[CP2K:7621] How to choose the NGRIDS in &MGRID

[hut... at chem.uzh.ch]

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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