An efficient method for calculating static polarizabilities from a correlated wave function

We present a method closely related to configuration interaction over single excitations ( CI‐S ) for the calculation of static dipole moments and polarizabilities, which we call modified configuration interaction over single excitations ( MCI‐S ). This method allows for orbital relaxation through the inclusion of single excitations on a correlated first‐order Rayleigh–Schrödinger wave function. An approximation to our MCI‐S results in an inexpensive method to calculate polarizabilities that yields good results when compared with the full configuration interaction ( CI‐F ) results. This method, called modified configuration interaction over single excitations without triple excitations ( MCI‐S_WT ), seems particularly suitable to applications for large systems. Examples are presented using the intermediate neglect of the differential overlap model Hamiltonian ( INDO/1 ), which indicates that the MCI‐S_WT procedure presents an error of less than 15% with respect to the CI‐F values in a fraction of the computational effort. © 1994 John Wiley & Sons, Inc.