Equilibration of a liquid/solid system

Dear LAMMPS users and developers,

I am new to LAMMPS and I have one quick question:

I am studying the interaction between a liquid molecule and a solid plate. To equilibrate the system, I did the following:

  • I equilibrated the solid plate alone by fixing the NVT ensemble at 300K,
  • then, I equilibrated the liquid molecule alone by fixing the NVT ensemble at 300K,
  • then, I put the solid plate and the liquid molecule together in the same simulation separated by a given distance.

My question is: Do I need to re-equilibrate the whole system again ? or is the equilibration of the 2 subsystems separately enough ?

Thank you for your help and sorry if this question is more about the concept of equilibration itself rather than about LAMMPS.

Best regards,

Yes, you need to equilibrate some more. In fact, whenever you manipulate your system you move it away from equilibrium and thus need to equilibrate until it is back and you can collect data for analysis.

Using pre-equilibrated parts can cut down on the time required, but it can also be a viable and effective approach to first build the full system and then apply minimization only to subgroups (by setting the force on all other atoms to zero with fix setforce) or equilibrate only subgroups by applying the time integration fix to only subgroups.

Which of the two approaches is more effective depends on how strongly the subsystems interact and what changes were made. If you pre-equilibrated with full periodic boundary conditions or the subsystems are interacting strongly, the alternative approach is usually more efficient.

Thank you so much for your prompt and detailed answer.