[Pw_forum] Problem with PAW vs. USP

Tue Oct 13 15:42:50 CEST 2015

Sorry, I've missed the isolated atom energy: -306.74916142 Ry

Giovanni

> On 13 Oct 2015, at 15:39, Giovanni Cantele <giovanni.cantele at spin.cnr.it> wrote:
> 
> A quick run using your input (that demonstrates that providing inputs results in quicker answers!!!) as is (so non lattice constant optimisation),
> gives:
> 
> bulk Zr : -614.4200082344 Ry
> isolated atom: 
> 
> where the isolated atom was reproduced using the same input with just one atom, a=12, c=15 (just more or less safe values for an isolated atom), nspin=2,
> starting_magnetization(1) = 0.5, KPOINTS { gamma }. The final magnetic moment on the isolated Zr atom is 2.00.
> 
> Cohesive energy: (-614.4200082344 - 2 * -306.74916142) / 2 * 13.605692 ~ -6.27 eV per Zr atom
> 
> So, the value matches what you reported from the USPP calculations. Are you sure that in calculating with PAW the cohesive energy you are using, for the
> isolated Zr atom, its PAW energy (so, did you remake the calculation for the atom and use that energy for calculating the cohesive energy)? Energy coming from
> different pseudo potentials cannot be used within the same formula, because they are referred to a different zero.
> 
> Hope this helps,
> 
>    Giovanni
> 
> 
> PS because you fix a quite strict criterion for the convergence of the scf cycle (conv_thr=1.0D-9) I stopped the calculation for the isolated atom when
> estimated scf accuracy    <          5.5E-09 Ry
> 
> 
>> On 13 Oct 2015, at 14:54, Vic Bermudez <c.h.bermudez at cox.net> wrote:
>> 
>> 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
>> 
>> 
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>> 
> 
> -- 
> 
> Giovanni Cantele, PhD
> CNR-SPIN
> c/o Dipartimento di Fisica
> Universita' di Napoli "Federico II"
> Complesso Universitario M. S. Angelo - Ed. 6
> Via Cintia, I-80126, Napoli, Italy
> e-mail: giovanni.cantele at spin.cnr.it
> Phone: +39 081 676910
> Skype contact: giocan74
> 
> ResearcherID: http://www.researcherid.com/rid/A-1951-2009
> Web page: http://people.na.infn.it/~cantele
> 
> 
> 
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-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele