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IPEAD = 1 | 2 | 3 | 4
Default: IPEAD = 4
Description: IPEAD specifies the order of the finite difference stencil used to compute the derivative of the cell-periodic part of the orbitals w.r.t. k, |∇kunk〉 (LPEAD=.TRUE.), and the derivative of the polarization w.r.t. the orbitals, δP/δ〈ψnk| for (LCALCEPS=.TRUE., or EFIELD_PEAD≠0).