MOLCAS manual: 2.5 Installation of CheMPS2–MOLCAS interface for DMRG calculations The CheMPS2–MOLCAS interface [10,11], based on the Block–MOLCAS interface [12], can support DMRG-SS-CASPT2 and DMRG-SA-CASPT2 calculations. It requires the following components: HDF5: https://support.hdfgroup.org/HDF5 CheMPS2 binary: https://github.com/SebWouters/CheMPS2/archive/v1.8.4.tar.gz HDF5 must be configured with Fortran, Fortran 2003, and C++ enabled: ./configure --enable-fortran --enable-fortran2003 --enable-cxx Make sure that both libhdf5_fortran.so and libhdf5.so are generated. For CheMPS2 binary installation, consult http://sebwouters.github.io/CheMPS2/sourcecode.html Note that only the version with the Open Multi-Processing (OpenMP) is supported, thus build CheMPS2 with: -DWITH_MPI=OFF In order to efficiently run the CheMPS2–MOLCAS interface, it is advisible to compile either serial or parallel MOLCASwith MPI. An example: ./configure -compiler intel -parallel -64 -mpiroot /path/to/mpi/root \ -mpirun /path/to/mpi/bin/mpirun -blas MKL -blas_lib -mkl=sequential \ -hdf5_inc /path/to/hdf5/include \ -hdf5_lib /path/to/hdf5/lib \ -chemps2 /path/to/chemps2/binary The CheMPS2–MOLCAS interface can also be activated with CMake: -DCHEMPS2=ON -DCHEMPS2_DIR=/path/to/chemps2/binary Before testing the CheMPS2–MOLCAS interface, make sure to increase stack size, export OMP_NUM_THREADS, the CheMPS2 binary, and all the required libraries for CheMPS2. ulimit -s unlimited [export OMP_NUM_THREADS=...] export PATH=/path/to/chemps2/binary:$PATH Verify the installation: molcas verify 850 851 molcas verify benchmark:970