- Date: 2020/06/09 22:34
- Name: Sergey
- 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?