# [gmx-users] Re:problem with interaction energy calculated by g_energy

Qiong Zhang qiongzhang928 at yahoo.com
Tue Mar 9 10:15:06 CET 2010

Hi dear Mark,

Please ignor my last mail replied to you. I made some mistake there.

Yes, you are right that I am using PME. The cutoff for the real space and
reciprocal space is 1.2nm.

The molecules I am simulating are carbohydrates. And I am using Glycam06 Force
Field.

I tried there
different ways to calculate the interaction energy:

The first approach is analyzed by directly using g_energy, summing up Coul_SR
and LJ_SR of two groups, since in the .mdp file I have defined in energygrps 1
2.

The interaction energy between 1 and 2 (E 1_2) = E
Coul_SR + E LJ_SR =-170.048+(-232.719)=-402.767 kJ/mol

The second approach is
using "mdrun -rerun" option with the exactly the same energygrps 1 2 defined
in .mdp, the same traj.xtc and the same index. Weird enough, this time, I got interaction
energy between 1 and 2 (E 1_2) = E Coul_SR + E LJ_SR
= -91.5234 + (-238.712) = -330.235 kJ/mol, which is quite far from the previously -402.767 kJ/mol!!!! But this -330.235 kJ/mol is the exact sum of the
contributions of subunits. The contributions of subunits are also calculated in
this approach with rerun. So the discrepancy I reported in my first mail is
solved.

But what is the reason for the huge discrepancy between
the interaction energy from the original run and the “rerun”?? I think they
should be exactly the same.

The third approach, in order to include the long range interaction, I've also
tried "mdrun -rerun" option with three
"reruns" carried out for molecule 1(1st), molecules 2 (2nd) and
molecule 1 and 2 (3rd). The interaction energy for molecule 1 and 2 is now
calculated by:

[Coul(SR+recip)+LJ(SR+Disper. corr.)]_3rd - [Coul(SR+recip)+LJ(SR+Disper.
corr.)]_2nd - [Coul(SR+recip)+LJ(SR+Disper. corr.)]_1st

=Delta(Coul_SR)+Delta(Coul_recip)+Delta(LJ_SR)+Delta(LJ_Disper.corr.)

=(-128.73) + (-30.33) +( -252.021) + (-39.9) = -450.217 kJ/mol

If we neglect the long-range interactions, namely, Delta(Coul_recip) and Delta(LJ_Disper.corr.),
we got the interaction energy -128.73
-252.021= -380.751 kJ/mol. We see here the long-range
contribution is not negligible. However, this short range energy -380.751 kJ/mol is neither close to the -330.235 kJ/mol nor -402.767 kJ/mol.

So Now I am confused. Which approach should be really
adopted in the calculation of interaction energy? And what approach do you use
in such interaction energy calculations?

Thank you very much!

Qiong

--- On Tue, 3/9/10, Qiong Zhang <qiongzhang928 at yahoo.com>
wrote:

From: Qiong Zhang <qiongzhang928 at yahoo.com>

Subject: Re:problem with interaction energy calculated by g_energy

To: gmx-users at gromacs.org

Date: Tuesday, March 9, 2010, 4:27 PM

Hi dear Mark,

Yes, you are right that I am using PME.

The molecules I am simulating are carbohydrates. And I am using Glycam06
Force Field.

The interaction energy I got previously is analyzed by directly using
g_energy, summing up Coul_SR and LJ_SR of two groups.

In order to include the long range interaction, I've also tried "mdrun
-rerun" option.  So three "reruns" were carried out for
molecule 1(1st), molecules 2 (2nd) and molecule 1 and 2 (3rd). This time, I
found the long range Coul_recip between molecule 1 and 2 is a quite positive
value. So when only Coul_SR is included, the electrostatic interaction
between molecule 1 and molecules 2 is much more negative (> 100 kj/mol)
than that when both Coul_SR and Coul_recip are included. I guess, for such
carbohydrate molecules, long range Coul_recip can not be excluded.

Am I right here?

For the second summing up problem, I am still checking all the input file,
especially the index file.

Thank you very much!

Qiong

----- Original Message -----

From: Qiong Zhang <qiongzhang928 at yahoo.com>

Date: Monday, March 8, 2010 20:35

Subject: [gmx-users] problem with interaction energy calculated by g_energy

To: gmx-users at gromacs.org

-----------------------------------------------------------

| > Dear gmx users,

>

> I am studying the adsorption behavior of a molecule ( molecule 1) on a surface
(molecules 2). Based on the production run, I calculated the interaction
energy between molecule 1 and molecules 2 by g_energy.

> Here comes the first question: Why only short range interactions between
1 and 2 are displayed, namely, Coul_SR and LJ_SR? So the interaction energy E
1_2 I calculated is just the sum of Coul_SR+LJ_SR. Will this bring about huge
errors?

Guessing wildly (since you've not told us the nature of your simulation
protocol) you're using PME, and so the long-range contributions cannot be
decomposed group-wise. This is probably a good thing - I'm not aware of any
force field that has been parameterized so that small chunks of atoms
interaction energies correlate to anything useful.

> After this, I'd like to know the individual contributions of the
components of molecule 1  to the interaction energy between 1 and 2. For
example, molecule 1 is composed of A, B, C and D resdues. So again, by
g_energy, I got interaction energy between A, B, C and D with 2,
respectively, denoted by E A_2, E B_2, E c_2 and E D_2.
Still, these interaction energies are the sum of

Coul_SR+LJ_SR.

> Then comes the second question: Why the sum of E A_2, E B_2, E c_2 and E
D_2 does not equal to E 1_2? I found there was big difference between them,
sometimes as large as 50 kJ/mol.

>

> Could anybody give me some hints or suggestions please?

They should add up. Check your index group definitions and use in the .mdp
file.

Mark

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