QM/MM Mulitpole Rcut

[mayank...@gmail.com]

Hi,

Are there any guidelines to set the RCUT parameter in the QM/MM Multipole section?

https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/QMMM/PERIODIC/MULTIPOLE.html  

Is the RCUT parameter here is equivalent to the FIST EWALD RCUT? In my case I have a larger MM box of 50*40*40 angstroms^3 and a smaller centered QM box of 40*30*30 angstroms^3, so in this case, the Multipole RCUT should be set to 19 angstroms (<R_min/2) or it is okay to set RCUT as same as FIST EWALD RCUT (~9-12 angstroms.)?

Best Regards,

Mayank Dodia

[mayank...@gmail.com]

Hi,

Sorry for reposting, but is there someone who can elaborate on the RCUT parameter in the QM/MM Multipole section?

https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/QMMM/PERIODIC/MULTIPOLE.html    

Best Regards,

Mayank Dodia

[Thomas Kühne]

Dear Mayank, 

it is an Ewald cutoff, i.e. for large cells values 

smaller than half the cell size is advised. 

Cheers, 

Thomas

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[Thomas Kühne]

Dear Mayank, 

the keyword EWALD_PRECISION in the same section already controls the real-space cutoff, 

i.e. RCUT is only necessary to explicitly override the override the value and as such only useful 

when you know what you are doing. 

Best, 

Thomas

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==============================

Thomas D. Kühne

Dynamics of Condensed Matter

Chair of Theoretical Chemistry

University of Paderborn

Warburger Str. 100

D-33098 Paderborn

Germany

tdku...@mail.upb.de

+49/(0)5251/60-5726

[mayank...@gmail.com]

Dear Thomas,

Thanks for your response!

My reasoning for a cutoff much lower than the half box size arises from these two papers:

Yonetani, JCP, 2006:  https://doi.org/10.1063/1.2198208

Spoel, Marren, JCTC,  2006: https://doi.org/10.1021/ct0502256

where it is suggested that the larger cutoff does not implies accuracy and can create "artifacts" in bulk water simulations, particularly if the force field was not parameterized for the larger ranges. I don't have experience in simulating larger bulk water box (such as the one I mentioned before), but does this reasoning makes sense or is there alternative explanation for this? Do you have any particular recommendations in such scenarios? Not specifically for QM/MM, but for FIST calculations as well.

Best Regards,

Mayank Dodia