Ammonia: the prototypical lone pair molecule

Abstract

Benchmark results for a variety of energetic properties of the ammonia molecule are presented and discussed. We attempt to converge to the exact self-consistent field and correlation energies, using correlation consistent basis sets and coupled-cluster methods. Specific values include the heat of formation, the inversion barrier, and the protonation and deprotonation energy. We also use coupled-cluster gradient methods to obtain harmonic vibrational frequencies and intensities, and the new equation-of-motion coupled-cluster methods to describe ammonia's photoelectron and electronic spectra. The latter considers both vertical and adiabatic excitation energies. For each example we present a brief summary of the theory applied, illustrating how it is the new theory that makes such applications possible, or simpler. All new calculations are performed with the aces-ii program system.