Approximately size extensive local multireference singles and doubles configuration interaction

Multi-reference Configuration Interaction (MRCI) is often used to predict the electronic structures and reaction energetics of small molecules with very high accuracy. Unfortunately, MRCI is inapplicable to large or even medium-sized molecules for two reasons: its computational cost scales poorly with molecule size and MRCI methods are not size extensive, leading to large energy errors. We have developed a new local (L) and approximately size extensive MRCI method that addresses both shortcomings. Truncating long-range electron correlation in a local orbital basis as well as efficient processing of two-electron integrals via Cholesky decomposition (CD) and integral screening reduce the computational cost to O(N-3) with a small prefactor. A priori and a posteriori size extensivity corrections are both considered. The Multi-reference Averaged Coupled-Pair Functional (MRACPF) provides approximate size extensivity by modifying the energy functional. The very inexpensive Davidson-Silver and Pople a posteriori schemes also produce quite accurate corrections over a large range of molecular size. Employing CD-LMRACPF is slightly more expensive than using a Davidson-type correction, but the former gives superior results. Molecules with up to 50 heavy atoms can be treated with our CD-LMRACPF method thus far.