diff --git a/unittest/commands/CMakeLists.txt b/unittest/commands/CMakeLists.txt
index 626301d404..d0793b707f 100644
--- a/unittest/commands/CMakeLists.txt
+++ b/unittest/commands/CMakeLists.txt
@@ -2,3 +2,8 @@
 add_executable(test_simple_commands test_simple_commands.cpp)
 target_link_libraries(test_simple_commands PRIVATE lammps GTest::GMock GTest::GTest)
 add_test(NAME SimpleCommands COMMAND test_simple_commands WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+
+add_executable(test_reset_mol_ids test_reset_mol_ids.cpp)
+target_compile_definitions(test_reset_mol_ids PRIVATE -DTEST_INPUT_FOLDER=${CMAKE_CURRENT_SOURCE_DIR})
+target_link_libraries(test_reset_mol_ids PRIVATE lammps GTest::GMock GTest::GTest)
+add_test(NAME ResetMolIDs COMMAND test_reset_mol_ids WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
diff --git a/unittest/commands/data.fourmol b/unittest/commands/data.fourmol
new file mode 120000
index 0000000000..ebbe325d24
--- /dev/null
+++ b/unittest/commands/data.fourmol
@@ -0,0 +1 @@
+../force-styles/tests/data.fourmol
\ No newline at end of file
diff --git a/unittest/commands/in.fourmol b/unittest/commands/in.fourmol
new file mode 120000
index 0000000000..cc47b5adb9
--- /dev/null
+++ b/unittest/commands/in.fourmol
@@ -0,0 +1 @@
+../force-styles/tests/in.fourmol
\ No newline at end of file
diff --git a/unittest/commands/test_reset_mol_ids.cpp b/unittest/commands/test_reset_mol_ids.cpp
new file mode 100644
index 0000000000..bc38c6404c
--- /dev/null
+++ b/unittest/commands/test_reset_mol_ids.cpp
@@ -0,0 +1,250 @@
+/* ----------------------------------------------------------------------
+   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
+   http://lammps.sandia.gov, Sandia National Laboratories
+   Steve Plimpton, sjplimp@sandia.gov
+
+   Copyright (2003) Sandia Corporation.  Under the terms of Contract
+   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
+   certain rights in this software.  This software is distributed under
+   the GNU General Public License.
+
+   See the README file in the top-level LAMMPS directory.
+------------------------------------------------------------------------- */
+
+#include "atom.h"
+#include "fmt/format.h"
+#include "info.h"
+#include "input.h"
+#include "lammps.h"
+#include "output.h"
+#include "update.h"
+#include "utils.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+#include <cstdio>
+#include <mpi.h>
+
+// whether to print verbose output (i.e. not capturing LAMMPS screen output).
+bool verbose = false;
+
+using LAMMPS_NS::utils::split_words;
+
+namespace LAMMPS_NS {
+using ::testing::MatchesRegex;
+
+#define GETIDX(i) lmp->atom->map(i)
+
+#define TEST_FAILURE(...)                \
+    if (Info::has_exceptions()) {        \
+        ASSERT_ANY_THROW({__VA_ARGS__}); \
+    } else {                             \
+        ASSERT_DEATH({__VA_ARGS__}, ""); \
+    }
+
+#define STRINGIFY(val) XSTR(val)
+#define XSTR(val) #val
+
+class ResetMolIDsTest : public ::testing::Test {
+protected:
+    LAMMPS *lmp;
+
+    void SetUp() override
+    {
+        const char *args[] = {"ResetMolIDsTest", "-log", "none", "-nocite", "-echo", "screen"};
+        char **argv        = (char **)args;
+        int argc           = sizeof(args) / sizeof(char *);
+        if (!verbose) ::testing::internal::CaptureStdout();
+        lmp = new LAMMPS(argc, argv, MPI_COMM_WORLD);
+        Info *info = new Info(lmp);
+        if (info->has_style("atom","full")) {
+            lmp->input->one("variable input_dir index " STRINGIFY(TEST_INPUT_FOLDER));
+            lmp->input->one("include ${input_dir}/in.fourmol");
+        }
+        if (!verbose) ::testing::internal::GetCapturedStdout();
+    }
+
+    void TearDown() override
+    {
+        if (!verbose) ::testing::internal::CaptureStdout();
+        delete lmp;
+        if (!verbose) ::testing::internal::GetCapturedStdout();
+    }
+};
+
+TEST_F(ResetMolIDsTest, Plain)
+{
+    if (lmp->atom->natoms == 0) GTEST_SKIP();
+
+    auto molid = lmp->atom->molecule;
+    ASSERT_EQ(molid[GETIDX(1)], 1);
+    ASSERT_EQ(molid[GETIDX(2)], 1);
+    ASSERT_EQ(molid[GETIDX(3)], 1);
+    ASSERT_EQ(molid[GETIDX(4)], 1);
+    ASSERT_EQ(molid[GETIDX(5)], 1);
+    ASSERT_EQ(molid[GETIDX(6)], 1);
+    ASSERT_EQ(molid[GETIDX(7)], 1);
+    ASSERT_EQ(molid[GETIDX(8)], 2);
+    ASSERT_EQ(molid[GETIDX(9)], 2);
+    ASSERT_EQ(molid[GETIDX(10)], 2);
+    ASSERT_EQ(molid[GETIDX(11)], 2);
+    ASSERT_EQ(molid[GETIDX(12)], 2);
+    ASSERT_EQ(molid[GETIDX(13)], 2);
+    ASSERT_EQ(molid[GETIDX(14)], 2);
+    ASSERT_EQ(molid[GETIDX(15)], 2);
+    ASSERT_EQ(molid[GETIDX(16)], 2);
+    ASSERT_EQ(molid[GETIDX(17)], 2);
+    ASSERT_EQ(molid[GETIDX(18)], 3);
+    ASSERT_EQ(molid[GETIDX(19)], 3);
+    ASSERT_EQ(molid[GETIDX(20)], 3);
+    ASSERT_EQ(molid[GETIDX(21)], 4);
+    ASSERT_EQ(molid[GETIDX(22)], 4);
+    ASSERT_EQ(molid[GETIDX(23)], 4);
+    ASSERT_EQ(molid[GETIDX(24)], 5);
+    ASSERT_EQ(molid[GETIDX(25)], 5);
+    ASSERT_EQ(molid[GETIDX(26)], 5);
+    ASSERT_EQ(molid[GETIDX(27)], 6);
+    ASSERT_EQ(molid[GETIDX(28)], 6);
+    ASSERT_EQ(molid[GETIDX(29)], 6);
+
+    // the original data file has two different molecule IDs
+    // for two residues of the same molecule/fragment.
+    if (!verbose) ::testing::internal::CaptureStdout();
+    lmp->input->one("reset_mol_ids all");
+    if (!verbose) ::testing::internal::GetCapturedStdout();
+
+    ASSERT_EQ(molid[GETIDX(1)], 1);
+    ASSERT_EQ(molid[GETIDX(2)], 1);
+    ASSERT_EQ(molid[GETIDX(3)], 1);
+    ASSERT_EQ(molid[GETIDX(4)], 1);
+    ASSERT_EQ(molid[GETIDX(5)], 1);
+    ASSERT_EQ(molid[GETIDX(6)], 1);
+    ASSERT_EQ(molid[GETIDX(7)], 1);
+    ASSERT_EQ(molid[GETIDX(8)], 1);
+    ASSERT_EQ(molid[GETIDX(9)], 1);
+    ASSERT_EQ(molid[GETIDX(10)], 1);
+    ASSERT_EQ(molid[GETIDX(11)], 1);
+    ASSERT_EQ(molid[GETIDX(12)], 1);
+    ASSERT_EQ(molid[GETIDX(13)], 1);
+    ASSERT_EQ(molid[GETIDX(14)], 1);
+    ASSERT_EQ(molid[GETIDX(15)], 1);
+    ASSERT_EQ(molid[GETIDX(16)], 1);
+    ASSERT_EQ(molid[GETIDX(17)], 1);
+    ASSERT_EQ(molid[GETIDX(18)], 2);
+    ASSERT_EQ(molid[GETIDX(19)], 2);
+    ASSERT_EQ(molid[GETIDX(20)], 2);
+    ASSERT_EQ(molid[GETIDX(21)], 3);
+    ASSERT_EQ(molid[GETIDX(22)], 3);
+    ASSERT_EQ(molid[GETIDX(23)], 3);
+    ASSERT_EQ(molid[GETIDX(24)], 4);
+    ASSERT_EQ(molid[GETIDX(25)], 4);
+    ASSERT_EQ(molid[GETIDX(26)], 4);
+    ASSERT_EQ(molid[GETIDX(27)], 5);
+    ASSERT_EQ(molid[GETIDX(28)], 5);
+    ASSERT_EQ(molid[GETIDX(29)], 5);
+}
+
+TEST_F(ResetMolIDsTest, DeletePlusAtomID)
+{
+    if (lmp->atom->natoms == 0) GTEST_SKIP();
+
+    auto molid = lmp->atom->molecule;
+
+    // delete two water molecules
+    if (!verbose) ::testing::internal::CaptureStdout();
+    lmp->input->one("group allwater molecule 3:6");
+    lmp->input->one("group twowater molecule 4:6:2");
+    lmp->input->one("delete_atoms group twowater compress no bond yes");
+    lmp->input->one("reset_mol_ids all");
+    if (!verbose) ::testing::internal::GetCapturedStdout();
+    ASSERT_EQ(lmp->atom->natoms, 23);
+    ASSERT_EQ(lmp->atom->map_tag_max, 26);
+
+    EXPECT_EQ(molid[GETIDX(1)], 1);
+    EXPECT_EQ(molid[GETIDX(2)], 1);
+    EXPECT_EQ(molid[GETIDX(3)], 1);
+    EXPECT_EQ(molid[GETIDX(4)], 1);
+    EXPECT_EQ(molid[GETIDX(5)], 1);
+    EXPECT_EQ(molid[GETIDX(6)], 1);
+    EXPECT_EQ(molid[GETIDX(7)], 1);
+    EXPECT_EQ(molid[GETIDX(8)], 1);
+    EXPECT_EQ(molid[GETIDX(9)], 1);
+    EXPECT_EQ(molid[GETIDX(10)], 1);
+    EXPECT_EQ(molid[GETIDX(11)], 1);
+    EXPECT_EQ(molid[GETIDX(12)], 1);
+    EXPECT_EQ(molid[GETIDX(13)], 1);
+    EXPECT_EQ(molid[GETIDX(14)], 1);
+    EXPECT_EQ(molid[GETIDX(15)], 1);
+    EXPECT_EQ(molid[GETIDX(16)], 1);
+    EXPECT_EQ(molid[GETIDX(17)], 1);
+    EXPECT_EQ(molid[GETIDX(18)], 2);
+    EXPECT_EQ(molid[GETIDX(19)], 2);
+    EXPECT_EQ(molid[GETIDX(20)], 2);
+    EXPECT_EQ(molid[GETIDX(24)], 3);
+    EXPECT_EQ(molid[GETIDX(25)], 3);
+    EXPECT_EQ(molid[GETIDX(26)], 3);
+
+    // now also check and reset the atom ids
+
+    EXPECT_GE(GETIDX(1), 0);
+    EXPECT_GE(GETIDX(2), 0);
+    EXPECT_GE(GETIDX(3), 0);
+    EXPECT_GE(GETIDX(4), 0);
+    EXPECT_GE(GETIDX(5), 0);
+    EXPECT_GE(GETIDX(6), 0);
+    EXPECT_GE(GETIDX(7), 0);
+    EXPECT_GE(GETIDX(8), 0);
+    EXPECT_GE(GETIDX(9), 0);
+    EXPECT_GE(GETIDX(10), 0);
+    EXPECT_GE(GETIDX(11), 0);
+    EXPECT_GE(GETIDX(12), 0);
+    EXPECT_GE(GETIDX(13), 0);
+    EXPECT_GE(GETIDX(14), 0);
+    EXPECT_GE(GETIDX(15), 0);
+    EXPECT_GE(GETIDX(16), 0);
+    EXPECT_GE(GETIDX(17), 0);
+    EXPECT_GE(GETIDX(18), 0);
+    EXPECT_GE(GETIDX(19), 0);
+    EXPECT_GE(GETIDX(20), 0);
+    EXPECT_EQ(GETIDX(21), -1);
+    EXPECT_EQ(GETIDX(22), -1);
+    EXPECT_EQ(GETIDX(23), -1);
+    EXPECT_GE(GETIDX(24), 0);
+    EXPECT_GE(GETIDX(25), 0);
+    EXPECT_GE(GETIDX(26), 0);
+
+    if (!verbose) ::testing::internal::CaptureStdout();
+    lmp->input->one("reset_ids");
+    if (!verbose) ::testing::internal::GetCapturedStdout();
+
+    ASSERT_EQ(lmp->atom->map_tag_max, 23);
+    for (int i = 1; i <= 23; ++i)
+        EXPECT_GE(GETIDX(i), 0);
+}
+
+// TEST_FAILURE(lmp->input->one("reset_mol_ids all"););
+// ASSERT_THAT(mesg, MatchesRegex(".*ERROR: Unknown command.*"));
+
+} // namespace LAMMPS_NS
+
+int main(int argc, char **argv)
+{
+    MPI_Init(&argc, &argv);
+    ::testing::InitGoogleMock(&argc, argv);
+
+    // handle arguments passed via environment variable
+    if (const char *var = getenv("TEST_ARGS")) {
+        std::vector<std::string> env = split_words(var);
+        for (auto arg : env) {
+            if (arg == "-v") {
+                verbose = true;
+            }
+        }
+    }
+
+    if ((argc > 1) && (strcmp(argv[1], "-v") == 0)) verbose = true;
+
+    int rv = RUN_ALL_TESTS();
+    MPI_Finalize();
+    return rv;
+}