An improved third order dipole moment surface for methane

In a previous article a dipole moment surface (DMS) of full-electron, multi-reference configuration interaction (MRCI) quality was obtained and used to calculate the rotational spectrum of methane vibrational ground state, by means of a combination of the mean field configuration interaction method (VMFCI) with a generalized perturbation theory. The theoretical line intensities were matching the experimental ones obtained at the SOLEIL synchrotron well within experimental uncertainties. However, not all third order terms were included in this DMS. In the present work, additional DMS points have been calculated and fitted using a complete third order expansion. The new results give R-branch intenSities systematically smaller by about 1% compared to those previously obtained by using the same ab initio method, so still within experimental errors. The relevance of this DMS to calculate intensities for excited vibrational states, in particular for the dyad, is addressed. (C) 2013 Elsevier Inc. All rights reserved.