The ABINIT Users Mailing List ( CLOSED )

[rabrol@us.ibm.com]

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