B_k Approximation Applied to the Multireference Configuration Interaction Method

The single-reference configuration interaction through quadrupoles CI SDTQ-B_k method has been extended to the general multireference CI method in the B_k approximation (MR CI-B_k). The computer code is based on the traditional graphical unitary group approach (GUGA)-CI technique, and it processes only the loops (coupling coefficients) that are necessary for the MR CI-B_k Hamiltonian matrix evaluation. The number of processed loops, and therefore also the cost of calculation, is considerably lower than in the case of the respective complete MR CI calculation. The method was tested on the ground-state potential curves of the N₂, F₂, and HF molecules, by comparing the calculated MR CI-B_k energies with the available literature data obtained by complete MR CI and full CI calculations.