[gmx-users] force

[Shuangxing Dai]

Hi, all,
    I was wondering about the force given by Gromacs. One is in the .trr  and .tpr file. When I read then with gmxdump, they are all zeros:

traj.trr frame 0:
   natoms=     24000  step=         0  time=0.0000000e+00  lambda=         0
   box (3x3):
      box[    0]={ 3.24900e+00,  0.00000e+00,  0.00000e+00}
      box[    1]={ 0.00000e+00,  5.62743e+00,  0.00000e+00}
      box[    2]={ 0.00000e+00,  0.00000e+00,  1.04120e+01}
   f (24000x3):
      f[    0]={ 0.00000e+00,  0.00000e+00,  0.00000e+00}
      f[    1]={ 0.00000e+00,  0.00000e+00,  0.00000e+00}
      f[    2]={ 0.00000e+00,  0.00000e+00,  0.00000e+00}
      f[    3]={ 0.00000e+00,  0.00000e+00,  0.00000e+00}
....

When I use mdrun -pforce 0 to get all the non-zero force, I get:
step 0  atom   4687  x    0.975    3.189    0.781  force  3.58158e+02
step 0  atom   4688  x    0.975    3.189    0.586  force  7.16316e+00
step 0  atom   4689  x    0.975    3.189    0.586  force  3.65324e+02
step 0  atom   4690  x    0.812    3.095    1.041  force  3.58167e+02
step 0  atom   4691  x    0.812    3.095    0.846  force  7.16314e+00
....

Why this happens? Thanks in advance.
Shuangxing Dai
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