Dear Quantum Espresso users,
I am trying to obtain the bulk modulus of MgH2 by fitting the total
energy versus volume curve to the Murnaghan equation of state. MgH2
has a tetragonal crystal system with P42/mnm symmetry. From the
literature, it appears that B0 should be around 40-50 GPa. However, I
am obtaining a value of 5 GPa. The input I am using is as follows:
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&CONTROL
title = 'ES-calculation' ,
calculation = 'relax' ,
restart_mode = 'from_scratch' ,
outdir = './' ,
pseudo_dir = './' ,
wf_collect = .FALSE. ,
disk_io = 'none' ,
/
&SYSTEM
ibrav = 6,
A = 4.00,
C = 2.672,
nat = 6 ,
ntyp = 2 ,
ecutwfc = 80 ,
ecutrho = 320 ,
input_dft = 'PBE' ,
occupations = 'smearing',
smearing = 'gaussian',
degauss = 0.02,
/
&ELECTRONS
conv_thr = 1.0d-8,
mixing_beta = 0.7,
/
&IONS
ion_dynamics = 'bfgs' ,
trust_radius_max = 0.2 ,
trust_radius_min = 1.0d-4 ,
/
ATOMIC_SPECIES
Mg 24.3050 Mg.pbe-spnl-kjpaw_psl.1.0.0.UPF
H 1.00000 H.pbe-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS crystal
H 0.19520000000000 0.80480000000000 0.50000000000000
H 0.69520000000000 0.69520000000000 -0.00000000000000
H 0.80480000000000 0.19520000000000 0.50000000000000
K_POINTS automatic
3 3 5 1 1 1
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The output of this example ends like this:
! total energy = -6.61308602 Ry
Harris-Foulkes estimate = -6.61308602 Ry
estimated scf accuracy < 5.1E-12 Ry
total all-electron energy = -6.613060 Ry
The total energy is the sum of the following terms:
one-electron contribution = -0.79116562 Ry
hartree contribution = 1.35759033 Ry
xc contribution = -3.89869029 Ry
ewald contribution = -3.27197209 Ry
one-center paw contrib. = -0.00084285 Ry
smearing contrib. (-TS) = -0.00800550 Ry
convergence has been achieved in 5 iterations
Forces acting on atoms (cartesian axes, Ry/au):
atom 1 type 2 force = -0.00001212 0.00001212 0.00000000
atom 2 type 2 force = -0.00000576 -0.00000576 0.00000000
atom 3 type 2 force = 0.00001212 -0.00001212 0.00000000
atom 4 type 2 force = 0.00000576 0.00000576 0.00000000
atom 5 type 1 force = 0.00000000 0.00000000 0.00000000
atom 6 type 1 force = 0.00000000 0.00000000 0.00000000
Total force = 0.000027 Total SCF correction = 0.000001
bfgs converged in 48 scf cycles and 47 bfgs steps
(criteria: energy < 1.0E-04 Ry, force < 1.0E-03 Ry/Bohr)
End of BFGS Geometry Optimization
Final energy = -6.6130860246 Ry
------------------------------------------------------------------------------------------------
The obtained values of volume (where V=a^2*c) and total energy are:
Vol (ang^3) Total energy (eV)
28.640500 -89.84334
33.836204 -89.93096
36.654496 -89.95390
39.625092 -89.96478
42.752000 -89.96531
46.039228 -89.95738
49.490784 -89.94272
56.902912 -89.89999
65.020448 -89.84715
------------------------------------------------------------------------------------------------
The fit to the Murnaghan equation seems to be correct, which makes me
question whether the calculation of the energies might be wrong.
According to literature, the total energy should be around -17.822 eV
after optimization. However, my values are around -89 eV (-6.61 Ry). I
cannot find where the problem lies. I have verified that ecutwfc,
ecutrho, and the k-points are converged. I can’t think of what else to
do. Could someone please verify the total energy of this structure or
guide me on where the issue might be?
Thank you in advance!
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