Determination of the reduced matrix elements using accurate ab initio wavefunctions: Formalism and its application to the vibrational ground state (000) of H216O

•Reduced matrix elements (RMEs) for transition moments.•Application to vibrational ground state of H2O.•Comparing RMEs from ab initio and effective Hamiltonian wavefunctions.•Effects of extrapolating effective Hamiltonian wavefunctions on RMEs.•Derivation of reduced element sum rules.

The calculations of the reduced matrix elements for 441 rotational collisional transitions for rotational quantum numbers of the lower state up to J″=20 in the vibrational ground state of H216O are presented using effective and ab initio wavefunctions. Effective wavefunctions are derived from a Watson A-reduced Hamiltonian with the effective parameters determined by Matsushima et al. [Matsushima et al., J Mol Struct 1995;352-353:371]. The ab initio wavefunctions used in this study are from the work of Partridge and Schwenke [Partridge, H, Schwenke, DW. J Chem Phys 1997;106:4618]. The comparison of the reduced matrix elements obtained by both methods is described. It is demonstrated that, even for the rotational band, the effective wavefunctions show problems for some states.