Multireference R12 Coupled Cluster Theory

Abstract

In order to account for static electron correlation, the explicitly correlated coupled cluster (CC) theory based on the R12 Ansatz is formulated with respect to a multi-determinantal reference using the Brillouin – Wigner (BW) approach. Though the latter avoids appearance of the intruder states, one pays for this desired feature by the loss of size extensivity. However, to some extent this can be remedied by an a posteriori correction. Since the BW-CC method offers simplest form of amplitude equations among Hilbert space MR CC ones, we have chosen it as the first step when developing MR CC-R12 approaches. It is shown that introducing of the basis set incompleteness correction via an explicit inclusion of the correlation factor into the wave function, separately for each reference, is easily realizable. Test calculations for the H4 model using the R12 optimized 9s6p4d3f basis and its subsets with increasing highest angular momentum show the potential of the MR CC-R12 approach. R12 results with meresp functions are very close to values obtained by using a conventional approach and the full 9s6p4d3f basis set. We stress, however, that care must be taken to treat the R12 corrections with comparable accuracy for all the reference determinants.