Inclusion of the quadrupole moment when describing polarization. The effect of the dipole-quadrupole polarizability.

A method to compute distributed dipole-quadrupole polarizabilities is suggested. The method is based on numerical differentiation of distributed quadrupole moments, using finite field perturbation calculations. It is tested using two different multicenter multipole expansions, and compared with results using polarizabilities obtained via the uncoupled Hartree-Fock approximation. The accuracy of these dipole-quadrupole polarizabilities are tested for different molecules and basis sets, by comparing the induced electrostatic potential of the Hartree-Fock density with the induced electrostatic potential of the polarization models. This is done by perturbing the molecules with an external homogeneous field and with an external dipole. It is found that inclusion of the dipole-quadrupole polarizability significantly improves the accuracy of the response of the molecule to these external perturbations. This suggests that inclusion of higher-order induced moments can be of importance when improving the description of intermolecular interactions using force fields. (c) 2008 Wiley Periodicals, Inc. J Comput Chem 2008