diff --git a/unittest/CMakeLists.txt b/unittest/CMakeLists.txt
index addc8bc244..26db526b60 100644
--- a/unittest/CMakeLists.txt
+++ b/unittest/CMakeLists.txt
@@ -10,6 +10,22 @@ set_tests_properties(RunLammps PROPERTIES
         ENVIRONMENT "TSAN_OPTIONS=ignore_noninstrumented_modules=1"
         PASS_REGULAR_EXPRESSION "^LAMMPS \\([0-9]+ [A-Za-z]+ 2[0-9][0-9][0-9]\\)")
 
+# check if the compiled executable will print the help message
+add_test(NAME HelpMessage
+         COMMAND $<TARGET_FILE:lmp> -h
+         WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+set_tests_properties(HelpMessage PROPERTIES
+         ENVIRONMENT "TSAN_OPTIONS=ignore_noninstrumented_modules=1;PAGER=head"
+         PASS_REGULAR_EXPRESSION ".*Large-scale Atomic/Molecular Massively Parallel Simulator -.*Usage example:.*")
+
+# check if the compiled executable will error out on an invalid command line flag
+add_test(NAME InvalidFlag
+         COMMAND $<TARGET_FILE:lmp> -xxx
+         WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+set_tests_properties(InvalidFlag PROPERTIES
+         ENVIRONMENT "TSAN_OPTIONS=ignore_noninstrumented_modules=1"
+         PASS_REGULAR_EXPRESSION "ERROR: Invalid command-line argument.*")
+
 if(BUILD_MPI)
   function(add_mpi_test)
     set(MPI_TEST_NUM_PROCS 1)
diff --git a/unittest/cplusplus/CMakeLists.txt b/unittest/cplusplus/CMakeLists.txt
index 90ef9a0282..aef69f3722 100644
--- a/unittest/cplusplus/CMakeLists.txt
+++ b/unittest/cplusplus/CMakeLists.txt
@@ -2,6 +2,7 @@
 add_executable(test_lammps_class test_lammps_class.cpp)
 target_link_libraries(test_lammps_class PRIVATE lammps GTest::GMockMain GTest::GTest GTest::GMock)
 add_test(LammpsClass test_lammps_class)
+set_tests_properties(LammpsClass PROPERTIES ENVIRONMENT "OMP_NUM_THREADS=1")
 
 add_executable(test_input_class test_input_class.cpp)
 target_link_libraries(test_input_class PRIVATE lammps GTest::GTest GTest::GTestMain)
diff --git a/unittest/cplusplus/test_lammps_class.cpp b/unittest/cplusplus/test_lammps_class.cpp
index c20cdf336a..6a0e6719cc 100644
--- a/unittest/cplusplus/test_lammps_class.cpp
+++ b/unittest/cplusplus/test_lammps_class.cpp
@@ -1,13 +1,17 @@
 // unit tests for the LAMMPS base class
 
+#include "comm.h"
+#include "info.h"
 #include "lammps.h"
-#include <cstdio> // for stdin, stdout
+#include <cstdio>  // for stdin, stdout
+#include <cstdlib> // for setenv
 #include <mpi.h>
 #include <string>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
+using ::testing::MatchesRegex;
 using ::testing::StartsWith;
 
 namespace LAMMPS_NS {
@@ -95,6 +99,7 @@ TEST_F(LAMMPS_plain, InitMembers)
         EXPECT_STREQ(LAMMPS::git_branch, "(unknown)");
         EXPECT_STREQ(LAMMPS::git_descriptor, "(unknown)");
     }
+    EXPECT_EQ(lmp->comm->nthreads, 1);
 }
 
 TEST_F(LAMMPS_plain, TestStyles)
@@ -229,6 +234,7 @@ TEST_F(LAMMPS_omp, InitMembers)
         EXPECT_STREQ(LAMMPS::git_branch, "(unknown)");
         EXPECT_STREQ(LAMMPS::git_descriptor, "(unknown)");
     }
+    EXPECT_EQ(lmp->comm->nthreads, 2);
 }
 
 // test fixture for Kokkos tests
@@ -318,10 +324,49 @@ TEST_F(LAMMPS_kokkos, InitMembers)
     }
 }
 
-// check help message printing
-TEST(LAMMPS_help, HelpMessage)
+// check if Comm::nthreads is initialized to either 1 or 2 (from the previous tests)
+TEST(LAMMPS_init, OpenMP)
 {
-    const char *args[] = {"LAMMPS_test", "-h"};
+    if (!LAMMPS::is_installed_pkg("USER-OMP")) GTEST_SKIP();
+
+    FILE *fp = fopen("in.lammps_empty", "w");
+    fputs("\n", fp);
+    fclose(fp);
+
+    const char *args[] = {"LAMMPS_init", "-in", "in.lammps_empty", "-log", "none", "-nocite"};
+    char **argv        = (char **)args;
+    int argc           = sizeof(args) / sizeof(char *);
+
+    ::testing::internal::CaptureStdout();
+    LAMMPS *lmp        = new LAMMPS(argc, argv, MPI_COMM_WORLD);
+    std::string output = ::testing::internal::GetCapturedStdout();
+    EXPECT_THAT(output, MatchesRegex(".*using 2 OpenMP thread.*per MPI task.*"));
+
+    if (LAMMPS_NS::Info::has_accelerator_feature("USER-OMP", "api", "openmp"))
+        EXPECT_EQ(lmp->comm->nthreads, 2);
+    else
+        EXPECT_EQ(lmp->comm->nthreads, 1);
+    ::testing::internal::CaptureStdout();
+    delete lmp;
+    ::testing::internal::GetCapturedStdout();
+
+    remove("in.lammps_empty");
+}
+
+// check no OMP_NUM_THREADS warning message printing. this must be the
+// last OpenMP related test as threads will be locked to 1 from here on.
+
+TEST(LAMMPS_init, NoOpenMP)
+{
+    if (!LAMMPS_NS::Info::has_accelerator_feature("USER-OMP", "api", "openmp"))
+        GTEST_SKIP() << "No threading enabled";
+
+    FILE *fp = fopen("in.lammps_class_noomp", "w");
+    fputs("\n", fp);
+    fclose(fp);
+    unsetenv("OMP_NUM_THREADS");
+
+    const char *args[] = {"LAMMPS_init", "-in", "in.lammps_class_noomp", "-log", "none", "-nocite"};
     char **argv        = (char **)args;
     int argc           = sizeof(args) / sizeof(char *);
 
@@ -329,7 +374,11 @@ TEST(LAMMPS_help, HelpMessage)
     LAMMPS *lmp        = new LAMMPS(argc, argv, MPI_COMM_WORLD);
     std::string output = ::testing::internal::GetCapturedStdout();
     EXPECT_THAT(output,
-                StartsWith("\nLarge-scale Atomic/Molecular Massively Parallel Simulator -"));
+                MatchesRegex(".*OMP_NUM_THREADS environment is not set.*Defaulting to 1 thread.*"));
+    EXPECT_EQ(lmp->comm->nthreads, 1);
+    ::testing::internal::CaptureStdout();
     delete lmp;
+    ::testing::internal::GetCapturedStdout();
 }
+
 } // namespace LAMMPS_NS