Field-Aware Interfaces in Continuum Solvation

Continuum models of solvation are widespread tools for the prediction of solvation free energies of small molecular compounds from first principles. However, the continuum approximation at the core of these approaches limits their accuracy for the modeling of the aqueous solvation of compounds with highly polarized residue or of charged species. This is due to the fact that straightforward definitions of the continuum interface do not account for the reorganization effects induced by these solutes on the positions of surrounding solvent molecules. This kind of problem is usually overcome by stretching the definition of the continuum model, i.e., by using chemical intuition to adjust (usually shrinking) the size of the solvation interface close to the polarized/charged atoms. Nonetheless, this strategy introduces a significant number of additional parameters that need to be tuned and, at the same time, deters the model's transferability. A transferable solution is instead represented by an improved definition of the continuum interface, able to automatically account for the polarization/charge state of the embedded system. Following recent approaches in the literature, the component of the solute's electric field normal to the interface can be used as an effective proxy for the net charge of the embedded system. Here we show a simple definition of this field-aware approach as applied to the recently proposed soft-sphere continuum solvation (SSCS) method. In this model, each soft sphere composing the interface is allowed to readjust as a function of the value of the field flux through its surface. This effect introduces a complex dependence of the interface function on both the electronic and the ionic degrees of freedom of the solute. To account for this dependence during optimization procedures (e.g., the SCF loop and geometry optimization algorithms), the analytic derivatives of the new interface are reported and validated with their numerical counterparts. Application of the field-aware procedure to molecular compounds showing pathological behaviors with the standard SSCS approach show that significant improvements can be achieved by specifically tuning the newly introduced parameters.