[Pw_forum] Defining an antiferromagnetic graphene nanoribbon

[Axel Kohlmeyer]

On Sat, 10 May 2008, Roberto G. A. Veiga wrote:

RV> Dear Stefano:
RV> 

RV> I'm using Ultrasoft PPs. My question was because this cutoff (20 Ry) 
RV> seemed to be appropriate for other carbonic systems.

dear roberto,

as stefano mentioned, what is appropriate depends on the individual
potential(s) and the properties that you are iterested in. while 
20-30ry is the usual wfc range for uspps, they also tend to require 
a higher density cutoff than the default 1:4 ratio that is adequate
for norm-conserving pseudopotentials. there is no "automatically good"
values and a "good beyond any doubt" setting is usually a huge waste
of cpu time. there have been attempts to encode cutoff requirements
into pseudopotential files to make this whole process a little bit 
less of a guesswork for unexperienced users, but for the reasons
outlined above, it is very difficult to make a choice. it is one
of the items, where you just have to know what you are doing and
run the required tests by yourself... :-(

cheers,
   axel.

RV> 
RV> []s,
RV> 
RV> Roberto
RV> 
RV> Stefano Baroni <baroni at sissa.it> wrote: 
RV> On May 9, 2008, at 10:42 PM, Roberto G. A. Veiga wrote:
RV> 
RV> Hi, Konstantin:
RV> 
RV> thank you very much. Such a large cutoff (80 Ry) is necessary for convergence? Or maybe I'm using one quite small (20 Ry)...
RV> 
RV> it depends on the PP you are using, I guess, and certainly NOT on the actual structure you are studying. 80 Ry seems like an appropriate cutoff for moderately hard norm-conserving PP's, whereas 20 Ry sounds like appriate for many US PP's. In either case, you better check the aprporiateness by yourself, through thorough convergence tests.
RV> 
RV> 
RV> Stefano
RV> 
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RV> 
RV> []s,
RV> 
RV> Roberto
RV> 
RV> Konstantin Kudin <konstantin_kudin at yahoo.com> wrote:  Hi Roberto,
RV> 
RV>  Here is a ribbon input that works for sure:
RV> https://kiev.princeton.edu/misc/rg_07.scf.in
RV>  (get PPs from the PWSCF website)
RV> 
RV>  Good luck!
RV>  Kostya
RV>  ---
RV> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
RV> [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
RV> 
RV> 
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RV> 
RV> 
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RV>  
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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
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