[gmx-users] OPLS parametrizaton

[Marcelo Silva]

Hi,

I am trying to simulate mixtures of ethanol and trifluoroethanol and I 
would like to use the same parameters used on OPLS parametrization.

On the OPLS1996 paper the authors starts by stating that they've used 
267 molecules and "The box size varied from approximately 26 26  26 Å 
for methanol to 37  37  37 Å for cyclohexane". In my simulations I am 
considering using 500 molecules (each box vector would be around 37 Å).

"In most cases, the intermolecular nonbonded interactions were truncated 
at 11Å based on roughly the center-of-mass separations with quadratic 
smoothing of the interaction energy to zero over the last 0.5 Å (...) A 
standard correction was made for the Lennard-Jones interactions 
neglected beyond the cutoff."

Trying to interpret what is written I think the correct way to treat 
cut-offs and energy corrections would be like this:

---------------------------------------
Cut-offs

rlist                    = 1.4
coulombtype              = Reaction-field-zero
rcoulomb                 = 1.1
rcuolomb_shift      = 0.6
vdwtype               = shift
rvdw                     = 1.1
rvdw_switch        = 0.6

Dispersion Correction

DispCorr             = EnerPress

---------------------------------------

However I have some doubts because I'm using a box with a different 
number of molecules/ different size (should I change the cutoffs to 15Å 
like it was for cyclohexane?) and I'm not sure about using 
Reaction-field-zero instead of the shift function for coulombic 
interactions.

Could you please help me confirm this parameters so I can run a 
simulation in the same conditions as OPLS was parameterized (although 
using MC and not MD).

Best regards,

Marcelo