[gmx-users] C6 attractive term off OPLSAA

[nishap.patel at utoronto.ca]

Hello,

    I have a concern regarding C6 attractive term in LJ potential. I  
am using OPLS-AA force field, and I wish to turn off attractive term  
C6 by setting the parameters to zero. One of the suggestion was to add  
a [nonbond_params] in my ffoplsaanb.itp file and set the C6 to zero  
between the non-bonded pair. In my system for example, which consists  
of one methanol in water, I wish to set C6 term to zero between my  
solute and solvent. Since OPLSAA is all atom force field it treats  
each atom individually and has sigma and epsilon for each atom, so I  
am not sure how I would actually set my nonbond_params in my nb.itp  
file. I realize I need to convert each sigma and epsilon to C6 and  
C12, so say for example for methanol in water my [nonbond_params]  
would look something like this?

[ nonbond_params ]
; i    j func          c6           c12
       CT        OW  1   0.00      calculated value for C12 here?
       CT        HW1  1  0.00
       CT        HW2  1  0.00

CT is the carbon in Methanol. OW, HW1 an HW2 correspond to atoms in  
TIP3P water model

Is that correct? Would I have to do that for each atom in methanol?

Any suggestions would be appreciated.

Thanks.

Nisha P