Six-dimensional ab initio potential energy surfaces for H3O+ and NH3:: Approaching the subwave number accuracy for the inversion splittings

New potential energy surfaces are calculated for the hydronium ion using high-order coupled cluster ab initio methods. Large basis sets are used especially for the inversion part of the full surface. Electronic energies obtained with different correlation consistent basis sets are extrapolated to the infinite basis set limit. Core-valence and first order relativistic effects are also included. The influence of these two contributions and basis set sizes on both the inversion barrier height and equilibrium geometry are investigated thoroughly. The same methods are also adopted for ammonia in order to further improve a recently published surface [J. Chem. Phys. 118, 6358 (2003)]. The vibrational eigenvalues are calculated variationally both for the symmetric and asymmetric isotopomers using exact six-dimensional kinetic energy operators and successive basis set contractions. With the new surfaces, the mean absolute deviations obtained for all experimentally observed inversion splittings for different isotopomers of H(3)O(+) (8 states) and (14)NH(3) (17 states) are 0.78 and 0.25 cm(-1), respectively. Inversion levels are calculated with almost similar accuracy. These results indicate that the calculated inversion barrier heights for H(3)O(+) and NH(3), 650 and 1792 cm(-1), respectively, are close to the real values. The value for ammonia is also close to the height determined from published experimental data in our previous work. The lowest energies for the high-frequency modes are computed with the mean absolute deviation being less than 2 cm(-1) for isotopomers of H(3)O(+) and less than 4.5 cm(-1) for (14)NH(3) with respect to experimental energies. (C) 2003 American Institute of Physics.