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Constrained relaxation
Date: 2020/06/09 22:34
Name: Sergey   <slisenk@zoho.com>

Dear all,

I'm testing OpenMX on simple ferroelectric material like BaTiO3. While for cubic structure I can get the values of lattice parameters comparable to other DFT studies, tetragonal phase is where I have problems.

1) If I choose "RFC5" option for cell relaxation and start with experimental structure (displacements only along z direction, c/a=1.01), then I end up with rhombohedral structure in the end. I understand this, because OpenMX does not use symmetry, and low temperature phase for BaTiO3 is rhombohedral.

2) If I choose "RFC5" and fix atomic positions for "x" and "y" directions, I still see that x,y atomic coordinates are changing despite the fact I froze them. So, it seems "RFC5" option is not compatible with MD.Fix. option.

3) If I choose "OptC5" and fix "x,y" coordinates, z-components are relaxed, and fix cell vectors components to allow only a11, a22 and a33 to change, I found out that indeed whatever I fixed is fixed, but my relaxed structure is way off - c/a = 2 (which is wayyyy to high) and atomic positions along z direction is way off as well.

Any suggestions what to do? I use 'recommended" PAO for Ba, Ti and O from the table ("precision" type).

On another question - is there any plans to adopt LibXC library in OpenMX?

Thanks,
Sergey
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Re: Constrained relaxation ( No.1 )
Date: 2020/06/10 12:38
Name: Naoya Yamaguchi

Hi,

1) I think that "OptC4" is most suitable for the tetragonal phase.
2, 3) Although I don't know the detail of the implementation of the cell optimization, I believe that all of atomic coordinates are converted to those in the unit of AU (atomic unit) in OpenMX. In this case, the atomic coordinates should be changed when the cell is reduced or extended, and MD.Fixed.XYZ should fix the coordinates in the unit of AU, not FRAC. I think that MD.Fixed.XYZ is not available for fixing the initial fractional coordinates, but it may look available in the case that atoms are fixed in a direction where the cell is fixed.

Regards,
Naoya Yamaguchi
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Re: Constrained relaxation ( No.2 )
Date: 2020/06/10 17:17
Name: T. Ozaki

Hi,

How about using OptC6, OptC7, RFC6, or RFC7?
http://www.openmx-square.org/openmx_man3.9/node54.html

Regards,

TO
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