[Wien] ,IRelast: incorrect structure for e=0.3 in C44 for TiO2

Mon Nov 14 15:19:26 CET 2022

Prof. Blaha,
I apologize for not noticing the exceeded size limit of the email.
Thank you for your comments, especially on calculating the low-symmetry structure in high symmetry.
Sincerely,
Ken

From: Wien <wien-bounces at zeus.theochem.tuwien.ac.at> on behalf of Peter Blaha <peter.blaha at tuwien.ac.at>
Date: Monday, November 14, 2022 at 3:29 AM
To: A Mailing list for WIEN2k users <wien at zeus.theochem.tuwien.ac.at>
Subject: [Wien] ,IRelast: incorrect structure for e=0.3 in C44 for TiO2
Your email must be smaller than 40 kB !!!!

A couple of comments:

Very small distortions are always problematic. When do you consider a
certain symmetry fulfilled ? Due to the limited digits of certain
numbers rounding problems (or factors like sqrt(2), or cos(45), ... may
cause such rounding problems, which later on cause problems.

I'm not surprised to see energy-jumps when comparing energies from a
tetragonal case with an orthorhombic one, where the lattice vectors have
been rotated. Such calculations would require "fully converged" meshes
(k-mesh, FFT mesh,..). You should do the "high-symmetry" case also in
the low-symmetry structure. For this, you have to initialize a
low-symmetry distorted structure and AFTER  init_lapw, change manually
the lattice parameters such that you obtain the high-symmetry case. Do
NOT rerun  init_lapw afterwards.

In essence: when symmetry (or better: the bravais lattice) changes, you
should either not include the e=0 case in the analysis or run the e=0
case as I indicated above.

PS: With a struct file, maybe the developer of IRestast could play
around ....

Best regards
Peter Blaha
---------------------
Von:
"Park, Ken" <Kenneth_Park at baylor.edu>
Datum:
13.11.2022, 16:18
An:
"wien at zeus.theochem.tuwien.ac.at" <wien at zeus.theochem.tuwien.ac.at>

Dear Wien2k developers and experts,

I have been using wien2k 21.1 and IRelast package to calculate the
elastic constants for rutile TiO2 at pressure =0.

When I used a small strain such as e = 0.3% was used for C44 (monoclinic
distortion), the script generated an incorrect structure causing a
crash. Below is a top portion of the structure with beta not equal to
90, for which wien2k expects alpha=beta=90.

[parkk at login001 tio2r]$ head -10 tio2r.struct

TiO2

CXZ LATTICE,NONEQUIV.ATOMS:  4 12 C2/m

MODE OF CALC=RELA unit=bohr

  12.390517 13.568280 12.390517 90.000000 90.000562155.460807

ATOM   1: X=0.00000000 Y=0.00000000 Z=0.00000000

           MULT= 1          ISPLIT= 0

Ti1        NPT=  781  R0=0.00005000 RMT= 1.78000     Z: 22.0

LOCAL ROT MATRIX:    1.0000000 0.0000000 0.0000000

                      0.0000000 1.0000000 0.0000000

                      0.0000000 0.0000000 1.0000000

The structures with e >= 0.5 seem working all right. Could you look into
this issue?

I also have been getting negative values of C44 and C66 in higher order
fit (4). So, initially I thought I needed higher rkmax in calculating
those elastic moduli, but then, I found out that the energy at e=0
remains higher compared to those up to about e < 1, despite of using
rkmax up to 10. (See the attached figure for C66.) With more data
points, it becomes clear that it is due to the e=0 data point lying
above the neighboring points by about 1.5 mRy. I believe that different
symmetry constraints are responsible for the energy discrepancy between
e=0 (tetragonal) and e != 0 (orthorhombic ) as the distortion for C66
lowers the symmetry from the tetragonal to the orthorhombic. This would
explain why I see a similar energy jump at e=0 also for C44, which
involves the deformation from tetragonal to monoclinic whereas the other
three distortions c11+c12, c33, and czz (all tetragonal) don’t show
such. So, I think that in the calculation of the elastic moduli with the
distortions involving symmetry change, the e=0 structure should not be
included. Could you comment on this?

If it may be convenient for you, I will be more than happy to provide
you the structure and outputeos files.

Respectfully,

Ken Park

--
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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