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Issue 41, 2014
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Spin–flip non-orthogonal configuration interaction: a variational and almost black-box method for describing strongly correlated molecules

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Abstract

In this paper, we report the development, implementation, and assessment of a novel method for describing strongly correlated systems, spin–flip non-orthogonal configuration interaction (SF-NOCI). The wavefunction is defined to be a linear combination of independently relaxed Slater determinants obtained from all possible spin–flipping excitations within a localized orbital active-space, typically taken to be the singly occupied orbitals of a high-spin ROHF wavefunction. The constrained orbital optimization of each CI basis configuration is defined such that only non-active-space orbitals are allowed to relax (all active space orbitals are fixed). A number of simplifications and benefits arise due to the fact that only a restricted number of orbital rotations are permitted, (1) basis states cannot coalesce during SCF, (2) basis state optimization is better conditioned due to a larger effective HOMO–LUMO gap, (3) smooth potential energy surfaces are easily obtained, (4) the Hamiltonian coupling between two basis states with non-orthogonal orbitals is greatly simplified. To illustrate the advantages over a conventional orthogonal CI expansion, we investigate exchange coupling constants of bimetallic complexes, the avoided crossing of the lowest singlet states during LiF dissociation, and ligand non-innocence in an organometallic complex. These numerical examples indicate that good qualitative agreement can be obtained with SF-NOCI, but dynamical correlation must be included to obtain quantitative accuracy.

Graphical abstract: Spin–flip non-orthogonal configuration interaction: a variational and almost black-box method for describing strongly correlated molecules

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Supplementary files

Article information


Submitted
26 Jun 2014
Accepted
15 Sep 2014
First published
15 Sep 2014

Phys. Chem. Chem. Phys., 2014,16, 22694-22705
Article type
Paper

Spin–flip non-orthogonal configuration interaction: a variational and almost black-box method for describing strongly correlated molecules

N. J. Mayhall, P. R. Horn, E. J. Sundstrom and M. Head-Gordon, Phys. Chem. Chem. Phys., 2014, 16, 22694
DOI: 10.1039/C4CP02818J

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