEPTON/pair_lepton.cpp
@@ -200,11 +200,15 @@ void PairLepton::coeff(int narg, char **arg)
   utils::bounds(FLERR, arg[0], 1, atom->ntypes, ilo, ihi, error);
   utils::bounds(FLERR, arg[1], 1, atom->ntypes, jlo, jhi, error);
 
-  std::string exp_one = arg[2];
   double cut_one = cut_global;
   if (narg == 4) cut_one = utils::numeric(FLERR, arg[3], false, lmp);
 
-  // check if the expression can be parsed and evaluated as needed without error
+  // remove whitespace and quotes from expression string and then
+  // check if the expression can be parsed and evaluated without error
+  std::string exp_one;
+  for (const auto &c : std::string(arg[2]))
+    if (!isspace(c) && (c != '"') && (c != '\'')) exp_one.push_back(c);
+
   try {
     auto epot = LMP_Lepton::Parser::parse(exp_one).createCompiledExpression();
     auto force = LMP_Lepton::Parser::parse(exp_one).differentiate("r").createCompiledExpression();
@@ -258,7 +262,7 @@ double PairLepton::init_one(int i, int j)
 void PairLepton::write_data(FILE *fp)
 {
   for (int i = 1; i <= atom->ntypes; i++)
-    fprintf(fp, "%d '%s' %g\n", i, expressions[type2expression[i][i]].c_str(), cut[i][i]);
+    fprintf(fp, "%d %s %g\n", i, expressions[type2expression[i][i]].c_str(), cut[i][i]);
 }
 
 /* ----------------------------------------------------------------------
@@ -269,7 +273,7 @@ void PairLepton::write_data_all(FILE *fp)
 {
   for (int i = 1; i <= atom->ntypes; i++)
     for (int j = i; j <= atom->ntypes; j++)
-      fprintf(fp, "%d %d '%s' %g\n", i, j, expressions[type2expression[i][j]].c_str(), cut[i][j]);
+      fprintf(fp, "%d %d %s %g\n", i, j, expressions[type2expression[i][j]].c_str(), cut[i][j]);
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/unittest/force-styles/tests/mol-pair-lepton.yaml b/unittest/force-styles/tests/mol-pair-lepton.yaml
index eaed580b02..56640e3834 100644
--- a/unittest/force-styles/tests/mol-pair-lepton.yaml
+++ b/unittest/force-styles/tests/mol-pair-lepton.yaml
@@ -14,17 +14,17 @@ input_file: in.fourmol
 pair_style: lepton 8.0
 pair_coeff: ! |
   * *    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.015;sig=3.1"
-  1 1    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.02;sig=2.5"
+  1 1    '4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.02;sig=2.5'
   1 2    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.01;sig=1.75"
-  1 3    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.02;sig=2.85"
-  1 4*5  "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.0173205;sig=2.8"
-  2 2    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.005;sig=1.0"
-  2 3    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.01;sig=2.1"
-  2 4    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.005;sig=0.5"
-  2 5    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.00866025;sig=2.05"
-  3 3    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.02;sig=3.2"
-  3 4    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.0173205;sig=3.15"
-  3 5    "4.0*eps*((sig/r)^12 - (sig/r)^6);eps=0.0173205;sig=3.15"
+  1 3    '4.0*eps*((sig/r)^12-(sig/r)^6);  eps=0.02;sig=2.85'
+  1 4*5  "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.0173205; 	sig=2.8"
+  2 2    "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.005;sig=1.0"
+  2 3    "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.01;sig=2.1"
+  2 4    "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.005;sig=0.5"
+  2 5    "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.00866025;sig=2.05"
+  3 3    "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.02;sig=3.2"
+  3 4    "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.0173205;sig=3.15"
+  3 5    "4.0*eps*((sig/r)^12-(sig/r)^6);eps=0.0173205;sig=3.15"
 extract: ! ""
 natoms: 29
 init_vdwl: 749.2370315373564