[gmx-users] REMD: A small bug in repl_ex.c and a related question

[Suman Chakrabarty]

Hello!

1. It seems I have encountered a minor bug (?) in repl_ex.c for
version 4.6.x (for NPT simulations only):

Line 880 (in version 4.6.5):
            fprintf(fplog, "  dpV = %10.3e  d = %10.3e\nb", dpV, delta + dpV);

should be:
            fprintf(fplog, "  dpV = %10.3e  d = %10.3e\n", dpV, delta + dpV);

The extra "b" results into lines like this in the log file:
bRepl ex  0    1    2    3    4    5    6    7    8    9   10   11
12 x 13   14 x 15   16 x 17   18   19   20 x 21

While the REMD itself runs fine, the lines containing "bRepl ex" are
never parsed by the demux.pl script. So, only half of the exchanges
are analyzed by the script.

2. The related question: In the log file, the dPV corrections are
reported only every alternate exchange attempts: (I have kept only a
relevant trimmed part below)

Replica exchange at step 2000 time 4
Repl ex  0    1    2    3    4    5 x  6    7 x  8    9 x 10

Replica exchange at step 3000 time 6
Repl 0 <-> 1  dE_term =  6.670e-01 (kT)
  dpV = -3.918e-05  d =  6.669e-01
bRepl ex  0 x  1    2    3    4    5    6    7    8 x  9   10

Why is this necessary? I hope it doesn't mean the correction is being
applied every alternate attempt (I have not checked this part of the
code yet).

Is it safe to use NPT REMD with the current version of the code? If I
just apply the above correction, and use demux.pl to analyze ALL
exchange attempts, is it still valid? Please confirm.

Thanks,
Suman.