Theoretical determination of the vibrational levels of NH₃⁺ and its isotopomers

The vibrational levels associated with the electronic ground state X-2 A " (2) of NH3+ have been determined up to 5000 cm(-1) by perturbation and variational calculations with full dimensionality of the molecule. For the variational part a new version of MULTIMODE was used which uses the ab initio electronic energy and its first derivative to der ne the potential energy function. These quantities were generated by the B97-1 density functional and RCCSD( T) approaches. For ND3+, ND2H+ and NDH2+ the vibrational levels were calculated only by perturbation theory. The rotational constants for all the isotopomers were determined and the first transition dipole moments for NH3+ and ND3+ were plotted. A critical comparison of the perturbation and variational techniques suggests a possible further modification to the MULTIMODE algorithm for large systems.