Explicitly correlated coupled cluster methods with pair-specific geminals

Abstract

Explicitly correlated MP2-F12 and CCSD(T)-F12 methods with orbital-pair-specific Slater-type geminals are proposed. The fixed amplitude ansatz of Ten-no is used, and different exponents of the Slater geminal functions can be chosen for core–core, core–valence, and valence–valence pairs. This takes care of the different sizes of the correlation hole and leads to improved results when inner-shell orbitals are correlated. The complications and the extra computational cost as compared to corresponding calculations with a single geminal are minor. The improved accuracy of the method is demonstrated for spectroscopic properties of Br₂, As₂, Ga₂, Cu₂, GaCl, CuCl, and CuBr, where the d-orbitals are treated as core.

Keywords:

• explicit correlation
• coupled-cluster
• core–valence correlation
• F12-methods

Acknowledgements

This work was funded by the DFG as part of a NSF-DFG grant and by the Stuttgart SimTech Cluster of Excellence. HJW also acknowledges generous support from the Fonds der Chemischen Industrie. We are grateful to Kirk Peterson and Grant Hill for providing their new basis sets prior to publication and for many helpful discussions. HJW thanks C. Hättig and D. Tew for providing test results for their (F12*) method, which were very useful for debugging our implementation.