Graphite MBD binding energy
Task
In this example you will determine the interlayer binding energy of graphite in its experimental structure using the MBD@rsSCS method of Tchatchenko et al. to account for van der Waals interactions.
Semilocal DFT at the GGA level underestimates long-range dispersion interactions. In the case of graphite, PBE predicts the interlayer binding energy of ~1 meV/atom which is too small compared to the RPA reference of 0.048 eV/atom [1]. In contrast, the pairwise correction scheme of Tkatchenko and Scheffler, overestimates this quantity strongly (0.083 eV/atom, see the Graphite TS binding energy example). Here we show that this problem can be eliminated by if many-body effects in dispersion energy are taken into account using the MBD@rsSCS method of Tchatchenko et al. (see Many-body dispersion energy).
Input
POSCAR
- Graphite:
graphite 1.0 1.22800000 -2.12695839 0.00000000 1.22800000 2.12695839 0.00000000 0.00000000 0.00000000 6.71 4 direct 0.00000000 0.00000000 0.25000000 0.00000000 0.00000000 0.75000000 0.33333333 0.66666667 0.25000000 0.66666667 0.33333333 0.75000000
- Graphene:
graphite 1.0 1.22800000 -2.12695839 0.00000000 1.22800000 2.12695839 0.00000000 0.00000000 0.00000000 20. 2 direct 0.00000000 0.00000000 0.25000000 0.33333333 0.66666667 0.25000000
INCAR
IVDW = 202 LVDWEXPANSION =.TRUE. NSW = 1 IBRION = 2 ISIF = 4 PREC = Accurate EDIFFG = 1e-5 LWAVE = .FALSE. LCHARG = .FALSE. ISMEAR = -5 SIGMA = 0.01 EDIFF = 1e-6 ALGO = Fast NPAR = 2
KPOINTS
- Graphite:
Monkhorst Pack 0 gamma 16 16 8 0 0 0
- Graphene:
Monkhorst Pack 0 gamma 16 16 1 0 0 0
Running this example
To run this example, execute the run.sh bash-script:
#
# To run VASP this script calls $vasp_std
# (or posibly $vasp_gam and/or $vasp_ncl).
# These variables can be defined by sourcing vaspcmd
. vaspcmd 2> /dev/null
#
# When vaspcmd is not available and $vasp_std,
# $vasp_gam, and/or $vasp_ncl are not set as environment
# variables, you can specify them here
[ -z "`echo $vasp_std`" ] && vasp_std="mpirun -np 8 /path-to-your-vasp/vasp_std"
[ -z "`echo $vasp_gam`" ] && vasp_gam="mpirun -np 8 /path-to-your-vasp/vasp_gam"
[ -z "`echo $vasp_ncl`" ] && vasp_ncl="mpirun -np 8 /path-to-your-vasp/vasp_ncl"
#
# The real work starts here
#
# Here the work starts
rm results.dat
drct=$(pwd)
for i in graphene graphite
do
cd $drct/$i
$vasp_std
done
cd $drct
# obtain total energy for graphite
en2=$(grep "free ene" graphite/OUTCAR |tail -1|awk '{print $5}')
# obtain total energy for graphene
en1=$(grep "free ene" graphene/OUTCAR |tail -1|awk '{print $5}')
# compute interlayer binding energy (eV/atom)
deltaE=$(echo print $en2/4 - $en1/2 |python)
echo "Binding energy (eV/atom): " $deltaE >results.dat
Note that the calculation is performed in two steps (two separate single-point calculations) in which the energy for bulk graphite and for graphene are obtained.
The binding energy is computed automatically and it is written in the file results.dat.
(N.B.: for the latter python needs to be available.)
The computed value of 0.050 eV/A is now fairly close to the RPA reference of 0.048 eV/atom [1].