No Explicit Solvent

There has been a great deal of work done to improve implicit solvent models. The Screened Coulomb Potential Implicit Solvent Model (SCPISM) has a solid theoretical foundation and is computationally efficient. Many variants of Generalized Born (GB) models have also been developed.

Years ago, prior to the establishment of better implicit models, a distance-dependent dielectric coefficient (RDIE: $\epsilon_r = r_{ij}$) was used to crudely approximate solvent screening without including explicit water molecules. The SHIFt option does a good job of monotonically damping a $1/r^3$ force (RDIE electrostatics) to zero, but the use of RDIE has been supplanted by the SCPISM and GB approaches.

In general, explicit-solvent simulations are considered the most accurate. With water molecules present, a constant dielectric (CDIE) is used with $\epsilon_r = 1$. Of course, the presence of water slows conformational searching.