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1 Computational Methods in Science and Technology

1 2003

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Increasing the stability of linear r12-calculations on the multi - reference level of theory

Cardoen W., Gdanitz R. J.

Computational Methods in Science and Technology

2003

Vol. 9, nr 1-2

31-43

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 o f 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 modiflcations 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 flrst-order Moller-Plesset perturbation theory wavefunction instead, might be a better choice.