The vibrational levels of ammonia.
A new six-dimensional potential energy function (PEF) of ammonia expressed in internal coordinates is determined by fitting to points evaluated by Density Functional Theory with the B97-1 functional. The C3v and D3h structures are treated on an equal footing. The inversion barrier is 1820 cm(-1), which is in very good agreement with the experimental value of 1834 cm(-1). The minimum 'reaction path' is well defined by the analytic function up to 40 degrees for the umbrella angle. Using this PEF, the vibrational levels are calculated variationally using three different methods. The first employs the internal kinetic energy operator developed for ammonia by Handy, Carter and Colwell (Mol. Phys. 96 (1999) 477). The second uses the code MULTIMODE (J. Chem. Phys. 107 (1997) 10458), which involves the kinetic energy operator as expressed in normal coordinates by Watson. The third uses an implementation of the reaction path hamiltonian (J. Chem. Phys. 72 (1980) 99) within the MULTIMODE code. All three approaches give similar energies for the vibrational energies of ammonia, and these agree with experiment to within 15 cm(-1) for the fundamental vibrations.