The dielectric self-consistent field method. II. Application to the study of finite range effects

The dielectric self-consistent field (DSCF) method is used to study finite range corrections for the electrostatic contribution to solvation free energies. Detailed results obtained from calculations using Ewald summation (EW) and a generalized reaction field (GRF) technique are reported for the solvation of a spherical ion, glycine and an alanine octapeptide in an ideal alpha -helical conformation. For the peptide EW calculations are carried out with both conducting ("tinfoil") dielectric boundary conditions and adjusted dielectric boundary conditions. The emphasis of this work is on solutes without net charge, but with a large dipole moment. It is shown that in this case-similarly to ionic solvation-the self-energy correction needs to be modified by a thermodynamic correction that accounts for the dielectric constant of the solvent. An analytical expression for this term is worked out. The results obtained for glycine and the alanine octapeptide demonstrate that its use improves the system-size independence of solvation free energies calculated with EW compared with just the self energy correction; the GRF results are less satisfactory. We further show the connection between finite range corrections and artifacts in the total electrostatic energy of a system resulting from the use of modified electrostatic interactions. The direct comparison of EW with GRF shows that at present EW is the best method to use in simulations with explicit solvent and periodic boundary conditions.(C) 2001 American Institute of Physics.