[Wien] Structural optimization, volume optimization

[Fecher, Gerhard]

The volume optimization is part of an optimization of the structure, but only one part of it.

To optimize the structure you need
- optimization of the lattice parameters (a,b,c, alpha, beta, gamma, or a set out of them depending on the space group)
- optimization of the atomic positions (if the sites have free parameters)

In case of a cubic system,. you have only one lattice parameter, namely a, and all angles 90°
as V=a^3, the optimized volume automatically gives you the optimization of ALL lattice parameters you need for a cubic system.

The optimization of the positions depend on their symmetry, indeed, you can have free parametrs in cubic systems
the optimization of the positions may be done in parallel to the volume (or say better all relevant lattice parameters that are not fixed in the crystal system, if your system is not cubic).

If you like to do a rough check of your structure, it depends on your choice whether you try to use one optimization after the other, they if you trast the lattice parametr but not the position.
Take care, in structures more complicated than cubic, you might run easily into an unwanted minimum, then you have to check for all stable or metastable states,
criterion is --as usual-- the lowest energy.

Ciao
Gerhard

DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
"I think the problem, to be quite honest with you,
is that you have never actually known what the question is."

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
and
Max Planck Institute for Chemical Physics of Solids
01187 Dresden
________________________________________
Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von Ashwani Kumar [ashwani.ism at gmail.com]
Gesendet: Mittwoch, 2. Mai 2018 09:02
An: wien at zeus.theochem.tuwien.ac.at
Betreff: [Wien] Structural optimization, volume optimization

Hi,
  Working with examples of TiO2 for structrual optimization from wien2k manual. Everything executed nicely. however i want to ask wether volume optimization (x optimize) has to be done first followed by structural optimization (mini_lapw port) or vice-versa. Working with hafnia, then one of the wien2k expert asked me for structural optimization.
i am trying to understand:
1. for a cubic systems, is both structural and volume optimization required.
2. For non-cubic systems, is both structural and volume optimization required. if Yes, then what need to be optimize first.

thanks
A kumar