Perturbatively selected CI as an optimal source for externally corrected CCSD

Perturbatively selected configuration interaction (PSCI) is used as a sourc e of higher than pair clusters in the externally corrected coupled cluster method with singles and doubles (ecCCSD). This significantly decreases the dimension of the standard multireference (MR) CISD that is employed by the so-called reduced MR (RMR) CCSD method, thus enabling the use of relatively large active spaces. The performance of the proposed PSCI CCSD method is i llustrated by considering the ground state potential energy curves of the H F molecule using DZP and cc-pVTZ basis sets (breaking of a single bond), an d of the N-2 molecule using a TZ basis set (breaking of a triple bond). It is shown that notwithstanding a large reduction in the dimension of the ext ernal source, the accuracy of the resulting ecCCSD energies is almost the s ame as that obtained when correcting with the full MR CISD wave function. ( C) 1999 American Institute of Physics. [S0021-9606(99)30424-4].