[gmx-users] calculation the energy of interaction

[Nicolas Sapay]

Hello everybody,

I have a system made of lipid and water and I want to calculate the 
components of the lipid-lipid, lipid-water and water solvent 
interactions (LJ+electrostatics).  I know there is already a lot of 
explanations about that kind of stuff on the mailing-list. I have 
carefully read all of them. Probably too carefully because I'm pretty 
confused now :(. Can someone tell me if my protocol is correct or not?

0. As a preliminary remark, I may need to specify my MD parameters:
vdwtype = Cut-off
rvdw = 1.4
coulombtype = PME
rcoulomd = 1.0
rlist=1.0

1. For the LJ interactions:
There is not much to do except running g_energy. For the Lipid-Water LJ 
interactions, I just have to sum LJ-SR:DOPC-SOL + LJ-LR:DOPC-SOL. Same 
thing for Lipid-Lipid and Water-Water interactions

2. For the Electrostatic interactions:
Still with g_energy, I can get the Coul-SR interactions. But Coul-LR 
need additional calculations because of the PME. I need to rerun my 
trajectory with
    - a. charge set to 0.00 on water molecules
    - b. charge set to 0.00 on POPC molecules
Then, the Coul-LR:DOPC-DOPC term is equal to the Coul.-recip. in run 'a' 
and the Coul-LR:SOL-SOL term is equal to the Coul.-recip. in run 'b'. 
The Coul-LR:DOPC-SOL is equal to the Coul.-recip. of the initial run. 
Now, I just have to sum Coul-SR and Coul-LR

3. Now, what if I want to measure the head group-water interactions 
(supposing the group is correctly defined in my .ndx)? For the LJ and 
the short range electrostatic interactions, it is the same thing than 
previously. For the long range electrostatic interactions, I need to do 
more rerun with :
  - c.  charge set to 0.00 on everything except lipid head groups
  - d.  charge set to 0.00 on everything except water molecules
  - e.  charge set to 0.00 on everything except head groups and water
  - f.  charge set to 0.00 on everything except acyl chains and water
  - g.  charge set to 0.00 on everything except acyl chains and head groups
  - h.  charge set to 0.00 on everything except acyl chains
Then, Coul-LR:headgroup-headgroup=Coul.-recip. of  '1' - 
('c'+'d'+'f'+'g'+'h')... At least that the value I'm looking for is 
simply the one calculated in 'e'?

I know the question has been asked many times. But I am still confused 
after have read all the answers... A tutorial about that would be great!

Thanks in advance for your comments

Nicolas



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