Ab initio CBS-QCI calculations of the inversion mode of ammonia

The complete basis set (CBS) extrapolation model chemistry of Petersson and co-workers was used to explore the potential energy surface of the ammonia inversion mode. The CBS-QCI theoretical energies were calculated using 41 points along the inversion surface at the MP2/6-311++G** geometries. A variety of techniques were explored to model the potential surface. Subsequent numerical solution of the one-dimensional Schrodinger equation produced energy levels for ammonia isotopomers in good agreement with experimental transitions. Accounting for the variable nature of the reduced mass with inversion coordinate is shown to be of significance. This study is an important first step in producing reliable methods for making ab initio thermodynamic corrections from Delta E(0 K) to Delta G(298 K) in other nitrogen-containing systems. Because no experimental methods generate data at 0 K, these corrections provide a crucial link between experimental thermochemical energies and ab initio theory.