LJ potential temperature

I am using metal units wit LJ potential.
How to setup temperature of simulation to 850 kelvin?

you can assign a suitable initial kinetic energy (i.e. temperature) with the velocity command and/or apply a thermostat algorithm in a fix (either combined with time integration or as a separate fix command). depending on the initial state of the system and when using correct simulation and force field parameters, the system will eventually equilibrate into the desired target temperature. if not, you have to correct your settings.