For some reason, the difference in minutes didn't come through, it seems, but I would guess that if it's a medium-large difference, then it has its roots in PGI7.2 vs. GCC rather than MPICH2 vs. OpenMPI.  Though, to be fair, I find GCC vs. PGI (for C code) is often a toss-up - one may beat the other handily on one code, and then lose just as badly on another.

  I think the biggest difference in performance with various MPICH2 install comes from differences in the 'channel' used..  I tend to make sure that I use the 'nemesis' channel, which may or may not be the default these days.  If not, though, most people would probably want it.  I think it has issues with threading (or did ages ago?), but I seem to recall it being considerably faster than even the 'ssm' channel.
 
  Sangamesh:  My advice to you would be to recompile Gromacs and specify, in the Gromacs compile / configure, to use the same CFLAGS you used with MPICH2.  Eg, "-O2 -m64", whatever.  If you do that, I bet the times between MPICH2 and OpenMPI will be pretty comparable for your benchmark case - especially when run on a single processor.

  Cheers,
  - Brian