Ad-hoc Fast Solvation Models
Another possibility is to use ad-hoc quick strategies to estimate solvation free energy. A first generation of fast implicit solvents is based on the calculation of a per-atom solvent accessible surface area. For each of group of atom types, a different parameter scales its contribution to solvation ("ASA-based model" described above).
Another strategy is implemented for the CHARMM19 force-field and is called EEF1. EEF1 is based on a Gaussian-shaped solvent exclusion. The solvation free energy is
The reference solvation free energy of i corresponds to a suitably chosen small molecule in which group i is essentially fully solvent-exposed. The integral is over the volume Vj of group j and the summation is over all groups j around i. EEF1 additionally utilizes a distance-dependent (non-constant) dielectric, and ionic side-chains of proteins are simply neutralized. It is only 50% slower than a vacuum simulation. This model was later augmented with the hydrophobic effect and called Charmm19/SASA.
Read more about this topic: Implicit Solvation
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