[lammps-users] water simulation

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1

Dear lammps users

does anyone of you have experience in setting up a water simulation using npt or press/berendsen? NPT works but does not lead to an decay in the pressure i.e. it fluctuates between -500 and +500 bar (also after a run of 1ns) as in an NVT ensemble at room temperature without pressure calibration. The press/berendsen however, interrupts always immediately with an “Out of range atoms - cannot compute PPPM” error right at the beginning of the simulation. I also tried to run first a NVT sim. and then use the restart file for press/berendsen to have a more feasible particle configuration, but with the same result. I used the attached input script and a starting configuration of water molecules on an periodic cubic fcc lattice (box length 29.6 A for 864 molecules).

Hope anyone can help me

Thank you

Sabine

in.wasser_npt (674 Bytes)

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The error is happening b/c atoms are moving too far between reneighborings.
If your box size is changing rapidly and dramtically due to NPT oscillations
then that could cause this problem. Reneighbor more often (until equilibrated)
or use the drag param in fix npt to damp volume oscillations.

Steve