[CP2K:4755] testing on simulation cell

[Marcella Iannuzzi]

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).
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).
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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