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

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.