[gmx-users] Frozen groups lead to a high energy

[Sam David]

Dear gromacs users
I'm simulation a hydrocarbon and water mixture placed on a slab of metal.
The metal surface is frozen in the z-direction. Energy minimization works
with no errors but I see the potential energy is too high as below.

Steepest Descents converged to machine precision in 1754 steps,
but did not reach the requested Fmax < 1.
Potential Energy  =  1.9604517e+08
Maximum force     =  3.1745122e+03 on atom 23433
Norm of force     =  3.8078518e+01

After energy minimization, I did short nvt followed by short npt then npt
production. I tried several times and different parameters to reduce the
energy of the system but still very high as below:

   Energies (kJ/mol)
           Bond          Angle    Proper Dih. Ryckaert-Bell.  Improper Dih.
    2.07807e+08    2.96662e+05    8.57456e+02    2.50326e+04    1.60567e+02
          LJ-14     Coulomb-14        LJ (SR)  Disper. corr.   Coulomb (SR)
    4.33776e+04   -7.73429e+03    3.05165e+11   -1.07225e+04   -1.24055e+07
   Coul. recip.      Potential    Kinetic En.   Total Energy    Temperature
    5.93613e+03    3.05360e+11    1.97149e+05    3.05361e+11    3.88708e+02
 Pres. DC (bar) Pressure (bar)   Constr. rmsd
   -2.71515e+02    3.08812e+10    4.72857e-06

However, when I remove frozen groups, potential energy becomes negative

Any ideas on how to reduce system energy while using freeze groups option!


Thank you