Dear QE users,
I am Michele Bianchi, a PhD student from Politecnico of Torino.
I am studying the Cu (111) surface using the meta-GGA SCAN exchange-correlation
functional. I am using QE 7.2 version compiled with libxc library (version
6.1.0).
Since I did not find a Cu pseudopotential optimized for SCAN functional, I am
using a Norm-Conserving pseudopotential optimized for PBE, as retrieved in the
Pseudo Dojo library.
I am aware of the numerical instability issues of the SCAN functional. For that
reason, I am using the improved r2SCAN version as proposed by Furness et al.
(J. Phys. Chem. Lett.2020, 11, 19, 8208–8215).
Preliminary tests on Cu bulk phase gave me no problems and optimized lattice
constant is reasonable.
Problems arise when I simulate a Cu (111) slab (dimension 4x4x6 with a vacuum
space of about 15 A). SCF accuracy fluctuates and does not relevantly decrease
(as visible below in the reported output file).
I tried to increase the ‘ecutrho’ parameter to get a denser FFT grid and use
other Norm-Conserving pseudopotentials developed for PBE, but no visible
improvements are achieved.
I believe that issues are associated with the use of the SCAN functional since
the same simulation with PBE functional converges. From previous posts (e.g.
https://lists.quantum-espresso.org/pipermail/users/2018-November/041688.html),
I am aware that systems with exponentially decreasing densities might be tricky
to simulate with SCAN functional.
Below I have reported a typical input file and an extract of the output file.
Any suggestion will be appreciated to solve my convergence problem.
Thank you in advance.
Michele Bianchi,
PhD student, Politecnico of Turin, Italy
TYPICAL INPUT SCRIPT:
&CONTROL
calculation = 'scf'
restart_mode='from_scratch',
prefix='Cu_4x4x6_r2scan',
pseudo_dir = '/pseudo/',
outdir='./tmp/'
/
&SYSTEM
ibrav= 4,
celldm(1) = 19.38,
celldm(3)= 2.5
nat= 96,
ntyp= 1,
ecutwfc = 80.0,
occupations='smearing',
degauss=0.005
smearing='gauss'
input_dft = 'r2scan'
/
&ELECTRONS
diagonalization='david'
conv_thr = 1.0d-8
mixing_mode = 'local-TF'
mixing_beta=0.2
diago_david_ndim=4
electron_maxstep=200
/
ATOMIC_SPECIES
Cu 63.55 Cu.ONCV.SR.v5.upf
ATOMIC_POSITIONS { angstrom }
………
K_POINTS { automatic }
3 3 1 1 1 1
EXTRACT OF OUTPUT:
Dense grid: 1524959 G-vectors FFT dimensions: ( 120, 120, 288)
Initial potential from superposition of free atoms
starting charge 1823.9693, renormalised to 1824.0000
negative rho (up,down): 3.017E-05 0.000E+00
Starting wfcs are 960 randomized atomic wfcs + 134 random wfcs
total cpu time spent up to now is 66.6 secs
Self-consistent Calculation
iteration # 1 ecut= 80.00 Ry beta= 0.20
Davidson diagonalization with overlap
ethr = 1.00E-02, avg # of iterations = 33.3
negative rho (up,down): 9.728E+02 0.000E+00
total cpu time spent up to now is 1189.7 secs
total energy = 283496.48467385 Ry
estimated scf accuracy < 3575348.00873053 Ry
iteration # 2 ecut= 80.00 Ry beta= 0.20
Davidson diagonalization with overlap
ethr = 1.00E-02, avg # of iterations = 54.0
negative rho (up,down): 5.521E+02 0.000E+00
total cpu time spent up to now is 3447.8 secs
total energy = -56652.42746379 Ry
estimated scf accuracy < 1421161.66580171 Ry
……
iteration # 22 ecut= 80.00 Ry beta= 0.20
Davidson diagonalization with overlap
ethr = 1.00E-02, avg # of iterations = 2.0
negative rho (up,down): 1.061E+01 0.000E+00
total cpu time spent up to now is 6239.9 secs
total energy = -35382.73785671 Ry
estimated scf accuracy < 661.23095918 Ry
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