Determination of vibrational polarizabilities and hyperpolarizabilities using field-induced coordinates

An analytical set of field-induced coordinates (FICs) is defined. It is sho wn that, instead of 3N-6 normal coordinates, a relatively small number of F ICs is sufficient to describe the vibrational polarizability and hyperpolar izabilities due to nuclear relaxation. The fact that the number of FICs doe s not depend upon the size of the molecule leads to computational advantage s. A method is provided for separating anharmonic contributions from harmon ic contributions as well as effective mechanical from electrical anharmonic ity. Hartree-Fock calculations on a dozen representative conjugated molecul es illustrate the procedures and indicate that anharmonicity can be very im portant. Other potential applications including the determination of zero-p oint vibrational averaging corrections are noted. (C) 2000 American Institu te of Physics. [S0021-9606(00)32137-7].