[lammps-users] Using NPT and NVT at the same time on two different groups


I have a question about the use of NPT and NVT dynamics. My system consists of two different materials. One of them is silica, and it is already in its equilibrated form. The other is an organosilicate material but it is not in its equilibrated form. What I do is, I form a void inside the silica and fill it with the ingredients of the organosilicate material, in other words, I put the molecules that make up the organosilicate material inside silica in a randomly distributed manner, and try to anneal and condense the organosilicate network while it is inside silica. The goal is to observe the effects of the interaction between silica and the organosilicate molecules on the final structure of the organosilicate network, in other words, answer the question of what happens to the organosilicate network structure when it is formed under constraints.

In this simulation, I separated silica and organosilicate material as two different groups. I want silica to stay at room temperature while the organosilicate network is being formed inside. I want to decrease the temperature of organosilicate material slowly from 700 K to 300 K so that the connectivity is formed between the molecules of the organosilicate material. For this reason I ended up using two different NPTs for silica and organosilicate material. For silica temperature is at 300 K and for organosilicate material temperature decreases from 700 K to 300 K.

The simulations ran without issues and I was able to obtain results that somewhat match well with experimental results, but I still wanted to ask if this procedure is right. I searched through the e-mail list but wasn’t able to find an example that would help me. I’m sorry beforehand if I’ve missed anything.

Thank you so much!
Karsu Kilic

I dont think LAMMPS would have allowed you to use two different fix npt commands at the same time. You can use one fix npt command on a group of atoms and to control the overall box size.
You can use as many additional thermostats as you want (e.g. fix nvt) at the same time of different groups of atoms, in your case one additional group. It is fine to use either thermostat
to ramp down the temperature of those atoms for 700K to 300K.