Selected versus complete configuration interaction expansions

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Abstract
Full CI (FCI) and selected multireference CI (MRCI) calculations have been carried out for the first row atoms (B to F) and hydrides (BH to FH). The multireference selected CI calculations were carried using the CIPSI algorithm (CIPSI stands for configuration interaction by perturbative iterative selection). An analysis of the relationship between variational and perturbational correlation energy contributions is also reported. By diagonalizing a matrix corresponding to a CI expansion selected from the full MRCI space, 99% of the total FCI correlation energy is obtained. The contribution of a large number of generated determinants to the correlation energy can be accounted for by second order perturbation theory. This contribution to the correlation energy is one order of magnitude smaller than the error with respect to the FCI value. The calculations were carried out using two different basis sets. The first one is based on the atomic natural orbital (ANO) contraction, and the second follows the segmented contraction scheme. Both sets contain 4s3p1d contracted functions for atoms B to F and 2s1p for the H atom.