[CP2K:4792] testing on simulation cell

[Huan Wang]

Dear Marcella,

Thank you very much for you suggestion. I will read these literatures
carefully.

Best regards,
Huan




On Tue, Oct 29, 2013 at 7:59 AM, Marcella Iannuzzi <marc... at pci.uzh.ch>wrote:

>
> Dear Huan,
>
> I think you should not use methods blindly, and read first the related
> literature.
> In the specific case, you should read more on the Martyna-Tuckerman
> Poisson solver and more on the GPW method,
> in order to understand how to prepare your setup for the simulations.
> The correct references are even given in the cp2k reference manual:
> Martyna, GJ; Tuckerman, ME. JOURNAL OF CHEMICAL PHYSICS, 110 (6),
> 2810-2821 (1999). <http://dx.doi.org/10.1063/1.477923>
> A reciprocal space based method for treating long range interactions in ab
> initio and force-field-based calculations in clusters.
>
> VandeVondele, J; Krack, M; Mohamed, F; Parrinello, M; Chassaing, T;
> Hutter, J.COMPUTER PHYSICS COMMUNICATIONS, 167 (2), 103-128 (2005).<http://dx.doi.org/10.1016/j.cpc.2004.12.014>
> QUICKSTEP: Fast and accurate density functional calculations using a mixed
> Gaussian and plane waves approach.
>
> In particular,  there you can find how to choose the cell size for the MT
> poisson solver,
> and how multi grids and relative cutoff are used within the GPW scheme.
>
> regards
> Marcella
>
>
> The structure of my benzene-water cluster was optimized by Gaussian09 and
> can be put into a cell with size as 5*5*5. I tried 7*7*7, 10*10*10,
> 15*15*15 and 20*20*20 cells for NVE calculation in CP2K with PBE
> functional. However, only cell with 20*20*20 can maintain the structure of
> benzene-water cluster. For the rest of results with smaller cell, all the
> structures break due to the dramatic increasing of temperature from 50 K to
> 10000 K after 30 steps.
>
> I have no idea about these testing results. Why could not use smaller cell?
>
> I consulted the CP2K reference and found a description on &MT section. *Sets
> up parameters of Martyna-Tuckerman poisson solver. Note that exact results
> are only guaranteed if the unit cell is twice as large as charge density
> (and serious artefacts can result if the cell is much smaller).
> *
>
>
> In addition, I noticed that the REL_CUTOFF value is related to the &MGRID
> section. *Specify the multiplicative factor for the CUTOFF keyword in
> MULTI_GRID section. The result gives the cutoff at which the 1/r
> non-periodic FFT3D is evaluated.Default is 2.0*
> *
> ***However, both of these lines do not have any detail explanations or
> examples. The CP2K reference is really too poor for a beginner to learn how
> to use CP2K, comparing the Gaussian reference. I do not understand what is
> the meaning of "if the cell is twice as large as charge density". How to
> check the charge density of my system? And also, is there any rule of thumb
> to choose the value of REL_CUTOFF?
>
> Best regards,
> Huan
>
>
>
>
> **
>
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