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["Cote Michel" <michel.cote@umontreal.ca>]

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
>