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[ren <opticalcase@gmail.com>]

Dear All:

I am doing geometry optimization for ZnO which has a hexagonal
geometry. The lattice a and b is equal. Therefore, a and b should
change by the same amount. Since it has a space group of P63 mc,
therefore, only the oxygen atoms is able to vary in its z (or u i
suppose) direction , internal coordinate.

I read up the optimization notes  by L. D. Marks, which is meant for
Wien2k, but i guess it should be applicable universally. What is done
is that, having 5 different volumes having the same a:b:c ratio, the
internal coordinates are optimized. Thereafter, for 5 c/a ratio, each
with 5 different volume, the internal coordinate are optimized.

At the moment, I tried the first step. But I tried exploring a larger
range of volumes (26 different volumes). By plotting the volumes vs
total energy, it appears that there is 2 local minimal (See the
picture attached. I supposed one would choose the minimal with the
lowest energy ? The volume obtained for the lowest energy is
313.992080 bohr^3. The other local minimal with a higher energy has a
volume of 294.765030 bohr^3

The problem occurs when I tried to use optcell=1 to see one could get
similar result. It appears that the result obtained has a optimized
volume similar to 294.765030 bohr^3 which has an energy similar to the
local minimal that has a higher energy as obtained from my first
study. I have also attached the email with both input files together.

For optcell=1, here is the options i used

optcell 1 # with optimization of volume only, do not modify rprim and
allow an homogeneous dilation of the 3 component of acell
ionmov 2 # Broyden Steps
ntime 40
dilatmx 1.05
ecutsm 0.5
tolmxf 5.0d-5
toldff 5.0d-6

I am using Troullier-Martins psp.

I tried to increase ntime, tolmxf to 5.0d-7 and toldff to 5.0d-7 but
the same results are obtained.

May I know if anyone can advised me what i have missed out that caused
this discrepancy?
Should using optcell=1 be more accurate than manually optimized the
internal coordinate with different volumes with fixed a:b :c ratio?

Lastly, i read from the mailing list and it is always recommended to
use optcell=2? Because of this spacegroup, does anyone have experience
that optcell=2 ensure that lattice a=b always expand and contract by
the same amount and that only the z internal coordinate is optimized?

Hope like to hear from your advices!!

Sorry for the lengthy email.

Cheers,
Yee Yan
School of Materials Science and Engineering
Nanyang Technological University (Singapore)

Attachment: cell-opt.tar.gz
Description: GNU Zip compressed data