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[Masayoshi Mikami <mmikami@rc.m-kagaku.co.jp>]

Dear Ravi,

Please let me suggest several things:
1) to confirm if your model has a "plausible" initial geometry
(e.g. molecules should not overlap), some GUI will help;
with an viewer option "Balls based on van der Waals radii".
Such option should be available on most GUIs. e.g. XCrySDen, Cerius2, 
....

2) about the van der Waals interaction: my post might be interesting ?
  http://www.abinit.org/wws/arc/forum/2003-12/msg00038.html
Geometries of C60 solid or graphite may be reproduced with LDA,
which seems very fortunate ... (this may be understood as "rules of 
thumb")
The geometries of C60/graphite cannot be reproduced with the present 
GGA's....
GGA may be useful for the description of (intermolecular) 
hydrogen-bonds...

Regards,
Masayoshi

On 2004/04/01, at 7:38, Cote Michel wrote:

> Dear Ravi,
>
> I did molecular crystals of naphthalene and pentacene with abinit and 
> I had no problem. For your naphthalene calculations, looking at your 
> input it seems that your box in to small. The molecule extent from 
> about -3 to 3 Angstroms, which is roughly 11 bohr but your box is only 
> 12 bohr. So your naphthalene molecule interacts with its periodic 
> images! Increase your box size and that should fix your problem.
>
> Also, don't expect to get the right lattice parameters with DFT. Since 
> DFT does not include van der Waals interactions, at the moment, the 
> calculated lattice parameters are quite off from the experimental 
> values.
>
> I hope it helps.
>
> Michel
>
> ***************************************************************
>   Michel Cote                          tel: +1 (514) 343-5628
>   Professeur adjoint                   fax: +1 (514) 343-2071
>   Département de physique
>   Université de Montréal
>   C.P. 6128, succ. Centre-ville
>   Montréal (Québec) H3C 3J7          Michel.Cote@umontreal.ca
>   Canada              http://www.phys.umontreal.ca/~michel_cote
> ***************************************************************
>
>
>
>
>
> > -----Message d'origine-----
> > De : rabrol@us.ibm.com [mailto:rabrol@us.ibm.com]
> > Envoyé : 31 mars, 2004 16:43
> > À : forum@abinit.org
> > Objet : [abinit-forum] molecular crystals...
> >
> >
> > Hi All,
> > I am starting to use AbInit and have finished all the
> > tutorials. I want to study molecular crystals like
> > naphthalene, pentacene.
> > Does anybody have some idea about how good the GGA
> > exchange-correlation functionals like PBE or RPBE
> > are for these van der Waals crystals?
> >
> > I tried using RPBE GGA functional for geometry optimization
> > of a single naphthalene molecule and the minimum energy
> > structure came out to be non-planar. The ground state
> > geometry being planar, do I need to use any constraints to keep
> > the molecule planar or I am doing something wrong.
> > Here's my input file:
> >
> > -----------------------start of input file---------------------
> > # Single naphthalene molecule
> > #
> > # Geometry optimisation
> >
> > # RPBE GGA functional
> > ixc  15
> >
> > ndtset 1
> >
> > # geometry optimization
> > ionmov 2
> > ntime 20
> > tolmxf 5.0d-4
> > nband 24
> >
> > #Definition of the planewave basis set
> > ecut 30.0         # Maximal kinetic energy cut-off, in Hartree
> >
> > toldfe  1.0d-6
> >
> > #Definition of the unit cell
> > acell 12 12 12
> >
> > #Definition of the atom types
> > ntypat 2          # There are two types of atoms
> > znucl 6 1         # The keyword "znucl" refers to the atomic
> > number of the
> >                   # possible type(s) of atom. The pseudopotential(s)
> >                   # mentioned in the "files" file must correspond
> >                   # to the type(s) of atom. Here, the types
> > are carbon and hydrogen
> >
> >
> > #Definition of the atoms
> > natom 18          # There are 18 atoms in 1 naphthalene molecule
> > typat 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 1
> >              # They are of type 1 or 2, that is, carbon or
> > hydrogen respectively
> > xangst       # This keyword indicate that the location of the atoms
> >              # will follow, one triplet of number for each atom
> >     0.51156     1.49914    -1.00626
> >     0.94747     2.32584    -0.85293
> >     0.08820     1.13232    -2.26318
> >     0.22250     1.75733    -3.01875
> >    -0.55004    -0.11049    -2.46435
> >    -0.82988    -0.35063    -3.37189
> >    -0.75471    -0.97534    -1.41169
> >    -1.19403    -1.81924    -1.52799
> >    -0.51156    -1.49914     1.00626
> >    -0.94747    -2.32584     0.85293
> >    -0.08820    -1.13232     2.26318
> >    -0.22250    -1.75733     3.01875
> >     0.55004     0.11049     2.46435
> >     0.82988     0.35063     3.37189
> >     0.75471     0.97534     1.41169
> >     1.19403     1.81924     1.52799
> >     0.32066     0.62614     0.10178
> >    -0.32066    -0.62614    -0.10178
> >
> >
> > #Definition of the SCF procedure
> > nstep 20          # Maximal number of SCF cycles
> > diemac 2.0
> > --------------------------end of input file-----
> >
> > Any help is greatly appreciated.
> > Thanks,
> > Ravi