Dynamical Discrete/Continuum Linear Response Shells Theory of Solvation Convergence Test for NH4+ and OH- Ions in Water Solution Using DFT and DFTB Methods

A new dynamical discrete/continuum solvation model was tested for NH4+ and OH- ions in water solvent The method is similar to continuum solvation models in a sense that the lineal response approximation is used However, different from pure continuum models, explicit solvent molecules are included in the inner shell which allows adequate treatment of specific solute solvent interactions present in the first solvation shell, the main drawback of continuum models Molecular dynamics calculations coupled with SCC-DFTB method are used to generate the configurations of the solute in a box with 64 water molecules, while the interaction energies are calculated at the DFT level We have tested the convergence of the method using a variable number of explicit water molecules and it was found that even a small number of waters (as low as 14) are able to produce converged values Our results also point out that the Born model, often used for long-range correction, is not reliable and our method should be applied for more accurate calculations