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[ren <opticalcase@gmail.com>]

Dear Prof Zwanziger, Dr Matthieu and Dr Dong:

Thank you for your valuable advices! My apology for the late reply.

I did some preliminary work on increasing the Ecut and no of kpts, the
results seems to be better.

I will work on using optcell=2 as well as strtarget to see how's the result.

Thanks once again!

Cheers
Yee Yan


On Sun, Mar 22, 2009 at 2:52 AM, St Druid <tq02ksu@gmail.com> wrote:
> 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 ]
>
>