[gmx-users] g_sas problem

[tangxuan]

Dear,all
When I calculate the accessible surface area(ASA) of interaction region
between two subunits A and B in my protein. First I calculated the ASA
of A and B by "g_sas -f .xtc -s .tpr -n index.ndx -o .xvg -n index.ndx"
respectively. After that, I combined A and B as the one group, then I
computed SAS of group A+B in the same way above. When I calculated the
hydrophobic SAS of interaction region between A and B by formula
(SAS=SAS(A)+SAS(B)-SAS(A+B)), at some specific time this result is
negative(the third ps in the result below). But in my opinion, this area
should be positive. What is wrong? I can show you the result of the
first 3 ps.
A:
         0     136.923     81.5686     218.491           0
         1     136.149     87.5994     223.748           0
         2     136.607     90.9957     227.603           0
B:      
         0     129.967     85.1524     215.119           0
         1     140.375      86.953     227.328           0
         2     138.37     90.9806      229.35           0
A+B:      
         0     264.215     169.394     433.608           0
         1     274.129     176.945     451.074           0
         2     276.924     180.027     456.951           0

Look forward to your explanation.

Thank you,

Tang Jiaowei