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 C3vand D3hstructures 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 1for the two/three-parameter extrapolation rules,which are in good agreement with other theoretical extrapolation and empirical values in the literature.