Correlations beyond CCSD(T) for accurate study of Hg$_2$ and Cp$_2$

TL;DR

This paper investigates high-order correlation effects beyond CCSD(T) to improve the accuracy of molecular constants for Hg₂ and Cn₂, achieving perfect agreement with experimental data for Hg₂.

Contribution

It introduces the evaluation of non-perturbative triple and quadruple cluster amplitudes for van der Waals dimers, enhancing the accuracy of theoretical predictions.

Findings

Non-perturbative triple and quadruple contributions significantly affect dissociation energies.

Inclusion of these effects yields perfect agreement with experimental data for Hg₂.

Corrections are around 6% for dissociation energy of Hg₂.

Abstract

Contributions from high-order (non-perturbative triple and quadruple) cluster amplitudes to the dissociation energies, equilibrium distances, and vibrational constants for the ground states of van der Waals dimers Hg$_2$ and Cn$_2$ are evaluated. The incorporation of these contributions into the results of large-scale CCSD(T) calculations leads to non-negligible corrections of the computed molecular constants for Hg$_2$ (6% for the dissociation energy), and enables one to attain perfect agreement with the experimental values.