The averaged coupled-pair functional (ACPF): A size-extensive modification of MR CI(SD)

Abstract

To account for unlinked-cluster effects at the multireference level, we propose a generalization (MR ACPF) of the “coupled-pair functional” (CPF). Comparison with full CI results for BeH 2, H 2O, CH 2, O 2 and the five lowest-lying 2A 1 states of CH 2 +, shows the superiority of MR ACPF over MR CI(SD) and MR LCCM. Large-scale calculations for F 2 (up to a [7s5p3d2flg] CGTO basis set and 6 references) show better convergence to the experimental D e values on basis set and reference space extension than does MR CI(SD).