[Pw_forum] Problem with PAW vs. USP

[Vic Bermudez]

Hello,

	I'm encountering some strange behavior in calculations for the lattice
constants and cohesive energy of Zr metal. All calculations are being done
using version 5.0.

(1) I first find the energy of the free Zr atom, taking care to obtain the
proper triplet ground state of the free atom.

(2) I then optimize the (hexagonal) Zr metal lattice constants using a
script that varies a0 over a range of values while doing vc_relax to
optimize c0 at each value of a0. From this I find the values of a0 and c0
that give an absolute-minimum total energy

	When I do this using PBE with a USPP for Zr (Zr.pbe-nsp-van.UPF), which I
obtained from the QE website, the results are very good. I find a0=3.235
Angstroms, c0=5.148 Angstroms and a cohesive energy of 6.23 eV/Zr, all of
which are very close to the experimental values. So far, so good.

	When I repeat this procedure using PBE with a PAW pseudopotential
(Zr.pbe-spn-kjpaw_psl.0.2.3.UPF), which I obtained from the THEOS website, I
get essentially the same lattice constants, but the cohesive energy is
wildly wrong (about 41 eV/Zr !!). I get the same results for
ecutwfc=40/ecutrho=320 and ecutwfc=70/ecutrho=560. Except for the choice of
PP, I'm using all the same input parameters for PAW as I did for USPP. Also,
I get the same ridiculous cohesive energy whether I use the PAW total
energies or the all-electron energies. Since the lattice constants are good,
and, according to THEOS, the pseudopotential is well-tested, I must be doing
something spectacularly wrong in obtaining or interpreting the energy
values.

	Any advice would be much appreciated. In case it will help, I'm inserting a
representative input file below (for relaxing with the optimized lattice
constants).

******************

&CONTROL
calculation='relax',
restart_mode='from_scratch',
title='single-crystal alpha-Zr - testing Zr PBE/PAW pseudopotential',
pseudo_dir='/lustre/cmf/scratch/b/bermudez/QE_PP/',
! outdir='/lustre/cmf/scratch/b/bermudez/aZr/',
etot_conv_thr=1.0D-5,
forc_conv_thr=1.0D-4,
nstep=10000,
wf_collect=.TRUE.,
verbosity='default'
/

&SYSTEM
ibrav=4,
a=3.24, c=5.147,
nat=2,
ntyp=1,
ecutwfc=40.0,
ecutrho=320.0,
occupations='smearing',
smearing='gaussian',
degauss=0.002
/

&ELECTRONS
electron_maxstep=200,
conv_thr=1.0D-9,
mixing_mode='plain',
mixing_beta=0.7D0
/

&IONS
ion_dynamics='bfgs',
upscale=10000.D0
/

ATOMIC_SPECIES
Zr 91.22   Zr.pbe-spn-kjpaw_psl.0.2.3.UPF

ATOMIC_POSITIONS {crystal}
Zr   0.000     0.000     0.000
Zr   0.333     0.667     0.500

K_POINTS {automatic}
10 10 6 0 0 0

*******************

Best Wishes,
Vic Bermudez

Victor M. Bermudez
E-mail: bermudez at alum.mit.edu