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Int. J. Mol. Sci. 2002, 3(6), 639-655; doi:10.3390/i3060639
Article

Multi-state Multireference Rayleigh–Schrödinger Perturbation Theory for Mixed Electronic States: Second and Third Order

Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Received: 12 December 2001 / Accepted: 20 March 2002 / Published: 30 June 2002
(This article belongs to the Special Issue Multireference Quantum Chemical Methods)
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Abstract

The formalism for multi-state multireference configuration-based Rayleigh-Schrödinger perturbation theory and procedures for its implementation for the second-order and third-order energy within a multireference configuration interaction computer program are reviewed. This formalism is designed for calculations on electronic states that involve strong mixing between different zero-order contributions, such as avoided crossings or mixed valence-Rydberg states. Such mixed states typically display very large differences in reference-configuration mixing coefficients between the reference MCSCF wave function and an accurate correlated wave function, differences that cannot be reflected in state-specific (diagonalize-then-perturb) multireference perturbation theory through third order. A procedure described in detail applies quasidegenerate perturbation theory based on a model space of a few state-averaged MCSCF functions for the states expected to participate strongly in the mixing, and can be characterized as a “diagonalize-then-perturb-thendiagonalize” approach. It is similar in various respects to several published methods, including an implementation by Finley, Malmqvist, Roos, and Serrano-Andrés [Chem. Phys. Lett. 1998, 288, 299–306].
Keywords: Perturbation theory; multireference; mixed states; avoided crossings; ab initio Perturbation theory; multireference; mixed states; avoided crossings; ab initio
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).
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Shavitt, I. Multi-state Multireference Rayleigh–Schrödinger Perturbation Theory for Mixed Electronic States: Second and Third Order. Int. J. Mol. Sci. 2002, 3, 639-655.

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