optimal lattice parameter GaAs
Posted: Fri May 04, 2012 8:41 am
Dear members of this forum,
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
Thanks,
Temok
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