[gmx-users] Electrostatics and VDW

Una Bjarnadottir una.bjarnadottir at ucd.ie
Fri Jan 27 11:27:27 CET 2006

Dear all,

Thank you David for your reply,

Now when I try to use rvdw = 1.0 and rlist 1.1 and rcoulomb = 1.0 I get 
the error; With vdwtype = Cut-off,rvdw must be >= rlist.
I don't have any rvdw-switch or rcoulomb-switch
I'm using version 3.2.1 and I thought it had been fixed according to the 
discussion on the list.

Is there some trich that one has to do to have rlist>rvdw?

Best wishes to everyone, Una

> With PME, you don't have to have rlist<rvdw. You might, however, want to
> have rlist slightly larger than rvdw. You don't have to, but since the
> neighbor list is only updated every nstlist steps, if rlist is the same as
> rvdw, you can run into the situation where an atom has moved inside the vdw
> cutoff in a couple of steps, but your calculation doesn't realize it has
> moved inside the cutoff until after nstlist steps. You can solve the
> problem, of course, by using nstlist=1, but that could slow things down a
> lot, so it may be better to simply use, say, rlist=1.1 and rvdw=1.0 so atoms
> appear in the neighbor list before getting counted for vdw interactions.
> David
> On 1/24/06, Una Bjarnadottir <una.bjarnadottir at ucd.ie> wrote:
>>Dear all,
>>Just to have the content of this discussion clear
>>and all other discussion on this matter.
>>When using coulombtype = PME  and vdw-type = Cut-off is it alright to
>>have the;
>>rlist=rvdw=rcoulomb = 1
>>rvdw-switch=rcoulomb-swithc = 0
>>Or does it have to be;
>>rlist=rcoulmb = 0.9 and rvdw = 1.4
>>Is the 2nd thing only for cut-off but not for PME?
>>Thanks in advance for any replies, this is confusing me a lot.
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