Relativistic CASPT2/RASPT2 Program along with DIRAC Software.

Exploring electronic states in actinide compounds is a critical aspect of nuclear science. However, considering relativistic effects and electron correlation in theoretical calculations poses a complex challenge. To tackle this, we developed the CASPT2/RASPT2 program along with the DIRAC program, enabling calculations of electron correlation methods using multiconfigurational perturbation theory with various relativistic Hamiltonians. Currently, we employ a method that combines the improved virtual orbital (IVO) approach and CASCI methodologies as reference functions, deviating from the traditional use of CASSCF. Additionally, we implemented the RASCI-RASPT2 method to treat larger active spaces and parallelized the entire program. Due to the intricate process of selecting orbital spaces in CASPT2 and RASPT2, we offer a GUI program to assist with input creation. All these programs and tutorials are freely accessible on GitHub for anyone to use. In our benchmark calculations, we demonstrated the efficiency of parallelization by utilizing 1-256 cores for CASCI-CASPT2 calculations on the UO molecule. Despite encountering some anomalies, we achieved commendable parallelization efficiency with CASCI and CASPT2 computational times. We also computed the vertical excitation energies of UO using the RASCI-RASPT2 approach. By adapting the IVO and setting the maximum number of holes and electrons to three for RAS1 and RAS3, we obtained trends consistent with those reported in previous studies using alternative methods. We plan to continue improving the program in the future, believing that its widespread use will contribute to further development in actinide chemistry.