[lammps-users] A simple example water MD input?

Dear Dr. David Schall,

Thanks for your example, your work helps me a lot, especially it suggested me that we can write a code to replicate molecules into any array we can imagine. I had roughly calculated the heat capacity of tip4p water based on the fluctuation of energy, I got 4.34 J/gK (exp. cv= 4.18 J/gK).
Since ice has a hexagonal structure, I wonder if I run NPT for water with a cubic structure at low temperature, say 200K, will it comes to hexagonal structure eventually?
Thank you very much.



In principle you should be able to do this. You’ll want to read up on how to do an npt simulation with a triclinic periodic cell because your system will want to change shape when it goes from cubic to hexagonal. I have done a similar simulation where I turned diamond into graphite. In practice, it may be difficult (I’m just guessing) to get the phase change for water, depending on how far from the correct structure you are. Since the temperature is relatively low, you may get stuck in a metastable state, or at best end up with a very defected structure. The best bet for examining the phase you are interested in is to generate the phase you are interested from scratch before you try to run MD.

Also, I would be interested in your input files for generating the heat capacity. How long did it take to converge to your answer? I have tried Green Kubo methods for thermal conductivity of tip4p liquid water and could never get the right answer. I’m just curious to see what you did for heat capacity.