Friday, May 18, 2012

Energy Decomposition Analysis in Solution Based on the Fragment Molecular Orbital Method

D. G. Fedorov, K. Kitaura, J. Phys. Chem. A 116 (2012) 704-719.

The solvent screening has long been described relying on the dielectric constants taken arbitrarily based on "experience". In this work the dielectric constants are derived from ab initio calculations using the charges induced by the solute on the solvent surface in the polarizable continuum model. These charges uniquely define the solvent screening energy, and also the dielectric constant.

Induced solvent charges δ surrounding two subsystems I and J.
[Reprinted with permission from the Journal of Physical Chemistry A.  Copyright (2012) American Chemical Society.]

For the rather trivial case of two interacting systems (sodium and chloride ions), the analysis provides rich physical insight into the solvation process, and the induced charge quenching effect is observed, caused by the partial loss of the solvation shell and the potential pressure of the nearby ion.
The real challenge for a physical interpretation is found in systems with multiple fragments, in which case strong many-body electrostatic effects lead to  very interesting and peculiar results, such as a partial solvent screening reduced by the pressure of the solute-induced electrostatic potential of nearby fragments.

The solvent screening of the electrostatic interactions between charged residues in a small protein chignolin (PDB: 1UAO) is discussed based on ab initio (FMO) calculations in solution.