Computation of quadratic electric dipole moment functions

A numerically reliable method for the calculation of molecular electric dipole moment functions is described. This scheme, which is an extension of a method due to Komornicki and McIver, is applied to the computation of the quadratic dipole moment function of water using many-body perturbation theory. A pronounced basis set dependence is observed for the stretch and stretch–stretch coefficients of the quadratic expansion of the dipole moment, while those involving the bending symmetry coordinate appear to be less sensitive. In addition, previously published ab initio dipole moment functions are discussed and compared to the present results.