Hello all, I'm attempting to use PhonoPy to analyze temperature-dependence of the phase diagram of a perovskite. Using VASP, I optimized the unit cell as recommended to high accuracy with little residual forces left, around 0.001 eV/Angstrom or less for each direction. I used 5x5x5 k-points (unit cell is nearly cubic with 5.75 angstrom sides). The ZPVE is -60.00010509 eV. Using a 2x2x2 supercell and 3x3x3 kpoints I got 72 displacements for each of which I did a single-point run. I made sure the sampling mesh was sufficiently dense for convergence.
My confusion lies with interpreting the thermodynamic outputs (attached). There, the ZPVE is reported as 193.5466325 kJ/mol. How can the value be positive, and why does it not match? I'm also getting strange results when using the free energy to generate the phase diagram, so I'm hoping that answering these questions will elucidate any conceptual issues I might be having.
Dear Sean,
Perovskite structures (BaTiO3, SrTiO3, CaSiO3) usually have imaginary frequencies in the phonon spectra (calculated by Phonopy), so your results may not agree with the available data. See, for example,
Article Ab initio prediction of structural phase-transition temperat...
Zero_point_energy written in the yaml file is zero-point energy of phonons (Fvib at 0 K) without ground state energy from the static calculation (F0), and it is always positive. To obtain "total" zero-point energy, it is necessary to add Zero_point_energy written in the yaml file to free energy (F) given in OSZICAR from your optimization (static calculation).
Hope you find it helpful
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