[gmx-users] Pres-XX and Pres-YY difference

[Johnny Lu]

Hi. The following seems a bit strange to me. It was extracted from the edr
file with g_energy of mixed precision gromacs 4.6.7.

Statistics over 31921501 steps [ 0.0000 through 31921.5000 ps ], 9 data sets
All statistics are over 319216 points

Energy                      Average   Err.Est.       RMSD  Tot-Drift
-------------------------------------------------------------------------------
Pres-XX                     1.00863       0.23    222.874 0.00505598  (bar)
Pres-XY                   -0.218743       0.33    138.143    2.17256  (bar)
Pres-XZ                   -0.233072       0.33    137.629    1.36597  (bar)
Pres-YX                   -0.262129       0.32    138.137    2.12662  (bar)
Pres-YY                    0.788658       0.23     225.34   0.928867  (bar)
Pres-YZ                  -0.0207522       0.23    138.913    1.08012  (bar)
Pres-ZX                   -0.276173       0.33    137.619    1.40069  (bar)
Pres-ZY                   -0.098337       0.23    138.913    1.14865  (bar)
Pres-ZZ                     0.60576       0.38    222.687  -0.259413  (bar)

I don't know the meaning of Pres-XX. But I guess it means the pressure at
some direction (some kind of tensor.).

Why the pressure at XX direction is different from other direction?
I thought Pres-XX should be same as Pres-YY and Pres-ZZ.

The box is a truncated dodecahedron, the barostat was set to 1 bar, and the
simulation was ran with mixed precision gromacs 4.6.7 with Parrinello-Rahman
barostat for NPT simulation of about 300 ns before this stretch of NPT
simulation.

Thank you again.