I am trying to find the melting point of gas hydrates at a certain pressure (solid made from water+gas). How can I do it using LAMMPS? One way to do it is to try different temperatures in NPT until I find a narrow range of T in which the solid melts, but I think there should be an automated method to find the Tmelt. Any suggestions?
Since you are doing it in NPT, the density will change as a function of temperature. Maybe, change the temperature step-wise and check for density (or simulation box volume) as a function of temperature. That should give you an idea of melting point.
To automate the process, you could set up a loop where the temperature is first increased in a certain interval, then equilibrated for another interval, and the cycle repeated to explore a given range. However, if you run that simulation in reverse (i.e. cooling down the melt or two-phase system), you will likely find a different condensation temperature due to hysteresis effects.
This protocol should give you an upper and lower bound for the transition temperature. Then you will need to explore this interval with a different method, such as the co-existence method at fixed volume or similar. Look carefully the literature for MD studies on phase transition temperatures, it’s a classic.