Tutorial t34 shows how to obtain the optimal lattice parameter for silicon,
using molecular dynamics algorithms,
http://www.abinit.org/documentation/helpfiles/for-v6.10/tutorial/lesson_3.html#34.
Such example uses a pspnc pseudopotential for Si.
In the same spirit and using the same kind of pseudopotentials (pspnc) for Ga and As,
I tried to obtain the optimal lattice parameter for GaAs. My result was:
10.680 Bohr, room temperature;
10.526 Bohr, LDA, in literature;
10.298 Bohr, my result.
Then I repeated the calculation using a simpler method, calculating the total energy for different values of acell,
in order to find the acell that gives the minimum value of total energy. I got the same result, 10.298 Bohrs.
Finally I plotted the bandstructure and most features for the valence bands are well reproduced,
but the first conduction band has a serious problem: it shows GaAs as an indirect band gap material.
The plot is attached in this post.
(If I plot the bandstructure using the usual value of acell=10.60 the plot is in good agreement)
From this I think I did a mistake in the relaxation of the lattice parameter.
The input file I used for GaAs is almost the same as tutorial t34 for Silicon, except by
acell, ntypat, znucl, natom, and typat.
Should I have added any other modification to the input file due to the fact that
GaAs has a significant spin orbit coupling, as compared to Si?
The input file is shown below
Code: Select all
# Crystalline GaAs : computation of the optimal lattice parameter
# Based on tutorial t34 for silicon
ndtset 4
#Optimization of the lattice parameters
optcell 1
ionmov 3
ntime 10
dilatmx 1.05
ecutsm 0.5
#Definition of the k-point grids
kptopt 1 # Option for the automatic generation of k points, taking
# into account the symmetry
nshiftk 4
shiftk 0.5 0.5 0.5 # These shifts will be the same for all grids
0.5 0.0 0.0
0.0 0.5 0.0
0.0 0.0 0.5
ngkpt1 2 2 2
ngkpt2 4 4 4
ngkpt3 6 6 6 # Optimal grid
ngkpt4 8 8 8
getwfk -1 # This is to speed up the calculation, by restarting
# from previous wavefunctions, transferred from the old
# to the new k-points.
#Definition of the unit cell
#acell 3*10.18 # Silicon
acell 3*10.60 # GaAs
rprim 0.0 0.5 0.5 # FCC primitive vectors (to be scaled by acell)
0.5 0.0 0.5
0.5 0.5 0.0
#Definition of the atom types
ntypat 2 # There are two types of atom
znucl 31 33 # 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 only type is Silicon.
#Definition of the atoms
natom 2 # There are two atoms
typat 1 2 # One atom is Ga, the other is As
xred # This keyword indicate that the location of the atoms
# will follow, one triplet of number for each atom
0.0 0.0 0.0 # Triplet giving the REDUCED coordinate of atom 1.
1/4 1/4 1/4 # Triplet giving the REDUCED coordinate of atom 2.
#Definition of the planewave basis set
ecut 30.0 # Maximal kinetic energy cut-off, in Hartree
#Definition of the SCF procedure
nstep 10 # Maximal number of SCF cycles
toldfe 1.0d-10 # Will stop when, twice in a row, the difference
# between two consecutive evaluations of total energy
# differ by less than toldfe (in Hartree)
# This value is way too large for most realistic studies of materials
diemac 12.0 # Although this is not mandatory, it is worth to
# precondition the SCF cycle. The model dielectric
# function used as the standard preconditioner
# is described in the "dielng" input variable section.
# Here, we follow the prescription for bulk silicon.
Thanks,
Temok