Equilibrium inversion barrier of NH3 from extrapolated coupled-cluster pair energies

The basis-set convergence of singlet and triplet pair energies of coupled-cluster theory including single and double excitations is accelerated by means of extrapolations based on the distinct convergence behaviors of these pairs. The new extrapolation procedure predicts a nonrelativistic Born-Oppenheimer inversion barrier of 1767 +/- 12 cm(-1) for NH3. An effective one-dimensional, vibrationally averaged barrier of 2021 +/- 20 cm(-1) is obtained when relativistic effects (+20 m(-1)), Born-Oppenheimer diagonal corrections (-10 cm(-1)), and zero-point vibrations (+244 cm(-1)) are accounted for. (C) 2001 John Wiley & Sons, Inc.