[CPMD-list] Trying to understand screened exchange B3LYP for PBC solids

[Axel Kohlmeyer]

On Thu, 1 Mar 2007 driver at mps.ohio-state.edu wrote:

KD> Hello CPMD users,

kevin,

KD> I am using CPMD-3.11 to do some PBC, many k-point calculations with the
KD> screened B3LYP functional for phases of silica.

hmmm... did you change the code for it?
do you see any signs in the output, that
you get a different answer from a plain BLYP
calculation?

to the best of my knowledge, HFX hasn't been 
implemented for k-points at all. 
on top of that, i was under the impression, that
screened exchange only makes sense, when your box is large 
enough that the screening is (almost) complete, which in 
your case is not at all the case.

cheers,
   axel.

KD> 
KD> What seems bizarre is that a phase calculation will always converge to the
KD> same total energy no matter what options I use with the SCREENED EXCHANGE
KD> keyword. I've tried out ERFC, EXP, and SELF screening while varying the
KD> screening parameter over several orders of magnitude (eg. 10^-4 to 10^2)
KD> for each case. It seems like all the long range Hartree Fock exchange is
KD> always omitted (or always included perhaps).
KD> 
KD> Is this the behavior I should expect when using SCREENED EXCHANGE, or is
KD> it an indication that I'm not using the code properly (input file is
KD> below)?
KD> 
KD> The energies I do get seem reasonable - that is, the phase energy
KD> differences I calculate with CPMD-screened-B3LYP agree with all-electron,
KD> unscreened-B3LYP calculations I've done in another code.
KD> 
KD> Kevin Driver
KD> The Ohio State University, Physics Dept.
KD> driver at mps.ohio-state.edu
KD> 
KD> ------------ Input file -----------------
KD> 
KD> &INFO
KD>  Stishovite SiO2
KD>  6x6x6 k-point mesh; 6 atoms (primitive cell)
KD>  400 Ryd planewave cutoff energy; B3LYP
KD> &END
KD> 
KD> &CPMD
KD>   OPTIMIZE WAVEFUNCTION
KD>   CONVERGENCE ORBITALS
KD>    1.0e-6
KD>   PRINT FORCES ON
KD>   MAXSTEP
KD>   1000
KD> &END
KD> 
KD> &DFT
KD>   FUNCTIONAL B3LYP
KD>   SCREENED EXCHANGE ERFC
KD>       0.1
KD> &END
KD> 
KD> &SYSTEM
KD>   ANGSTROM
KD>   CELL VECTORS
KD>    4.19311248230   0.00000000000   0.00000000000
KD>    0.00000000000   4.19311248230   0.00000000000
KD>    0.00000000000   0.00000000000   2.68292300360
KD>   CUTOFF
KD>   400.0
KD>   POINT GROUP
KD>   AUTO
KD>   KPOINTS MONKHORST-PACK
KD>   6 6 6
KD>   TESR
KD>   8
KD> &END
KD> 
KD> &ATOMS
KD> *SI_SG_BLYP  KLEINMAN-BYLANDER
KD>    LMAX=P
KD>    2
KD>    0.000000000000  0.000000000000  0.000000000000
KD>    2.096556241150  2.096556241150  1.341461501800
KD> 
KD> *O_SG_BLYP KLEINMAN-BYLANDER
KD>    LMAX=P
KD>    4
KD>    1.283524462631  1.283524462631  0.000000000000
KD>   -1.283524462631 -1.283524462631  0.000000000000
KD>   -0.813031778519  0.813031778519  1.341461501800
KD>    0.813031778519 -0.813031778519  1.341461501800
KD> &END
KD> 
KD> 
KD> _______________________________________________
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KD> CPMD-list at cpmd.org
KD> http://cpmd.org/mailman/listinfo/cpmd-list
KD> 

-- 
=======================================================================
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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