High excitations in coupled-cluster series: vibrational energy levels of ammonia

Abstract

The ammonia molecule containing large amplitude inversion motion is a revealing system in examining high-order correlation effects on potential energy surfaces. Correlation contributions to the equilibrium and saddle point geometries, inversion barrier height and vibrational energy levels, including inversion splittings, have been investigated. A six-dimensional Taylor-type series expansion of the Born–Oppenheimer potential energy surface, which is scaled to different levels of theory, is used to determine vibrational energy levels and inversion splittings variationally. The electronic energies are calculated by coupled-cluster methods, combining explicitly correlated R12 theory (which includes the interelectronic coordinate in the electronic wave function) with a conventional approach including excitations up to the pentuple level. Finally, the electronic correlation contribution is scaled to the full configuration interaction limit. Corrections due to relativistic and non-Born–Oppenheimer effects are also included. Special emphasis is put on the convergence of the high-order contributions with respect to the size of the atomic basis set. To achieve an accuracy of 1 cm−1, it is essential to be at the basis set limit, include all the subtle effects and also include highly excited configurations—even up to the pentuple level in the coupled-cluster expansion.