A General Route to Include Pauli Repulsion and Quantum Dispersion Effects in QM/MM Approaches.

A methodology to account for nonelectrostatic interactions in Quantum Mechanical (QM)/Molecular Mechanics (MM) approaches is developed. Formulations for Pauli repulsion and dispersion energy, explicitly depending on the QM density, are derived. Such expressions are based on the definition of an auxiliary density on the MM portion and the Tkatchenko-Scheffler (TS) approach, respectively. The developed method is general enough to be applied to any QM/MM method and partition, provided an accurate tuning of a small number of parameters is obtained. The coupling of the method with both nonpolarizable and fully polarizable QM/fluctuating charge (FQ) approaches is reported and applied. A suitable parametrization for the aqueous solution, so that its most representative features are well reproduced, is outlined. Then, the obtained parametrization and method are applied to calculate the nonelectrostatic (repulsion and dispersion) interaction energy of nicotine in aqueous solution.