drift of center of mass

[Wei Lai]

Matt,

Thanks for the suggestions.  Examining the force noise suggests that higher 
cutoff values are needed.  Increasing the cutoff values does stabilize the 
center of mass, although the statistics of fluctuation are different from 
those from VASP.  But I guess this is probably due to different types of 
basis sets.

Wei

On Thursday, January 5, 2017 at 10:21:24 AM UTC-5, Matt W wrote:
>
> 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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