Efficient Implementation of Approximate Fourth Order <i>N</i>-Electron Valence State Perturbation Theory.

ABSTRACT: In this work, the implementation of a partial fourth order N-electron-valence perturbation theory (NEVPT) is reported and numerically evaluated. The method, termed NEVPT4(SD), includes the internally contracted functions that span the first-order-interacting space (FOIS) and evaluates their contribution to second-order in the wave function and fourth order in the energy. The triple- and quadruple excitations that would additionally enter the second-order-interacting space (SOIS) are not included. As discussed by Grimme [Chem. Phys. Lett. 2001, 334, 99-106] in order to obtain a size-consistent method, it is necessary to also drop the fourth-order renormalization term if the quadruple excitations are dropped. The NEVPT4(SD) method is demonstrated to be perfectly s

SUBMITTER: Kempfer EM 

PROVIDER: S-EPMC12020360 | biostudies-literature | 2025 Apr

REPOSITORIES: biostudies-literature