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[St Druid <tq02ksu@gmail.com>]

Hello users:

Invariance of lattice parameter a and b is determined by the symmetry,
unless you set the "nsym = 1 ",  according to the origin of the E-V
curve, I would like to suggest that, setting optcell = 2, and serial
strtarget values to obtain deferent volumes, than plot it.

Regards.

On Sat, Mar 21, 2009 at 10:14 AM, ren <opticalcase@gmail.com> wrote:
> 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)
>



-- 
B.W Dong
State Key Laboratory of Superhard Material
jilin university, Changchun Jilin, China. 130012

tq02ksu@gmail.com [ tq02ksu@email.jlu.edu.cn ]