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

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,

  Thanks very much for your reply.

  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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