Accurate equilibrium inversion barrier of ammonia by extrapolation to the one-electron basis set limit

A scheme based on treating uniform singlet-pair and triplet-pair interactions is suggested to extrapolate electron correlation energy of ammonia, calculated at two basis-set levels of ab initio theory in the infinite one-electron basis-set limit. The dual-level method is tested on the extrapolation of the full correlation in coupled-cluster singles and doubles and in the case also a noniterative perturbative correction for connected triple energies for the C_3v and D_3h structures of ammonia, with correlation-consistent basis sets of the type cc-pVXZ (X=D,T,Q,5,6) and aug-cc-pVXZ (X=D,T,Q,5). For testing and comparison purposes, the energies reported by Klopper J. Comput. Chem. 22 1306 (2001) have been taken. From a corresponding extrapolation of CCSD(T)/AVXZ energies for X =4, 5, we obtain total inversion barriers of 1833.87 cm^-1/1832.33 cm^-1 for the two/three-parameter extrapolation rules, which are in good agreement with other theoretical extrapolation and empirical values in the literature.