Dear Wei Hu
This is an example input file to set things going, but you should keep
in
mind that Van der Waals interactions are subtle ones (a much smaller
energy scale than most other interactions) and thus demand quite
some care in the more technical approximations. In addition to
tolerances and cutoffs, the convergence with basis set will be more
demanding than in other situations. (From what I see you are using a
default automatic basis set). Poor basis sets will tend to overbind in
these systems (e.g. basis set superposition error).
Another point: Although LDA and GGAs do not include the non-local
correlations essential to VdW, they do tend to bind (and even overbind)
in these situations due to some (exponentially-decaying, but not small)
spurious attraction mediated by the proposed exchange form.
Emilio
On 7 Jun 2011, at 08:50, Wei Hu wrote:
Dear,
I have done some tests about graphite using VDW-DRSLL in siesta-
trunk-367.The input and initial structures comes from trunk-367/
Tests/graphite_vdw_df. I test three functional LDA,GGA and VDW.
However,their results are not very consistent with the experimental
data.
For the calculated three interlayer equilibrium distance, LDA,
GGA and VDW are all smaller than the the experimental data 0.3354nm.
The LDA result is even smaller that 0.3100nm. The interlayer
distance is 0.3250nm and 0.3225nm for GGA and VDW,respectively.
What is the matter with my calculations?
SystemName Graphite
SystemLabel graphite
NumberOfAtoms 4
NumberOfSpecies 1
%block ChemicalSpeciesLabel
1 6 C.vdw
%endblock ChemicalSpeciesLabel
LatticeConstant 1 Ang
%block LatticeParameters
2.4610 2.4610 6.7080 90.0000 90.0000 120.0000
%endblock LatticeParameters
AtomicCoordinatesFormat NotScaledCartesianAng
%block AtomicCoordinatesAndAtomicSpecies
0.00000000 0.00000000 0.00000000 1 C 1
0.00000000 0.00000000 3.35400000 1 C 2
1.23050004 0.71042953 3.35400000 1 C 3
0.00000000 1.42085905 0.00000000 1 C 4
%endblock AtomicCoordinatesAndAtomicSpecies
XC_functional VDW
XC_authors DRSLL
%block PS.lmax
C.vdw 2
%endblock PS.lmax
--
Emilio Artacho
Department of Earth Sciences, University of Cambridge
Downing Street, Cambridge CB2 3EQ, UK
Tel. (+44/0) 1223 333480, Fax (+44/0) 1223 333450
[email protected], http://www.esc.cam.ac.uk/~emilio
