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 cm−1), Born–Oppenheimer diagonal corrections (−10 cm−1), and zero-point vibrations (+244 cm−1) are accounted for. c © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1306–1314, 2001