Close coupling calculations on rotational excitation and inversion of NH3 by collisions with Ar

State-to-state total cross sections for rotational excitation and inversion of NH3 by collisions with Ar have been calculated within the accurate close coupling framework. The inversion motion in NH3 was included both via a delta function model and by taking the inversion coordinate explicitly into account. We used an ab initio potential and a potential in which one term in the angular expansion of the ab initio potential is scaled in order to reproduce spectroscopic data. At the energies of these calculations the delta function model is found to be in nearly quantitative agreement with the ‘‘exact’’ inversion results. Comparison with experiment shows the original ab initio potential to be better than the scaled one. The state-to-state cross sections for ortho-NH3 are in general accord with the measurements. For para-NH3 the agreement is good also, but the relative magnitudes of the cross sections for transitions to the ± inversion states of the same rotational level are not reproduced correctly for all levels.