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

Question on input files/tags, interpreting output, etc.

Please check whether the answer to your question is given in the VASP online manual or has been discussed in this forum previously!

Moderators: Global Moderator, Moderator

Post Reply
Message
Author
ccrook
Newbie
Newbie
Posts: 3
Joined: Tue May 18, 2021 6:24 pm

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

#1 Post by ccrook » Wed May 19, 2021 6:36 pm

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
You do not have the required permissions to view the files attached to this post.

andreas.singraber
Global Moderator
Global Moderator
Posts: 231
Joined: Mon Apr 26, 2021 7:40 am

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

#2 Post by andreas.singraber » Thu May 20, 2021 4:44 pm

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
Newbie
Newbie
Posts: 3
Joined: Tue May 18, 2021 6:24 pm

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

#3 Post by ccrook » Thu May 20, 2021 6:23 pm

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

Post Reply