[gmx-users] NAN at high temperatures and high pressures

[Popov Konstantin]

Hello,

I run simulations with polyethylene molecules with harmonic potential for C-H bonds and Morse potential for C-C bonds. The system consists of 1000 carbon atoms and 2002 hydrogen atoms. The simulation runs with PBC at constant density (~0.8 g cm-3) and no pressure coupling. At the temperature 2900 K about 25% of simulations end up with NAN (not a number) records in their .log and .gro files. Simulations for the same system at the same temperature in vacuum don't have this problem. Simulations run with the same density but lower temperature (2500 K) don't have this problem either.
Below is a fragment of a .log file when reasonable numbers turn to NANs:


           Step           Time         Lambda
          14780       14.78000        0.00000

   Energies (kJ/mol)
           Bond          Morse          Angle Ryckaert-Bell.        LJ (SR)
    2.62634e+04    1.84883e+04    5.57324e+04    6.74611e+03    5.36911e+03
   Coulomb (SR)      Potential    Kinetic En.   Total Energy    Temperature
    6.11740e+02    1.13211e+05    1.12145e+05    2.25356e+05    2.99729e+03
 Pressure (bar)
    1.49525e+04

           Step           Time         Lambda
          14790       14.79000        0.00000

Group rest with mass  1.40287e+04, Ekrot  8.82852e+00 Det(I) =  1.54313e+12
  COM:      1.56520       1.58669       1.58357
  P:       -0.00208       0.00121       0.00119
  V:       -0.00000       0.00000       0.00000
  J:      560.09625    -303.20575      13.91409
  w:        0.02418      -0.01353       0.00097
Inertia tensor (3x3):
   Inertia tensor[    0]={ 1.13658e+04, -2.64121e+02,  1.90793e+02}
   Inertia tensor[    1]={-2.64121e+02,  1.17820e+04, -2.87465e+02}
   Inertia tensor[    2]={ 1.90793e+02, -2.87465e+02,  1.15395e+04}
   Energies (kJ/mol)
           Bond          Morse          Angle Ryckaert-Bell.        LJ (SR)
    2.48452e+04    1.71570e+04    5.62121e+04    6.85057e+03    5.75860e+03
   Coulomb (SR)      Potential    Kinetic En.   Total Energy    Temperature
    6.03772e+02    1.11427e+05    1.13892e+05    2.25319e+05    3.04398e+03
 Pressure (bar)
    2.24204e+04

           Step           Time         Lambda
          14800       14.80000        0.00000

   Energies (kJ/mol)
           Bond          Morse          Angle Ryckaert-Bell.        LJ (SR)
            nan            nan            nan            nan    0.00000e+00
   Coulomb (SR)      Potential    Kinetic En.   Total Energy    Temperature
    0.00000e+00            nan            nan            nan            nan
 Pressure (bar)
            nan

Does anybody have any idea what might cause this?

Thanks

K. Popov