The atomization energy and proton affinity of NH3. An ab initio calibration study

The total atomization energy and proton affinity of NH3 have been subjected to an extensive convergence study involving basis sets of up to spdfgh quality. Our best extrapolated Sigma D-o = 276.5 kcal/mol lies only 0.2 kcal/mol below the experimental value. Our recommended value for PA(298), 203.9+/-0.3 kcal/mol, is in excellent agreement with the most recent experimental value but has a smaller error margin. It is found that augmented basis sets are desirable when determining proton affinities using the empirical correction for further basis set expansion proposed in J. Chem. Phys. 97 (1992) 5012.