diff --git a/unittest/formats/test_file_operations.cpp b/unittest/formats/test_file_operations.cpp
index 9c75029130..d48dee34b2 100644
--- a/unittest/formats/test_file_operations.cpp
+++ b/unittest/formats/test_file_operations.cpp
@@ -469,6 +469,8 @@ TEST_F(FileOperationsTest, write_data)
 
     TEST_FAILURE(".*ERROR: Cannot open file noexist.data: No such file or directory.*",
                  command("read_data noexist.data"););
+    TEST_FAILURE(".*ERROR: Unknown read_data keyword xxx.*",
+                 command("read_data noexist.data xxx"););
 
     BEGIN_HIDE_OUTPUT();
     command("pair_style zero 1.0");
@@ -501,6 +503,116 @@ TEST_F(FileOperationsTest, write_data)
     delete_file("triclinic.data");
 }
 
+#define GETIDX(i) lmp->atom->map(i)
+TEST_F(FileOperationsTest, read_data_fix)
+{
+    ASSERT_EQ(lmp->restart_ver, -1);
+    BEGIN_HIDE_OUTPUT();
+    command("echo none");
+    command("atom_modify map array");
+    command("fix MoleculeIDs all property/atom mol");
+    command("region box block -2 2 -2 2 -2 2");
+    command("create_box 1 box");
+    command("create_atoms 1 single 1.0 0.0 0.0");
+    command("create_atoms 1 single 0.0 1.0 0.0");
+    command("create_atoms 1 single 1.0 0.0 1.0");
+    command("create_atoms 1 single 0.0 1.0 1.0");
+    command("mass 1 1.0");
+    command("set atom 1*2 mol 1");
+    command("set atom 3*4 mol 2");
+    command("write_data test_mol_id.data");
+    lmp->atom->molecule[0] = 5;
+    lmp->atom->molecule[1] = 6;
+    lmp->atom->molecule[2] = 5;
+    lmp->atom->molecule[3] = 6;
+    lmp->atom->tag[0] = 9;
+    lmp->atom->tag[1] = 6;
+    lmp->atom->tag[2] = 7;
+    lmp->atom->tag[3] = 8;
+    lmp->atom->map_init(1);
+    lmp->atom->map_set();
+    command("write_data test_mol_id_merge.data");
+    command("clear");
+    END_HIDE_OUTPUT();
+    TEST_FAILURE(".*ERROR: Cannot use read_data add before simulation box is defined.*",
+                 command("read_data test_mol_id.data add append"););
+
+    BEGIN_HIDE_OUTPUT();
+    command("atom_modify map array");
+    command("fix MoleculeIDs all property/atom mol");
+    command("read_data test_mol_id.data fix MoleculeIDs NULL Molecules");
+    command("read_data test_mol_id_merge.data add merge fix MoleculeIDs NULL Molecules");
+    END_HIDE_OUTPUT();
+
+    EXPECT_EQ(lmp->atom->natoms, 8);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(1)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(2)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(3)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(4)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(6)], 6);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(7)], 5);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(8)], 6);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(9)], 5);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(1)], 1);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(2)], 2);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(3)], 3);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(4)], 4);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(6)], 6);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(7)], 7);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(8)], 8);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(9)], 9);
+
+    BEGIN_HIDE_OUTPUT();
+    command("clear");
+    command("atom_modify map array");
+    command("fix MoleculeIDs all property/atom mol");
+    command("read_data test_mol_id.data fix MoleculeIDs NULL Molecules");
+    command("read_data test_mol_id.data add append fix MoleculeIDs NULL Molecules");
+    END_HIDE_OUTPUT();
+    EXPECT_EQ(lmp->atom->natoms, 8);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(1)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(2)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(3)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(4)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(5)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(6)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(7)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(8)], 2);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(1)], 1);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(2)], 2);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(3)], 3);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(4)], 4);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(5)], 5);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(6)], 6);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(7)], 7);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(8)], 8);
+
+    BEGIN_HIDE_OUTPUT();
+    command("clear");
+    command("atom_modify map array");
+    command("fix MoleculeIDs all property/atom mol");
+    command("read_data test_mol_id.data fix MoleculeIDs NULL Molecules");
+    command("read_data test_mol_id.data add 6 4 fix MoleculeIDs NULL Molecules");
+    END_HIDE_OUTPUT();
+    EXPECT_EQ(lmp->atom->natoms, 8);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(1)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(2)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(3)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(4)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(7)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(8)], 1);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(9)], 2);
+    EXPECT_EQ(lmp->atom->molecule[GETIDX(10)], 2);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(1)], 1);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(2)], 2);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(3)], 3);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(4)], 4);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(7)], 7);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(8)], 8);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(9)], 9);
+    EXPECT_EQ(lmp->atom->tag[GETIDX(10)], 10);
+}
+
 int main(int argc, char **argv)
 {
     MPI_Init(&argc, &argv);