Improved and simplified orthogonalisation scheme and connected triples correction within the internally contracted multireference coupled-cluster method

Abstract

The internally contracted multireference coupled-cluster theory (icMRCC) is reviewed, providing additional insights that we have gained over recent years. In addition, several modifications are presented that lead to an overall improvement in the method. These include a simpler and size extensive way to generate the excitation manifold and therefore determine the cluster amplitudes. Modifications and approximations are also introduced to the perturbative triples correction, icMRCCSD(T), which lead to a small improvement over the previous manifestation, at a reduced computational cost. With these reformulated methods, we can approach sub-chemical accuracy results.

GRAPHICAL ABSTRACT

Keywords:

• Multireference
• coupled-cluster
• perturbative triples

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was in part funded by the German Research Foundation [DFG, Grants KO 2337/4-1 and KO 2337/4-2].