[gmx-users] Accessing the distance_restraints energy term

[Chris Neale]

Thanks Mark for your assistance.

I have discovered the source of my problems in getting distance_restraints working. 
Without disre=simple in the mdp file, disre=no is assumed and, according to the manual:

"no distance restraints (ignore distance restraint information in topology file)"

While I admit that this information was readily available to me, it is so counter-intuitive to me
that a defined distance restraint would simply be ignored that I am posting the solution here 
in case anybody else gets confused on this topic.

Chris.

-- original message --

>/ Hello,
/>/ 
/>/ I can't seem to get the energetic contribution from distance restraints 
/>/ in gmx 3.3.1, 3.3.3, or 4.0
/>/ 
/>/ I am using Mark Abraham's incantation of the charmm ff in gromacs. Here, 
/>/ distance_restraints are used to deal with Charmm's Urey-Bradley 
/>/ potentials.
/
Not true. Yuguang Mu's version did so because at the time he wrote his, 
the native U-B was not available in GROMACS. Mine uses the native U-B in 
GROMACS. My documentation (such as it is) does note this.

>/ My colleague is using some non-standard charmm parameters, 
/>/ so I also used the scripts from the gromacs site to convert our 
/>/ parameter files to gromacs format and included them in ffcharmm. I 
/>/ believe that I have done everything correctly, but I want to check that 
/>/ the energy of a given snapshot is the same when calculated by gromacs as 
/>/ it is in NAMD (which reads the charmm files directly). Most energies are 
/>/ looking close enough to be rounding errors, but my angle energy is off. 
/>/ I suspect that the distance restraint energy plus the angle energy will 
/>/ be the same as the angle energy as stated by NAMD, but I do need to be 
/>/ sure here. I don't see any indication of the distance restraint energy 
/>/ in the .log file or by g_energy.
/
Caveat that I haven't tested with GROMACS 4.0 yet, if you're using my 
stuff correctly, then there will be an explicit U-B term. If you're not, 
then I can only guess what you've done wrong. Either you're actually 
using Yuguang's version and not using some -DDISRES correctly, or you 
are using mine but ignoring some warnings and/or not applying the Perl 
scripts correctly.

>/ I know others have been using the charmm forcefield, and I am wondering 
/>/ how they solved this problem. Perhaps I have done something incorrectly, 
/>/ but if the distance restraint energy is in fact not accessible, then 
/>/ this is also a suggestion to include that in the .edr file for gmx 4.1.
/
Distance restraint energies do get written when they are present. I 
think the issue here is that they are not present.

Mark