Modelling and calculation of dipole moment functions for XH bonds

Abstract

Two studies of aspects of modelling dipole moment functions of XH bonds in small molecules for use in calculating overtone intensities have been undertaken. The first study deals with the fitting of ab initio calculations of the dipole moment at discrete points to a functional form. The two methods that are compared are the use of least- squares regression and the use of interpolating polynomials. The interpolating polynomial method is deemed superior due to its greater efficiency in terms of the number of points necessary to obtain reasonable results. The second study attempts to explain the indifference of calculated overtone intensities to the inclusion of electron correlation in the ab initio calculation of the dipole moment function. It is found that in most cases the influence of electron correlation can be modelled as a function with a very small matrix element, which results in a very small contribution to the overtone intensity.