[gmx-users] Setting the C6 LJ term for OPLSA FF

[Luca Bellucci]

Dear all
I need to change sigma and epsilon for non-bonded parameters of the OPLSA FF.
In particular I want to set the attractive part of the LJ potential to zero 
(C6=0). 
In doing this I have read the manual but unfortunately the reported 
explanation did not help me. To understand how it works in a reliable way, 
I followed the Berk suggestions available at
http://lists.gromacs.org/pipermail/gmx-users/2010-December/056303.html
and i decided to report a simple example.

The main rules are in forcefield.itp file and for OPLSA FF they are: 
 ; nbfunc   comb-rule   gen-pairs     fudgeLJ     fudgeQQ
     1               3                  yes                   0.5        0.5

The non-bonded force field parameters for two atoms are in ffnonbonded.itp 
file and they look like:

[ atomtypes ]
; name   bond_type  mass       charge  ptype    sigma       epsilon
 opls_1   C     6        12.01100   0.500        A       sig_1        esp_1
 opls_2   O     8       15.99940   -0.500       A       sig_2        eps_2

From these values I am going to define the non-bonded parameter between a 
couple of atoms as:
  
[ nonbond_params ]
  i      j     func              SIG_ij                  EPS_ij 
opls_1 opls_2  1    (sig_1*sig2)^1/2  (eps_1*eps_2)^1/2 ; Normal behavior

However, if I want the attractive term C6 of LJ potential equal zero, I should
set sig_12=-sig_12

[ nonbond_params ]
  i      j     func                SIG_ij                 EPS_ij 
opls_1 opls_2  1   -(sig_1*sig_2)^1/2  (eps_1*eps_2)^1/2  ; -sig_ij -> C6=0

It is right?

Thanks
 Luca