[CPMD-list] VDW / molecular crystals !!

[Axel Kohlmeyer]

On Thu, 27 Dec 2007, Belabbas Imad wrote:

BI> Dear CPMD users,

dear imad,

BI> I am currently trying to perform calculations with Van der Waals 
BI> correction on some molecular crystals.

BI> I am just wondering if the VDW correction is implemented in CPMD for 
BI> crystals (Where the point group is fixed and multiple k-points 
BI> sampling is involved !!!) ???

it should be. you can test this in a very simple way:

- set up a (small) system where you do a single point 
calculation (with forces) using the gamma point code and
then compare to kpoints with a 1x1x1 unshifted grid (i.e.
also only gamma). it should give the same result (with
little numerical differences).

- do the same calculations with different k-point grids
  with and without the VDW correction off and you should 
  get the same differece in the total energy (and forces).

one more comment: if you have rather small unit cells
(or else you would not need k-points in the first place),
you have to pay attention to the TESR parameter and the
equivalent VDW-CELL parameter(s), so that you have a 
reasonable distance before you cut off the realspace
coulomb/vdw interactions.

BI> 
BI> I have done some tests on molecules (Wave function optimization ) 
BI> and at the end when the energy decomposition is given, no explicit 
BI> value is given for the VDW contribution !!??? is it normal ?

unfortunately yes. please feel free to make this more obvious
by changing the code and sending in your changes.

cheers,
    axel.

BI> 
BI> Thank you in advance,
BI> Imad
BI> 
BI> 
BI>              
BI> ---------------------------------
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-- 
=======================================================================
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.