## Summary `external/muparser/CMakeLists.txt` sets `ENABLE_OPENMP` to `${GMX_OPENMP}`, which will set the `MUP_USE_OPENMP` define used in `muParserBase.cpp` to determine whether to try to `#include <omp.h>`. However, the `muparser` CMake target is not configured to use any of the other logic that the gromacs targets use to find OpenMP details. This means that muparser may fail to compile while the rest of gromacs compiles (when `omp.h` is not available through some global mechanism). ## Possible fixes The recommended fix would be to `target_link_libraries(muparser PRIVATE OpenMP::OpenMP_CXX)`. I'm not sure whether this should be done in `src/external/muparser/CMakeLists.txt` or whether that will cause problems merging updates.