First let us consider the latter case, where electrostatics are truncated at
the cutoff distance. Using this scheme, all electrostatic interactions
beyond a specified distance are ignored, that is, assumed to be zero.
If switching is set to on, rather than having a discontinuity
in the potential
at the cutoff distance, a shifting function is applied to the electrostatic
potential as shown in Figure 2. As this figure shows, the
shifting function shifts the entire potential curve so that the curve
intersects the x-axis at the cutoff distance.
This shifting function
is based on the shifting function used by X-PLOR.
See section 5.1.4 for more details.
Next, consider the case where full electrostatics are calculated. In this
case, the electrostatic interactions are not truncated at any distance. In
this scheme, the cutoff parameter has a slightly different meaning
for the electrostatic interactions -- it represents
the local interaction distance, or distance within which electrostatic
pairs will be directly calculated every timestep. Outside of this distance,
interactions will be calculated only periodically using DPMTA. These forces
will be applied using a multiple timestep integration scheme as described in
section 4.5.4.
