Let's say you optimize an experimental geometry of a structure with very large basis sets, strict optimization settings with different XC functionals and after calculating phonons of this structure and you get imaginary frequencies. Does this mean that this structure not stable at all? If so, how...

I am writing a simple Lennard-Jones fluid simulation in LAMMPS. So far, I have outputted the temperature, volume, density, total energy, enthalpy, pressure, and all of it seems to be believable every 500 steps. This leads me to believe that my simulation is doing the right physics. thermo_style ...