<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
Hello,<br>
<br>
Thank you very much for your reply. I actually carried out very
basic MDs of a peptide in vacuum (no pbc, cut-off for electrostatics
and vdw = "infinity", niter = 10^6). The computing results are given
in the two tables below. As they are identical, I am assuming that
there is only one neighbor list that is created for the calculation
of both the electrostatic and vdW interactions. Since I want to
calculate all vdW terms, I suppose that all Coulomb terms are
automatically calculated too. Am I right ?<br>
<br>
<br>
What if I create a group with all the nul-charge atoms and, by some
means (e.g., a different fudgeQQ value - or something else- for that
group ???), avoid to compute the electrostatic interactions with all
the other charges particules ? <br>
<br>
<br>
Similarly to the problem I reported in my mail, I also want to add
some kind of charged virtual sites. In that case, those sites
should not contribute to the vdW interactions of the system. I
found no problem at all to define such virtual sites and to carry
out MD simulations, but I suspect vdW terms to be calculated even if
epsilon and sigma are set equal to zero for such "particles".<br>
<br>
Thank you in advance for any help<br>
<br>
Laurence<br>
<br>
<br>
<br>
For the regular MD (all atoms bear a non-zero charge) :<br>
<font face="Courier New, Courier, monospace">Computing:
M-Number M-Flops % Flops<br>
-----------------------------------------------------------------------------<br>
All-vs-All, Coul + LJ 19404.019404
737352.737 72.1<br>
Outer nonbonded loop 197.000197
1970.002 0.2<br>
1,4 nonbonded interactions 518.000518
46620.047 4.6<br>
Bonds 105.000105
6195.006 0.6<br>
Angles 363.000363
60984.061 6.0<br>
Propers 674.000674
154346.154 15.1<br>
Virial 24.200242
435.604 0.0<br>
Stop-CM 19.700197
197.002 0.0<br>
Calc-Ekin 197.000394
5319.011 0.5<br>
Lincs 96.000288
5760.017 0.6<br>
Lincs-Mat 468.001404
1872.006 0.2<br>
Constraint-V 192.000384
1536.003 0.2<br>
Constraint-Vir 9.600096
230.402 0.0<br>
-----------------------------------------------------------------------------<br>
Total
1022818.053 100.0</font><br>
<br>
<br>
For the modified system (most of the atoms, i.e. 169 over 197, bear
a nul charge) :<br>
<font face="Courier New, Courier, monospace">Computing:
M-Number M-Flops % Flops<br>
-----------------------------------------------------------------------------<br>
All-vs-All, Coul + LJ 19404.019404
737352.737 72.1<br>
Outer nonbonded loop 197.000197
1970.002 0.2<br>
1,4 nonbonded interactions 518.000518
46620.047 4.6<br>
Bonds 105.000105
6195.006 0.6<br>
Angles 363.000363
60984.061 6.0<br>
Propers 674.000674
154346.154 15.1<br>
Virial 24.200242
435.604 0.0<br>
Stop-CM 19.700197
197.002 0.0<br>
Calc-Ekin 197.000394
5319.011 0.5<br>
Lincs 96.000288
5760.017 0.6<br>
Lincs-Mat 468.001404
1872.006 0.2<br>
Constraint-V 192.000384
1536.003 0.2<br>
Constraint-Vir 9.600096
230.402 0.0<br>
-----------------------------------------------------------------------------<br>
Total
1022818.053 100.0<br>
-----------------------------------------------------------------------------</font><br>
<br>
<br>
<br>
<br>
On 04/06/2012 16:37, Mark Abraham wrote:
<blockquote cite="mid:4FCCC829.3040609@anu.edu.au" type="cite">On
5/06/2012 12:08 AM, Laurence Leherte wrote:
<br>
<blockquote type="cite">Dear Gromacs users,
<br>
<br>
I am using the Amber99 FF in MD simulations of peptides (and
proteins). In a first stage to the design a different charge
distribution, most of the atomic charges were set equal to zero
(i.e., all charges but the C and O backbone atoms).
<br>
It appeared that the calculation times observed for the original
all-atom charges and the modified system are similar.
<br>
<br>
My question is thus the following one. In order to save
calculation time (and whatever the FF is), how is it possible to
avoid that the atoms bearing a nul charge are considered in
electrostatic calculations ? I should specify here that I want
these atoms to be considered in the vdW non-bonding
interactions.
<br>
</blockquote>
<br>
IIRC GROMACS neighbour searching already identifies atoms with
zero charge and/or LJ parameters and uses non-bonded code that
does not compute contributions that are known to be zero. You
should be able to see this from the differences in the flop
accounting at the end of your .log files when you have different
numbers of zero-charge atoms. If the total calculation times are
similar, then the number of atoms for which time was saved was
negligible. This would be normal for a peptide in a much larger
quantity of water. You will have to judge the truth of this from
the timing and flop breakdown at the end of the .log file.
<br>
<br>
Mark
<br>
</blockquote>
<br>
<br>
<pre class="moz-signature" cols="72">--
Laurence Leherte
Laboratoire de Physico-Chimie Informatique
Unité de Chimie Physique Théorique et Structurale
Facultés Universitaires Notre-Dame de la Paix (University of Namur)
Rue de Bruxelles, 61
B-5000 Namur
Belgique (Belgium)
</pre>
</body>
</html>