Comparison of perturbatively corrected energy results from multiple reference double-excitation configuration-interaction method calculations with exact full configuration-interaction benchmark values

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Abstract
Perturbative corrections of various types are employed in conjunction with variational configuration-interaction (CI) treatments in order to obtain estimates of the energy eigenvalues of a large series of full CI benchmark calculations recently reported by Bauschlicher et al. It is found that the best agreement is generally obtained when an extrapolation procedure based on the results of truncated secular equations is employed to estimate the zero-threshold multiple reference double-excitation configuration-interaction (MRD-CI) energy values, after which these intermediate results are then corrected for effects of higher excitations by means of a multireference analogue of the Davidson–Langhoff correction D1. A root-mean-square (rms) deviation of 0.94 kcal mol−1 relative to the full CI energy results for 66 cases is obtained with this perturbative method, which has been in general use now for most of the last decade. By comparison the T=0 variational MRD-CI energy values give a higher rms deviation of 3.19 kcal mol−1. This behavior is shown to result from a systematic cancellation of errors in the two types of perturbative corrections employed in the above scheme. The advantages of employing a multireference set of configurations in generating singles and doubles CI spaces are also underscored by the present results, and the advisability of employing a nearly uniform value of ∑mpc2p for all states and nuclear conformations of a given atomic or molecular system is stressed.