A global electric dipole function of ammonia and isotopomers in the electronic ground state

A global analytical representation of the electric dipole hypersurface for ammonia and isotopomers is developed as a function of bond lengths and bond angles. Its simple and general form allows for the simultaneous description of all three dipole moment components using a small number of parameters. The parameter values are determined by adjustment to dipole moment values obtained from ab initio calculations at the second-order Moller-Plesset and multiconfiguration self-consistent field level of theory. The dipole function is used to calculate six-dimensional transition moments for NH3, using wave functions obtained from a variational calculation and a global analytical representation of the potential hypersurface. The comparison with experiment demonstrates a fairly reliable description of the electric dipole hypersurface for molecular structures with potential energy equivalent to up to 10000 cm(-1). At higher excitations a qualitatively correct asymptotic behavior of the dipole function is assured by construction. (C) 2003 American Institute of Physics.