[Octopus-users] Potential Energy fluctuation during Ehrenfest-TDDFT MD simulation

[Yasumitsu Suzuki]

Dear Matthieu,

Thank you very much for your advice.
Now, I tested several situations of Ehrenfest-TDDFT molecular dynamics 
simulations, and I found the bellow things:

(1) When I set  the center of mass of oxygen molecule at  the center of 
simulation box initially, i. e.,
   %Coordinates
    "O"  |  0   |  1.1397 |  0
    "O"  |  0   | -1.1397 |  0
   %
 potential energies fluctuated slightly during the simulation. (I think 
the total energy was conserved enough.)
And I found this fluctuation corresponds to the stretching vibration of O2.
And the center of mass did not move from (0,0,0).

(2) When I set the center of mass of O2 at (0,0, 3.78) initially, O2 
started the translational motion toward the center of simulation box 
(0,0,0).
I cannot understand the reason of this!
When I changed the size of box from (Lsize=13.0 Bohr) to (Lsize=15.0 
Bohr), this  translational motion still occurred with the same velocity.

(3) It was found that when a molecule moves translationally, potential 
energies fluctuated largely.

(4) I think that the reason of this fluctuation derives from the 
difference of the time scale between nuclear and electronic movement..?
 
(5) Potential energies fluctuated around a certain value but never 
diverged when molecule moves translationally.
So is it okay to consider the average value of these as the true 
potential energy?

Please give me some comments about this matter.
Please help me.

Sincerely,

Yasumitsu Suzuki


> It looks like your kinetic energy peaks when the center of mass has
> largest speed, whereas the O2 vibrations probably induce a much
> smaller oscillation of the total energy (need to zoom in a bit).
>
> You will need to test the parameters yourself, to know if the box is
> large enough and time step small enough. How about 1 atom? There is
> even less reason to move the center of mass, there. Is the COM
> initially at 0,0,0?
>
> Matthieu
>
>