[gmx-users] 1-4 columb interaction

chris.neale at utoronto.ca chris.neale at utoronto.ca
Wed Sep 20 22:35:06 CEST 2006

You are right that the LJ fudge values don't get applied if  
gen-pairs=no. However, FudgeQQ is always applied. However, one would  
hope that Qiao was correct and the main .itp files at least list the  
correct scaling of LJ 1-4 interactions. Force fields that use  
gen-pairs=no explicitly list 1-4 LJ parameters in the pairtypes  
section. This appears to be the case in the gromos96 force field. For  

[ nonbond_params ]
; i    j func          c6           c12
       OM        O  1  0.0022619536  7.4149321e-07
[ pairtypes ]
; i    j func          c6           c12
       OM        O  1  0.0022619536  7.4149321e-07

So a 1-4 (pairtypes) and a 1->5 or nonbonded LJ are going to be the same.

The treatment of 1-4 interactions is quite well described in the  
gromacs manual.

   ----- Original Message -----
Thanks Qiao for your reply.

But If I am using genpair = no (GROMOS 96 force field) then there is  
no meaning of  fudgeLJ and fudgeQQ (correct me if I am wrong).

So more specific if I am using GROMOS96 force field then is there any  
scaling for 1-4 columb interaction??
if yes then how?

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