[lammps-users] Melting Quartz to obtain amorphous silica

I am just wondering. By doing this... Should the density of the final amorphous phase automatically adjust itself to
that of amorphous silica?

Not necessarily. That assumes the force field is "correct" which none really are. And it assumes you've run the simulation for long enough to get to amorphous silica, which may not be computationally reasonable.

I mean I am starting from a quartz phase with a density of about 2.6. Should that density naturally drop to about 2.2
for amorphous silica after the quench in NPH or is it not that straightforward at all?

When using NPH, ideally, yes, it should approach the density of amorphous silica. You might need to do a slower quench, or use a "better" force field if any exist. Or maybe go back to playing with those damping constants ...

Should I rather start with a crystalline phase of density already modified to 2.2 to get closer to the real amorphous
phase density?

There's no guarantee that that would help, but you could certainly try it. I doubt that it would help much.

what would be more realistic you think?

I think that what you're already using as a starting point should be good enough.