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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. IBRION=7 does not apply symmetry, whereas IBRION=8 uses symmetry to reduce the number of displacements. The output is similar to the previous section (Sec. 6.22.6). The only exception is that the ionic relaxation contributions to the elastic moduli are presently not determined. Born effective charges and piezoelectric constants, and the ionic contribution to the dielectric tensor can be calculated additionally by specifying LEPSILON=.TRUE. (see also Sec.6.72.6)

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