[CPMD-list] question about CUTOFF

[Axel Kohlmeyer]

On Sat, 23 Feb 2008, Mark Kosmowski wrote:

mark,

MK> I've been choosing cutoff by making an unoptimized single energy vs. cutoff
MK> energy plot (I'll usually use 25 Ry to 250 Ry in 25 Ry increments) and
MK> deciding how "flat" I want the curve to be at my cutoff of choice.  This is
MK> done knowing that the calculation as a whole will take longer to perform.

while this is a convenient way of checking for cutoffs and in most cases 
it will mean, that will err to the side of having too large a cutoff. 
but more importantly, i would still recommend to make 1-2 small checks 
w.r.t. the property of interest on your final system (or some simpler 
version of it) since the convergence behavior of different properties 
can be very different from the convergence of the total energy. as 
stated before forces (for MD and geometry optimization) converge in
general better. for ultrasoft pseudopotentials, one also has to consider
the relation between (wavefunction) CUTOFF and DENSITY CUTOFF (by 
default a factor of 4. see DUAL). in most cases good convergence 
and energy conservation with ultrasoft pseudpotentials require the 
density cutoff to be relatively larger, ranging from DUAL 5 up to 
DUAL 10 for a stress tensor calculation (if somebody would actually
fix the calculation of the stress tensor for USPPs in CPMD, that is).

MK> Since cutoff choice is based primarily on atomic species, I do not 
MK> repeat this analysis for a new species unless the new species 
MK> contains elements not previously examined in this fashion.

right.

MK> >From my limited experiences, if I am concerned about a 5 Ry cutoff
MK> difference, I am probably not on the "flat" enough part of the curve.
MK> 
MK> For the Goedecker pseudopotentials available from the CPMD website I 
MK> have been using a cutoff of 150 Ry as an acceptable compromise.  
MK> The systems I am looking at are small molecule solids.

as you have pointed out yourself. the choice depends on the atomic 
species in question. so while 150ry seems like a good guess for 
your systems there are goedecker type potentials that work well 
enough at 60ry and less and others that require over 200ry. so 
_any_ statement about using a "standard" or default cutoff shows 
at best a lot of experience, confidence or trust in other people's 
work, and at worst ignorace, i.e. that there were no checks made.

it always puzzles me, that while it is quite difficult to test for
basis set completeness and balance with atomic (gaussian) basis sets
and not to mention BSSE errors, people tend to do these tests, 
whereas with plane waves, where BSSE is not an issue and the test 
would be easy as you essentially only have a single parameter that 
can be systematically variied, people don't do this...

if anybody can clue me in on why this happens, i would
be very thankful as it would help enormously to address
this issue and provide better guidelines to people to 
use CPMD correctly. there is nothing worse than having
a powerful tool at hand and not using it right.

cheers,
   axel.


MK> 
MK> Good luck!
MK> 

-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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If you make something idiot-proof, the universe creates a better idiot.