Phonons from finite differences

First of all to run a phonon calculation a sufficiently large super cell is needed.

The phonon calculations are simply carried out by setting IBRION=5 or IBRION=6 in the INCAR file. IBRION=5, is available as of VASP.4.5, IBRION=6 starting from VASP.5.1. Both flags allow to determine the Hessian matrix (matrix of the second derivatives of the energy with respect to the atomic positions) and the vibrational frequencies of a system. Important: Only zone centered (Γ-point) frequencies are calculated.