SWITch specifies that the truncation of the electrostatic interaction should be handled by a smooth change of Ecoulomb from CTONNB to CTOFNB, which in your case is a 0.5A interval. This is not a good idea because over that short interval the energy will vary rapidly, leading to a large, artificial force at a fairly long distance.
"OPTIONS THAT ARE NOT RECOMMENDED (OR REALLY BAD):
[ ATOM ] [ CDIElec ] [ SWITched ] NO! Very bad - do not use!
For a better description of these methods and how they perform, see:
P.J. Steinbach, B.R. Brooks: "New Spherical-Cutoff Methods for Long-Range
Forces in Macromolecular Simulation," J. Comp. Chem. 15, 667-683 (1994)."
See also Norberg, J., and Nilsson, L. (2000). On the Truncation of Long-Range Electrostatic Interactions in DNA. Biophysical Journal 79, 1537-1553.
EPSilon is the dielectric constant used in Coulomb's law to evaluate pairwise electrostatic interactions, and it should be 1 for explicit solvent simulations.