[gmx-users] Re: What is the purpose of the [ pairs ] section?

[Andrew DeYoung]

Hi Javier, Mark, Justin, and Bogdan,

Thank you SO much for your time!  The responses that you gave are INCREDIBLY
helpful to me.  I think that they will be very useful, too, to future users
as helpful canonical answers. 

If you have time, I have another question related to the [ pairs ] section.
I generated an .itp file for my large molecule (containing 492 atoms) using
g_x2top, in the OPLS-AA force field.  g_x2top, it seems, specifies all 1-4
interactions in the [ pairs ] section.  

The [ pairs ] section generated by g_x2top for my 492-atom molecule contains
2225 pairs.  I have not checked all 2225 of these pairs (and I should), but
from looking at a subset of them, it appears that these all correctly
correspond to 1-4 interactions, which as Bogdan pointed out, in the OPLS-AA
would be the correct behavior.  (That is, in OPLS-AA, 1-4 interactions --
and only 1-4 interactions -- are specified in [ pairs ], assuming that
nrexcl = 3, which in my case it is.)

Only the indices are specified in my [ pairs ] section, so I have set
gen-pairs = yes to generate (calculate) the parameter combinations, as
Bogdan explained.

But what confuses me is the grompp output, which says:

Generated 276 of the 276 non-bonded parameter combinations 
Generating 1-4 interactions: fudge = 0.5 
Generated 276 of the 276 1-4 parameter combinations

Why is the number of nonbonded interactions 276, and to what does this
refer?  In a previous post in this thread, Justin said that this 276 number
refers only to 1-4 interactions.  If this is the case (and I DO completely
trust Justin!), then why doesn't it say 2225 instead of 276?

Maybe the 276 number refers to the number of UNIQUE combinations found
within the 2225 pairs, but I don't see how this could be since my molecule
only contains four atomtypes (two different carbon atomtypes, one oxygen
atomtype, and one hydrogen atomtype).  Does anyone have any thoughts on
this?

Thank you so much for all of your EXTREMELY helpful explanations, everyone!

Andrew DeYoung
Carnegie Mellon University