Temperature control in hybrid simulations (GCMC and NVT)

LAMMPS LAMMPS General Discussion
1

Hello all,

I am doing a simulation of polymers with considering water as a solvent. Firstly, I wanted to add water to the system of polymer chains; for that, I used widom’s insertion method to compute the chemical potential of water. After that, I am using fix gcmc with chemical potential found in widom’s method to insert water in the system along with the NVT ensemble.

Currently, I am facing two problems. 1) pressure control of the system. 2) temperature control of the system.

  1. To some extent, I have solved this problem using a very slow insert allowing inserted water to relax in the system. I am writing this problem to see if someone has a better suggestion I can use. I will be happy to hear that.
  2. I used coupling of a reservoir and system to have the same temperature as suggested in fix gcmc. However, I still face the same problem: the temperature goes very high in the slow or fast insertion.

I really appreciate your time and any suggestions to fix my simulation problems.

Thank you.

2

Hi @muzaffar_rafique,

Without more technical background it is kind of hard to give relevant insight on your issue. It would be useful to be sure that you use the last lammps version but the rest of your description is rather vague.

  1. How do you control the pressure and how do you do the insertion? Insertion of molecules in bulk material is tricky as you can find yourself inserting atoms very close to present atoms or locking a molecule on a polymer loop putting very high and unrealistic mechanical constraints that can’t relax. There are some techniques in the literature to bypass these issues but only you can decide if they suit your needs.

  2. Similarly, without any information on the way you control the temperature and how you insert your new molecules, there is no way to give meaningful comments on the reason your temperature increase and how to fix this.