TREATING LARGE INTERMEDIATE SPACES IN THE CIPSI METHOD THROUGH A DIRECT SELECTED CI ALGORITHM

A general direct selected configuration interaction algorithm has been implemented and coupled to the second-order multireference many-body perturbation theory CIPSI algorithm. The new direct selected Cl code is highly vectorizable and able to handle any list of determinants selected to describe a given electronic state of any spin multiplicity. In the present work selection of determinants has been carried out through the CIPSI algorithm but this is not a constraint of the direct selected Cl code. The largest case treated so far involves a Cl expansion containing 215 260 determinants selected from single and double excitations from 371 references. In this case there were 8 active electrons in 28 molecular orbitals for NH3 in a DZP basis set. The direct selected Cl calculation needs only 14 Mb of central storage (for the 215 260 determinants case) and takes 406 seconds per iteration on an IBM 3090/600J with vector facility.