[CP2K:8263] Cutoff distance question

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

Hi

1) Use 0.5*Cell_size unless you have very large cells.
2) vdW pair potentials can correctly use multiple cells.
   There is no problem in having a cutoff >> Cell_size.
   The long range correction should only be used for nearly isotropic systems.
3) vdW non_local calculates the interaction potential using FFTs,
   including long range corrections.

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
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-----cp... at googlegroups.com wrote: -----To: cp2k <cp... at googlegroups.com>
From: Yiyang 
Sent by: cp... at googlegroups.com
Date: 10/17/2016 11:02PM
Subject: [CP2K:8263] Cutoff distance question

Hello,
While reading the page demonstrating the Hartree-Fock exchange:
https://www.cp2k.org/exercises:2015_pitt:hfx

and reading the 2009 paper of Guidon, I came up with a few questions:

1. How can we determine the CUTOFF_RADIUS of the truncated electrostatic potential?    In [2009 Guidon] it is suggested that CUTOFF_RADIUS < 0.5*Cell_size    guarantees no spurious self-exchange parts enter the gamma-point calculation.    Does this mean we should always use 0.5*Cell_size when we are not using long range correction?    Because [2009 Guidon] also reported that accurate HF energy is obtained more easily without long range correction.

2. For the dispersion correction, the VDW_POTENTIAL section, if we use type PAIR_POTENTIAL, then    we need to specify the R_CUTOFF for the pair potential.     As shown in the example script (https://www.cp2k.org/exercises:2015_pitt:hfx):    R_CUTOFF [angstrom] 16    so the cutoff used is 16*2 =32 A.    However the cell size is    ABC [angstrom] 12.42 12.42 12.42    I am not sure why we can use the cutoff much bigger than the cell size.    Is there any rule about setting this cutoff distance?

3. Also in the VDW_POTENTIAL section, using type PAIR_POTENTIAL we have option LONG_RANGE_CORRECTION.    However, using type NON_LOCAL there is no option for long range correction.    Is the correction built in the KERNAL_FILE?     
Thank you for your time.
Regard,Yiyang



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