I'm trying to use LAMMPS to perform a relaxation of a structure and find its minimum energy configuration. My plan of attack was to allow the system to run for a reasonably long time with constant volume and energy (fix/nve) and then, once the temperature appears relatively stable), begin removing kinetic energy until the temperature drops to near 0 K.
To reduce the kinetic energy, I wanted to use fix/nvt. However, experimenting with this command has left me confused. It seems that the Tdamp parameter specifies how long the temperature ramp should take. I tried (metal units, so time is in ps):
fix 1 all nvt 0.0001 100.0 10.0
10.0 ps is 10,000 steps at 0.001 ps/step. However, it isn't until about step 79000 in my 8-atom Silicon system that the temperature reaches the correct order of magnitude (it hits 62 K at that step, then shoots up into the thousands for a few thousand iteration and rapidly drops back down to near 0.)
It's as if it's taking 10X as long as it should, roughly. Am I misunderstanding something here?