Pair‐correlated configuration interaction method and its approximate version for solving the electron correlation problem in molecules

The pair-correlated configuration interaction (PCCI) method has been developed in this work to be an alternative to the traditional CI method for solving the correlation problem in closed-shell, ground-state molecules. The PCCI expansion is well defined on the localized molecular orbital description and can be truncated according to how many electron pairs are explicitly correlated (namely, the PCCI-n expansion includes all configurations up to n-pair correlation). However, PCCI-n calculations are formidable for most of chemical systems, and therefore further approximations are required. After the separated electron pair approximation is adopted, the PCCI method reduces to the simplified PCCI (SPCCI) method, which is computationally feasible for moderate molecular systems by using matrix element expressions presented in this work. We have tested the SPCCI-n method by applying it to solve the ground state of the Pariser-Parr-Pople (PPP) Hamiltonian for-networks of a series of conjugated polyenes. The most significant features of the SPCCI-n method have been brought to light from our test calculations: (1) The SPCCI-n (nD 2, 3) method can recover more than 90% of the total ground-state correlation energy for conjugated polyenes with up to 30 -electrons. It is far superior to the conventional configuration interaction that includes all singly and doubly excited configurations (CI-SD) method in systems with more than 6-electrons. (2) The SPCCI-n method works well in the moderately and strongly correlated regions but less satisfactorily in the weakly correlated domain. Finally, the size consistency error of the general SPCCI-n method is analyzed and a size-consistent coupled-cluster-like approach is proposed. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 153-167, 2000

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