[gmx-users] Van der Waals: switch attractive term off

Pradip Kumar Biswas p.biswas at csuohio.edu
Mon Jul 3 16:42:30 CEST 2006

Hi Mark,

On Jul 1, 2006, at 3:24 AM, Mark Abraham wrote:

>> It seems that you can modify the following line in the function
>> *mk_nbfp() in force.c and achieve what you want.
>> 	C6(nbfp,atnr,i,j)   = idef->iparams[k].lj.c6;      (it is in line 117
>> of force.c in version 3.3)
>> and change it to
>>          C6(nbfp,atnr,i,j)   =  0;
>> This will set the attractive interaction between atom i & j to 0.
> More correctly, it will set the coefficient of the r^6 term to zero.

That's the goal.

> This
> alters the shape of the function for *all* r, both in the "attractive" 
> and
> "repulsive" regions,

I could not get that. How could the setting C6(nbfp,atnr,i.j) = 0 can 
affect the repulsive interaction term, keeping in mind that even if  
you  are using a ff that uses 'sigma' or 'epsilon'  to construct C6 and 
C12 and explicit the minimum of the potential, you are not affecting 

> such that the potential function no longer has a
> local minimum.

That's true and that's possibly be the concern of the user who wanted 
only to switch-off  the attractive (c6) vdw without asking for its 

> As Erik pointed out, this is very different from having
> zero force after a certain r.
> Mark


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Pradip K. Biswas, PhD.
Research Associate, Department of Chemistry;
Cleveland State University, Ohio-44115
Phone: 1-216-875-9723
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