I am currently doing a simulation of the behaviour of various sizes of water droplets (~100-1000 water molecules) in vacuum. I am using NAMD with the spherical boundary conditions to keep the droplet in place, and using the CHARMM parameters for simulation.
The CHARMM parameter stream files contain a list of options which contain the recommended non-bonded cut offs for vdW forces and electrostatics:
NONBONDED nbxmod 5 atom cdiel shift vatom vdistance vswitch - cutnb 14.0 ctofnb 12.0 ctonnb 10.0 eps 1.0 e14fac 1.0 wmin 1.5 !adm jr., 5/08/91, suggested cutoff scheme
(Explanation—pair-list distance= 14 Å, cut-off distance= 12 Å, dielectric const.= 1, 1,4-interaction scaling= 1)
However, these values are only read by the CHARMM software, NAMD manual mentions that NAMD will ignore them, and set the values according to what is set inside the NAMD configuration file.
If I understand correctly, force fields are usually parameterised considering a liquid system (i.e. with 3D periodic boundary conditions) therefore, these cut offs are also likely optimised for a liquid simulation.
What cut-offs should be used if a system in vacuum has to be simulated with CHARMM parameters? (and why?)
My guess here is that because the CHARMM non-bonded parameters were developed with that cut-off, the cut-off should be kept the same, even if the simulation is done in vacuum. However, a more-experienced colleague has told me that I should use a very large cut-off distance (much larger than the droplet, say ~500Å).
So I am confused about what cut-off I should use, and how much it would affect the system if I use a very large cut-off.
(Note that in the simulation I am not trying to measure any dynamic property of the system like diffusion or timescales etc. I am only interested in overall positions of the water molecules, their distributions, effect of a different molecule in the droplet etc.)