Freezing of SPC water

Dear LAMMPS users,

I have tried to run a simulation of a water box where water molecules are defined with a Flexible SPC model.

Initially, I ran with 0.5 fs under NVT for 500 ps and then continued simulation under NPT for another 500 ps.

Then, I increased the timestep to 1 fs and continued simulation under NPT for 20 ns more.

Surprisingly, I found that water molecules are getting frozen after 6-7 ns.

I must have been making some mistake which made my water freeze.
As I am new user I am not allowed to attach script files in this forum.

Your suggestions will be highly appreciated.


Could it be an occurrence of the flying ice cube effect ? Are you using the Berendsen thermostat? If so, you may want to consider another thermostat, or/and try to cancel any center of mass motion that may occur.

You can still quote the relevant sections of your input files and read the forum guidelines. Also, knowing which temperature and units were used would be useful.

A time step of 1fs for flexible water is bound to give wrong results, even 0.5fs is “ambitious”. The virbrational modes of the OH bonds are too fast to be accurately represented by such larger timesteps.