Quasi-degenerate second-order perturbation theory for occupation restricted multiple active space self-consistent field reference functions

Abstract

A multi-configuration quasi-degenerate second-order perturbation method based on the occupation restricted multiple active space (ORMAS-PT/ORMAS) reference wavefunction is presented. ORMAS gives one the ability to approximate a complete active space self-consistent field (CASSCF) wavefunction using only a subset of the configurations from the CASSCF space. The essential idea behind ORMAS-PT is to use the multi-reference Møller-Plesset formalism to correct the ORMAS reference energy. A computational scheme employing direct CI methodology is presented. Several tests are presented to demonstrate the performance of the ORMAS-PT method. A multi-configuration quasi-degenerate second-order perturbation method based on the occupation restricted multiple active space (ORMAS-PT/ORMAS) reference wavefunction is presented. OR-MAS gives one the ability to approximate a complete active space self-consistent field (CASSCF) wavefunction using only a subset of the configurations from the CASSCF space. The essential idea behind ORMAS-PT is to use the multi-reference Møller-Plesset formalism to correct the ORMAS reference energy. A computational scheme employing direct CI methodology is presented. Several tests are presented to demonstrate the performance of the ORMAS-PT method.

Improving the calculation of magnetic coupling constants in MRPT methods

“…A second inconvenience of CASPT2 is that for the coupling in binuclear complexes the single‐state orbitals obtained from a minimal active space does not provide equally good results as observed in the present work for the Ni complexes. To circumvent the drastic increase of the size of the CAS when considering system with more unpaired electrons, it is often recommended to leave behind the concept of a complete active space and rely on restricted active space SCF (RASSCF), occupation restricted multiple active space or variants of the generalized active space SCF approach. This would indeed widen the applicability range of ab initio wave function based methods, but the drawbacks lined‐out for CASPT2 will also show up for RASPT2, the implementation of MRPT2 with a RASSCF reference wave function …”

Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table

“…We should first start by saying that multiconfigurational perturbation theory isn't uniquely defined, and several different methods exist, such as complete or restricted active space second‐order perturbation theory (CASPT2/RASPT2), quasi‐degenerate multireference perturbation theory, n‐electron valence perturbation theory (NEVPT), and occupation‐restricted multiple active space perturbation theory (ORMAS‐PT) . While methods such as NEVPT have been parallelized, to the best of our knowledge, this is the first fully parallel implementation of the CASPT2 method.…”

Investigation of the water exchange mechanism of the Plutonyl(VI) and Uranyl(VI) ions with quantum chemical methods

“…To assess the accuracy of the DFT energies, the latter were also computed with WFT using general multiconfiguration quasi-degenerate second-order perturbation theory (GMC-QDPT2) [26][27][28][29], whereby the 6s and 6p MOs of U and Pu were included in the PT2 treatment. The multiconfiguration self-consistent-field (MCSCF) and the GMC-QDPT2 calculations (with krot = .t., kszdoe = .t., and thrde = 0) were based on the occupationally restricted multiple active space (ORMAS) [30] technique to realize a maximum excitation level of two within the MCSCF active space.…”