The R(-7) Dispersion Interaction in the General Effective Fragment Potential Method.

The R(-7) term (E7) in the dispersion expansion is developed in the framework of the general effective fragment potential (EFP2) method, formulated with the dynamic anisotropic Cartesian polarizability tensors over the imaginary frequency range. The E7 formulation is presented in terms of both the total molecular polarizability and the localized molecular orbital (LMO) contributions. An origin transformation from the center of mass to the LMO centroids is incorporated for the computation of the LMO dipole-quadrupole polarizability. The two forms considered for the damping function for the R(-7) dispersion interaction, the overlap-based and Tang-Toennies damping functions, are extensions of the existing damping functions for the R(-6) term in the dispersion expansion. The R(-7) dispersion interaction is highly orientation dependent: it can be either attractive or repulsive, and its magnitude can change substantially as the relative orientation of two interacting molecules changes. Although the R(-7) dispersion energy rotationally averages to zero, it may be significant for systems in which rotational averaging does not occur, such as rotationally rigid molecular systems as in molecular solids or constrained surface reactions.