The CIPT2 method: Coupling of multi-reference configuration interaction and multi-reference perturbation theory. Application to the chromium dimer

Celani, P, Stoll, H, Werner, H.J. and Knowles, Peter ORCID: https://orcid.org/0000-0003-4657-6331 2004. The CIPT2 method: Coupling of multi-reference configuration interaction and multi-reference perturbation theory. Application to the chromium dimer. Molecular Physics 102 (21-22) , pp. 2369-2379. 10.1080/00268970412331317788

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Abstract

The potential energy function of is computed using the MRCI+Q (multi-reference configuration interaction with Davidson correction) and CASPT2 (complete active space second-order multi-reference perturbation theory) methods and very large basis sets. In addition, a new hybrid method denoted CIPT2, in which excitations solely from the active space are treated by MRCI and the remaining ones by perturbation theory, is proposed and applied. These methods are used to analyse valence and core correlation effects on the potential energy function. MRCI calculations which fully include correlation of the 3p, 3d, and 4s electrons yield excellent agreement with experimental values for the equilibrium distance Re and the harmonic vibrational constant ω e . Correlation of the (3p) orbitals is found to be very important. At the MRCI+Q level this reduces Re by 0.08 Å and increases ω e by about 200 cm−1. These effects are not correctly reproduced in CASPT2 and CIPT2. It is shown that in CASPT2 the valence correlation effects are significantly overestimated and the distance dependence of the core correlation is incorrect. The basis set limit of the dissociation energies for MRCI+Q and CASPT 2 is estimated to be 1.3 eV and 1.9 eV, respectively. The remaining error (≈ 0.2 − 0.3 eV) of the MRCI+Q dissociation energy is attributed to unlinked cluster and higher excitation effects, which are only approximately accounted for by the Davidson correction.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Chemistry
Subjects:	Q Science > QD Chemistry
ISSN:	1362-3028
Last Modified:	18 Oct 2022 12:23
URI:	https://orca.cardiff.ac.uk/id/eprint/9631

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