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 vari ational calculations with full dimensionality of the molecule. For the vari ational part a new version of MULTIMODE was used which uses the ab initio e lectronic energy and its first derivative to der ne the potential energy fu nction. These quantities were generated by the B97-1 density functional and RCCSD( T) approaches. For ND3+, ND2H+ and NDH2+ the vibrational levels wer e 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 MULT IMODE algorithm for large systems.