SCPH Relaxation of structures with different symmetry

[RicardoCecconello]

Dear Dr. Tadano,

We're doing the relaxation calculation on the SCPH level to obtain structures and free energies as a function of temperature. Our system has a monoclinic symmetry, and we're exploring a phase transition to an orthorhombic phase. However, during the relaxation, the angles don't stay at 90 degrees (the variations are small). My question is: How do we ensure that the space group stays fixed during the relaxation? Since we need the free energy for both phases, we need to run the relaxation for these two different space groups, but that wouldn't occur if the angles, for instance, change, or the structure evolves to another phase. In your paper on BaTiO3, that's exactly what you have done: obtained the free energy as a function of temperature for the various space groups.

[ttadano]

In the case of BaTiO3, we were able to compute the free energy of the cubic BaTiO3 even below the phase transition temperature because cubic BaTiO3 stayed cubic if we did not add initial perturbation (initial displacements and strains).

I am not sure if the same approach works for your system.
In SCPH or QHA calculations, the space group symmetry should not be lowered from the initial distorted structure (if the implementation is correct).

[RicardoCecconello]

Thank you for the reply!