[CP2K:7622] How to choose the NGRIDS in &MGRID

[Huan Wang]

Dear Prof. Hutter,

Thank you very much for your reply and instruction.

In the second step, do you mean increasing the grids might spend more time
on the FFT calculation?

Regards,

Huan
.

On Tue, Mar 29, 2016 at 11:33 AM, <hut... at chem.uzh.ch> wrote:

> Hi
>
> the three inputs "CUTOFF", "NGRIDS", and "REL_CUTOFF" are important
> for accuracy and efficiency. The problem is that both, "accuracy and
> efficiency" depend on the combination of chosen values.
>
> In an optimal setting you can proceed the following way:
>
> 1) choose a high enough value for REL_CUTOFF
>    this is system and basis set independent. It determines the
>    minimal cutoff used to bring a Gaussian function on the real space grid.
>    High values mean high accuracy, low values faster execution.
>
> 2) choose your cutoff and number of grids. This will determine the real
> space grids
>    for your calulcation, e.g. cutoff=400 ngrids=4
>    grid1 = 400, grid2 = 200, grid3 =100, grid4 = 50 (not actual numbers)
>    The cutoff should be high enough to take on the Gaussian with the
> largest exponent,
>    i.e. exponent*REL_CUTOFF <= cutoff (where exponent is for a product
> Gaussian)
>    If you choose more grids the mapping (Gaussian->Grid) is better but you
> have more
>    overhead from more grids (mostly FFTs).
>
> regards
>
> Juerg Hutter
>
> --------------------------------------------------------------
> Juerg Hutter                         Phone : ++41 44 635 4491
> Institut für Chemie C                FAX   : ++41 44 635 6838
> Universität Zürich                   E-mail: hut... at chem.uzh.ch
> Winterthurerstrasse 190
> CH-8057 Zürich, Switzerland
> ---------------------------------------------------------------
>
> -----cp... at googlegroups.com wrote: -----To: cp2k <cp... at googlegroups.com>
> From: huan... at mail.huji.ac.il
> Sent by: cp... at googlegroups.com
> Date: 03/27/2016 10:16PM
> Subject: [CP2K:7621] How to choose the NGRIDS in &MGRID
>
> Dear CP2K developers and users,
> I found a usage hint in the BASIS_MOLOPT file."NGRIDS 5" (section &MGRID)
> will deal more efficiently (2x speedup) with the diffuse nature of the
> basis.Whereas in the CP2K manual, the default value of the NGRIDS is 4.
> My question is that is there any rule of thumb for choosing the NGRIDS?For
> example, lager basis set needs lager value of NGRIDS? Or how many Gaussian
> functions mapped on each grid would be efficient and accurate for the
> simulations?
> I noticed that the CP2K website gave an example on setting the CUTOFF and
> REL_CUTOFF, https://www.cp2k.org/howto:converging_cutoffwhich might
> related to the NGRIDS.
> However, the descriptions made me confused. It might lead to another
> question.
> In the introduction, it said:If CUTOFF is too low, then all grids will be
> coarse and the calculation may become inaccurate.
> While at the end of "Converging CUTOFF" section, it goes,The reader may
> also notice that as CUTOFF increases, the number of Gaussians being
> assigned to the finest grids decreases.
>
> It looks like the "NG on grid 1" is the finest grid, since as the
> description of the above sentence, the number of Gaussians decreased
> significantly on grid 1.However, this would be contrary to the
> introduction. Because the introduction indicates that the lower CUTOFF, the
> coarser the grid. As a result, the higher CUTOFF should lead to finer grid.
> But the example showed that increasing the CUTOFF pushed the Gaussian to
> the coarse grid.
> I appreciate it if anyone could correct me if I am wrong.
> Regards,Huan.
>
>
>
>
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