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.