Most polarizable force fields for water assign a fixed dipole polarizability to every water molecule despite the fact that there exist substantial <i>geometri</i>c and <i>environmental</i> effects on this fundamental quantity in the condensed-phase. Within the framework of linear response coupled cluster theory including single, double, and triple excitations (LR-CCSDT), we have computed the dipole polarizability surface (DPS) of the water molecule to quantify the geometric effect. In doing so, we find that the variation of the polarizability can be quite substantial in non-equilibrium geometries, and provide an analytical formula to capture this effect in a polarizable force field. In addition to this geometric effect, there are significant environmental effects on the molecular polarizability, which result from redistribution of the molecular charge density due to interactions with neighboring molecules in the condensed phase. In this work, we also develop a first-principles based approach for computing the dipole polarizability of a condensed-phase water molecule that includes both environmental and geometric effects, and introduce a practical yet highly accurate on-the-fly scheme for assigning molecular polarizabilities that is straightforward to use in current polarizable force fields. Accurate higher-order (beyond dipole) polarizabilities are also essential in accurate force fields and dispersion corrections. To obtain these response quantities, we have implemented a highly efficient version of the LR-CC method in NWCHEM that provides a straightforward route for computing multipole polarizabilities for atoms, ions, and molecules at the complete basis set limit. With this method, we have investigated the non-additivity of the multipole polarizabilities (and associated dispersion coefficients) for a wide range of fullerene nanostructures. Our results confirm that the multipole polarizabilities of fullerenes grow much faster than a simple pairwise model would suggest and provide the most accurate assessment of these highly non-additive quantities to date. Knowledge of this non-additivity is crucial for understanding the complex set of intermolecular forces at play across the nanoscale.

Multipole polarizabilities and dispersion coefficients for gas- and condensed-phase molecules and nanostructures

256th National Meeting (2018)

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