Colleagues,
I am working through a problem that I have assigned to my Computational
Materials Science class - computation of the transition pressure for the
diamond-cubic to beta-Sn structures in diamond using pw.x. The problem I have
is that pw.x produces answers that are inconsistent with those produced by
VASP, and those published in the literature. I have used similar ultrasoft
pseudopotentials in both calculations (both based on PW91 correlation). There
may be differences, between the two cases, but I have tried a number of
pseudopotentials in pw.x and find essentially the same results). I have
installed the QE software version 4.2.1 using both gfortran and ifort, using
entirely internal libraries and with MKL (32 bit) libraries, both openMPI and
single processor mode, and the tests of pw.x all conclude successfully). The
software is installed on a Mac OSX 10.6 laptop. I have compiled in 32 and 64
bit mode for gfortran. If memory serves me correctly, all produce essentially
the same results.
Though I have generated a number of discrepancies, I think the most simple
manifestation is the difference in total energy per atom for the equilibrium
structures as predicted by VASP. More specifically, I find:
(1) Diamond cubic structure in VASP. Relaxed structure (pressure ~ 0).
K=Point mesh is 12 x 12 x 12, symmetrized. The lattice parameter comes out to
be 5.456 Ang. Ultrasoft pseudopotential, PW91 exchange-correlation, cutoff
chosen to be 400 eV. Total energy of relaxed cell = -10.866660 eV.
(2) Diamond cubic structure run using beta-Sn POSCAR with c/a = sqrt(2).
Produces the same lattice parameter and the same total energy per relaxed cell
= -10.866649 eV.
(3) Beta-Sn structure also relaxed using the same potential yields a lattice
parameter a = 4.832 Ang, and the ratio c/a = 0.547. Using the same k-point
mesh (perhaps not completely converged, but close enough for our purposes) one
finds a total energy of this phase to be -10.191998 eV.
(4) Using these numbers, and noting that there are two atoms per unit cell, one
finds that the energy difference per atom between the structures is given by
E(BetaSn Si)- E(DC Si) = (-10.191998+10.866660)/2
eV/atom
= 0.34 eV/atom
This value is in very good agreement with published results. We can now take
these structural parameters and run pw.x from QE to compute the energy
difference. I will use the beta-Sn unit cell for both calculations (the
natural diamond cubic cell produces essentially the same results). I also used
a MP grid 12x12x12 and an energy cutoff of 400 eV (12*400 eV for the charge
density).
(5) The total energy of the beta-Sn structure, per atom, predicted by QE is
-130.192 eV. The total energy of the dc structure, per atom, predicted by QE
is -130.982 eV. The difference in energy is, therefore, 0.79 eV/atom, more
than a factor of two larger than predicted by VASP. I've attached the input
files and the output files for your perusal.
Sorry about the size of this message. Any insight you can lend will be more
than appreciated.
Thank you,
Daryl C. Chrzan
Professor, Materials Science and Engineering
University of California
Berkeley, CA 94720
and
Materials Sciences Division
Lawrence Berkeley National Laboratory
Berkeley, CA 94720
ph./fax: 510 643 1624
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