[gmx-users] Irregular relative entropy in free energy calculation

[Patrick armstrong]

Hi all,
   I am calculating the solvation energy of a LJ system, where one bead is
solvated in 1200 beads in a box. There is only LJ interaction between beads
and the interactions between beads are all the same. I calculated the free
energy with BAR method. For LJ 12-6 interaction it works well. While when
the LJ 9-6 interaction  was applied it showed irregular values for the
relative entropy as below.

  lam_A  lam_B      DG   +/-     s_A   +/-     s_B   +/-   stdev   +/-
     0      1   -0.17  0.00    0.00  0.00   -0.00  0.00    0.01  0.00
     1      2   -0.19  0.00   -0.00  0.00   -0.00  0.00    0.01  0.00
     2      3   -0.20  0.00    0.00  0.00    0.00  0.00    0.01  0.00
     3      4   -0.22  0.00    0.00  0.00    0.00  0.00    0.01  0.00
     4      5   -0.25  0.00   -0.00  0.00   -0.00  0.00    0.01  0.00
     5      6   -0.29  0.00   -0.00  0.00    0.00  0.00    0.01  0.00
     6      7   -0.35  0.00   -0.00  0.00    0.00  0.00    0.01  0.00
     7      8   -0.46  0.00   -0.00  0.00    0.00  0.00    0.02  0.00
     8      9   -0.74  0.00    0.00  0.00    0.00  0.00    0.04  0.00
     9     10   -1.12  0.03 26835097.52 1029204.76    3.58  0.03    6.34
0.09

I think maybe it is due to the tabulated potential. Did anyone have similar
problem previously? The GMX version is 5.1.2. Thanks in advance,

Patrick