Why is energy not constant in an NVE VASP MD simulation?

[ccrook]

I have a VASP MD simulation of a 2x2x1 supercell of Al2O3 totaling 120 atoms. The supercell was initially relaxed and then run for 1500 time steps (0.1 fs time step, 1e-7 EDIFF) in the NVE ensemble (MDALGO=1, ANDERSEN_PROB=0). Velocities were initialized to 500 K (TEBEG=500). As a sanity check, I ran the same MD simulation with TEBEG=0 and the energy does remain constant. I'm struggling to understand why there is an initial jump in the energy. My intuition is that the energy should be more or less constant as in classical MD. Is there a reason for this?

FPMD NVE Al2O3.PNG

[andreas.singraber]

Hello!

Which quantities are you showing in your plot? The OUTCAR file contains a section

Code: Select all

ENERGY OF THE ELECTRON-ION-THERMOSTAT SYSTEM (eV)

for each time step where you will find values for TOTEN, EKIN (kinetic energy) and ETOTAL. The conserved energy is the total energy ETOTAL = TOTEN + EKIN. Please correct me if I am wrong but I suspect that your plot is showing TOTEN, i.e. the potential energy, only. Then the initial jump is due to the fact that your system is not equilibrated at the temperature you have set via TEBEG.

[ccrook]

Ah yes!!! Thank you. I've only been doing static relaxations up until now so I overlooked that.