LPEAD= .TRUE. | .FALSE.
IPEAD= 1 | 2 | 3 | 4
Default: LPEAD=.FALSE. and IPEAD=4 (available as of VASP.5.2).
The derivative of the cell-periodic part of the orbitals w.r.t. , , may be written as:
It may also be found as the solution to the following linear Sternheimer equation:
Alternatively one may compute from finite differences (see Eqs. (96) and (97) in Ref. ):
As mentioned in Ref.  one may derive analoguous expressions
using higher-order finite
When LPEAD=.TRUE., VASP will compute using the aforementioned finite difference scheme. The order of the finite difference approximation can be specified by setting the IPEAD-tag.
These tags may be used in combination with LOPTICS=.TRUE. (Sec. 6.72.1) and LEPSILON=.TRUE. (Sec. 6.72.4).