IBRION=7 and IBRION=8 is only supported starting from VASP.5.1. It determines the Hessian matrix (matrix of second derivatives) using density functional perturbation theory. As for IBRION=5, IBRION=7 does not apply symmetry, whereas IBRION=8 uses symmetry to reduce the number of displacements. The output is similar to the previous case, although with the exception of the ionic relaxation contributions to the elastic moduli, elastic moduli are presently not determined. Born effective charges and piezoelectric constants can be calculated by specifying LEPSILON=.TRUE. (see also Sec. 6.65.6)