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Volume 9 (1) 2003, 31-43

INCREASING THE STABILITY OF LINEAR r12-CALCULATIONS ON THE MULTI-REFERENCE LEVEL OF THEORY*

Cardoen Wim 1, Gdanitz Robert J. 2

1 Henry Eyring Center for Theoretical Chemistry, Chemistry Dept., University of Utah,
Salt Lake City, UT 84112, U.S.A.
2Physics Dept., North Carolina State University,
Marteena Hall, Rm. 101, Greensboro, NC 27411, U.S.A.

Received:

Rec. 16 February 2004

DOI:   10.12921/cmst.2003.09.01.31-43

Abstract:

We investigate the performance of two approaches to remedy the notorious instability problems on explicitly correlated multi-reference averaged coupled-pair functional (r12-MR-ACPF) level of theory for the cases of HF molecule and Ne dimer. These two approaches are: i) the restriction of the unitary orbital invariance while retaining a chemically meaningful ansatz; ii) the contraction of associated r12-terms to geminals using amplitudes from the first-order perturbation theory wavefunction. We observe that our modifications outperform the extremal pair approach, which is commonly used on explicitly correlated coupled-cluster (CC-R12) level of theory and conclude that using geminals, which are e.g. taken from the first-order Møller-Plesset perturbation theory wavefunction instead, might be a better choice.

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