drift of center of mass

Matt W MattWa... at gmail.com
Thu Jan 5 16:21:24 CET 2017


Dear Wei,

this drift indicates that you are not calculating forces accurately enough. 
You can see forces in the output file by adding 
https://manual.cp2k.org/cp2k-4_1-branch/CP2K_INPUT/FORCE_EVAL/PRINT/FORCES.html

Possible solutions are:

(i) use the GAPW method (add METHOD GAPW in the &QS section) - might be 
slower
(ii) Increase the CUTOFF - try 600 or 800 ry, at some point GAPW might be 
better
(iii) Use smoothing on the XC function
(iv) just subtract the COM motion to remove the drift (
https://manual.cp2k.org/cp2k-4_1-branch/CP2K_INPUT/MOTION/MD.html#list_COMVEL_TOL
)

Matt

On Wednesday, January 4, 2017 at 1:29:39 AM UTC, Wei Lai wrote:
>
> Dear cp2k users and developers,
>
>  
>
> I am testing a NVE simulation with velocity scaling for crystalline CaO 
> (300 K) and having questions regarding the drift of center of mass.
>
>  
>
> In a regular NVE simulation, I am expecting that (1) the simulation box is 
> fixed and positions of atoms can be in fractional/scaled or Cartesian 
> units, (2) if atoms moved out of the simulation box, they can be folded 
> back to the box or left unchanged, (3) the mass-averaged position centers 
> oscillate around x, y, z direction.  I am showing the center of mass 
> positions from VASP, which are consistent with the expectation.
>
>
>
> <https://lh3.googleusercontent.com/-tRDdhXVYalE/WGxP2ScXNBI/AAAAAAAAhnk/8bGLtoxRwxINxHoXywkIJFBPOkR4IipMwCLcB/s1600/cp2k-vs-VASP.TIF>
>
>
>
>  
>
> If I do the same for cp2k (input file enclosed), I am clearly seeing drift 
> of center of mass.  If I analyze the dynamics of the trajectory (now 
> shown), it appears that both Ca and O atoms are diffusing instead of 
> vibrating, as obtained from VASP trajectory.
>
>  
>
> I am wondering if you have seen similar behaviors or if I have the wrong 
> settings in the input file.
>
>  
>
> Thanks, Wei
>
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