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 Mřller-Plesset perturbation theory wavefunction instead, might be a better choice.
|Increasing the Stability of Linear r12-Calculations on the Multi-Reference Level of Theory