The significance of fluctuating charges for molecular polarizability and dispersion coefficients

Y. Cheng, T. Verstraelen
The Journal of Chemical Physics
Volume 159, Issue 9


The inŕuence of ŕuctuating charges or charge ŕow on the dynamic linear response properties of isolated molecules from the TS42 database is evaluated, with particular emphasis on dipole polarizability and C6 dispersion coefficients. Two new descriptors are deőned to quantify the charge-ŕow contribution to response properties, making use of the recoupled dipole polarizability to separate isotropic and anisotropic components. Molecular polarizabilities are calculated using the “frequency-dependent atom-condensed Kohn-Sham density functional theory approximated to second orderž, i.e. the ACKS2ω model. With ACKS2ω, the charge-ŕow contribution can be constructed in two conceptually distinct ways, which appear to yield compatible results. The charge-ŕow contribution is signiőcantly affected by molecular geometry and the presence of polarizable bonds, in line with previous studies. We show that the charge-ŕow contribution qualitatively reproduces the polarizability anisotropy. The contribution to the anisotropic C6 coefficients is less pronounced, but cannot be neglected. The effect of ŕuctuating charges is only negligible for small molecules with at most one non-hydrogen atom. They become important and sometimes dominant for larger molecules or when highly polarizable bonds are present, such as conjugated, double or triple bonds. Charge ŕow contributions cannot be explained in terms of individual atomic properties, because they are affected by non-local features such as chemical bonding and geometry. Therefore, polarizable force őelds and dispersion models can beneőt from the explicit modeling of charge ŕow.