[gmx-users] md of only water - obtaining different pressure in the x , y and z directions

[Diez Fernandez, Amanda]

Hi,

I am running a simulation which contains only water (using the spc216.gro coordinates and TIP3P model) in an NPT ensemble.
 I get different values for the average pressure in the x y and z directions.
I am running the simulation for up to 10s in a 4nm side cube unit cell.
I am using a Nose Hoover thermostat and a Parrinello-Rahman barostat applied isotropically, with target pressure 1 bar.

The averages after 1ns are:


Energy                      Average   Err.Est.       RMSD  Tot-Drift

-------------------------------------------------------------------------------

Temperature                 299.999    0.00098    6.07906 0.00504098  (K)

Pressure                     1.3174      0.066    350.061   -0.14372  (bar)

Box-X                        4.0396    0.00013 0.00874048 -5.7234e-06  (nm)

Box-Y                        4.0396    0.00013 0.00874048 -5.7234e-06  (nm)

Box-Z                        4.0396    0.00013 0.00874048 -5.7234e-06  (nm)

Pres-XX                     1.11668          2     485.28   0.288761  (bar)

Pres-YY                   -0.762405          1    427.866    -6.2632  (bar)

Pres-ZZ                     3.59792        2.4    410.546    5.54328  (bar)


The averages over the 10 ns are :


Energy                      Average   Err.Est.       RMSD  Tot-Drift

-------------------------------------------------------------------------------

Temperature                     300      8e-05    4.00951 0.000535848  (K)

Pressure                    1.22566     0.0067    275.993 -0.0289005  (bar)

Box-X                       4.03929    5.1e-05 0.00823878 0.000126042  (nm)

Box-Y                       4.03929    5.1e-05 0.00823878 0.000126042  (nm)

Box-Z                       4.03929    5.1e-05 0.00823878 0.000126042  (nm)

Pres-XX                     1.29387       0.73    385.989  0.0898956  (bar)

Pres-YY                     1.35691       0.74    378.841    3.13618  (bar)

Pres-ZZ                     1.02621       0.84    377.022   -3.31278  (bar)



Why is the average pressure in the in each direction different?

And why is the drift of the XX direction so much lower than in the YY and ZZ directions?


Many thanks!