Application of explicitly correlated coupled-cluster methods to molecules containing post-3d main group elements

Abstract

A benchmark of explicitly correlated CCSD(T)-F12 methods for the dimers Ga₂, As₂, Br₂ and diatomic molecules AsN, BrO, HBr, GaF, GaCl, GaBr, AsF, AsCl, BrF, and BrCl is presented. Equilibrium distances, harmonic vibrational frequencies, and dissociation energies are compared with extensive conventional CCSD(T) calculations using a variety of orbital basis sets and different ansätze for the explicitly correlated wavefunctions. Correlation of the 3d electrons has a strong effect, in particular on the equilibrium distances, and it is shown that this can be recovered very efficiently by the explicit correlation treatment. It is found that CCSD(T)-F12 calculations with new F12-specific cc-pVnZ-F12 basis sets give comparable accuracy to standard CCSD(T) calculations with very much larger aug-cc-pwCV(n+2)Z basis sets. The effects of higher order valence electron correlation (up to CCSDTQP) are also investigated in conventional calculations and are found to be significant in some cases.

Keywords:

• CCSD(T)-F12
• explicit correlation
• core–valence correlation
• heavy elements

Acknowledgments

KAP and JGH gratefully acknowledge the support of the US National Science Foundation (Grant No. CHE-0723997). HJW acknowledges support from the Deutsche Forschungsgemeinschaft (NSF-DFG project). The project has also been supported by the SimTech Cluster of Excellence at the University of Stuttgart. The authors would also like to thank Dr. David Dixon (Alabama) for helpful discussions in regards to the experimental dissociation energies of the gallium and arsenic species, and to Dr. Gerald Knizia, who developed the open-shell UCCSD(T)-F12x codes.

Notes

Note

1. Ansatz 3 is equivalent to Ansatz 2; the older Ansatz 1 is less accurate and not used any more, so that for modern methods it would not be necessary to specify the ansatz at all. However, it is included here for consistency with previous work.