# [lammps-users] How fix heat affect the temperature

Hi, all lammps-users
I’m using fix heat to simulate the ultrafast heating process of nanoparticle. Below is the setting of fix heat:

fix 21 all heat 1 1046.41
fix 22 all nph iso 0 0 1.0
run 15000
unfix 21
unfix 22
The time unit is ps, energy unit is eV, timestep is 0.001ps. Hence, fix heat add 1046.4115=15696.15eV energy to the nanoparticle. The nanoparticle contains 30855 atoms. According to the definition of temperature, the energy euqal to T=2KE/3/N/kB=2*15696.15/3/30855/kB=3936.95K. However, in my simulation, the temperature just increase ~1370K. Does anyone can tell me why the temperature increase is smaller to the theoretical calculation.
Any suggestion will be appreciated.

Jocab

when atoms move faster because of the added kinetic energy, they can come closer to each other and that means that their potential energy increases.
also, you are using fix nph (i hope your system is not an isolated NP but rather one embedded in a fluid or solid, otherwise fix nph does not make much sense) so energy can also be used for work associated with expanding the box.

bottom line. if you start a bulk system on its ideal lattice points, i.e. in the minimum potential energy, and then add kinetic energy, you can expect that about half the kinetic energy on average will be converted to potential energy. if your simulation settings result in good energy conservation and you use fix nve to do time integration and no thermostat method, then your total energy should be preserved (on average and modulo energy losses from using cutoffs in the pairwise interactions)

axel.