NpT - grid GPW

[simin pahlavi]

Hello Matt,
Thank you for your helpful comment.
I have a question and I appreciate it if you reply me with the answer.
If during the NPT simulation, The box length and volume decreases, one 
should set the CELL_REF to the final small box length? or just this 
CELL_REF works in the situation  where the Box length increases during the 
simulation?
Thanks in advance 
Simin
On Saturday, June 23, 2012 at 8:10:39 AM UTC+2, Matt McGrath wrote:
>
> Hi Marc.
>
> The default method is to use a constant grid density, using grid sizes 
> that correspond to the "magic" numbers for FFTs.  You can change this by 
> putting in the CELL_REF flag, e.g
>
>     &CELL
>       ABC 17.28  17.28  17.28
>       &CELL_REF
>         ABC 18.0 18.0 18.0
>       &END CELL_REF
>     &END CELL
>
> Now the cell will always have the same number of grid points as a cell of 
> size 18x18x18 using the default grid density, regardless of the actual size 
> of the cell.
>
> You probably don't have to use a cutoff of 1200 Ry.  In later studies, we 
> use a cutoff of 600 Ry (you can look up the papers by McGrath, Kuo, and 
> Siepmann), and it seems to work.  Of course, you should also do some tests 
> for your system.  We reported I think in at least one paper that increasing 
> the cutoff (or, at least, using a more converged basis set) can actually 
> push the result away from the experimental properties.  This is a 
> well-known probably that arises from the lack of a variational principle in 
> DFT once a form for the exchange-correlation functional has been decided 
> on, though I caution you that I'm not an expert in this.  I wasn't an 
> author of the paper you cite, so I'm not sure why they chose 700.
>
> Good luck!
>
>                                                  Cheers, Matt
>
>
> On Friday, June 22, 2012 5:31:29 PM UTC+9, marc wrote:
>>
>> Dear all,
>>
>> I have a question concerning the GPW method, more specific about how the 
>> regular grid used in the computation of the electronic density, is 
>> generated.  In the following reference:
>> ChemPhysChem, 2005, 6, 1894
>> it is found that when using an (almost) constant density for the grid 
>> points, one should take the cut-off high enough (i.e. a 1200 Ry) to avoid 
>> discontinuities in the potential energy vs. volume curve.  However, when 
>> employing a constant number of grid points, by making use of the reference 
>> cell method, these jumps can be avoided (but in the paper the authors use 
>> also the very high 1200 Ry-cutoff in this method). 
>> Knowing this, I have two questions about this:
>> i) what is the 'default'-method the generate the grid in CP2K: constant 
>> density of points or constant number of points?
>> ii) if the default-method is to generate a constant number of points, is 
>> it then also necessary to take such a big cut-off of 1200 Ry? Or is it then 
>> good enough to go a bit smaller? In this paper: J. Phys. Chem. B, 2009, 
>> 113, 11959, the authors take 700 Ry for the reference cell method, but why 
>> exactly this value? It is 500 Ry less then the proposed 1200 Ry as it was 
>> proposed in the first reference...
>>
>> many thanks
>> marc
>>
>
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