anisotropic crystal & fixed volume energies with castep

[stanroozen]

Hello QHA developers,

I would like to use QHA to get thermodynamic properties for the mineral tourmaline (trigonal). I am a complete novice to the subject to i apologies if these are very basic questions. I use the software CASTEP for my geometry optimizations and VDOS calculations. I have a question regarding the 'static' input file that was generated using quantum expresso (a software i do not have any experience with).

1. the examples given of silicon and ice VII are both cubic systems, however i saw that the code was used for orthorhomic systems (fosterite) as well. What do i have to change in the input files for anisotropic crystal systems? (my system is trigonal however there are disordered endmembers in the tourmaline system which need supercells to describe them which have P1 symmetry)

2. In castep i can do fixed volume and fixed cell optimizations. which of these two do i have to use to get an static energy at fixed volume? So far i tried

• initial geometry optimisation of the endmember cell
• expand and contract the lattice parameter manually and use a fixed volume geometric optimisation to get new static energies at those fixed volumes
  However the fixed volume optimisation generates an internall stress in the cell which is anisotropic:
  

3. what does the pressure mean in the 'static' input file. Is this the internal pressure the cell, eg in the image above the -11.0495 GPa?

Greetings Stan Roozen