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c*****************************************************************
c*****************************************************************
c*****************************************************************
c GGA
c The Generalized Gradient Approximation
c GGA is a way to improve the local spin density approximation
c by adding nonlocal gradients in the evaluation of the exchange-
c correlation energy and potential. These functions and subroutines
c are based on programs made by John Perdew, Tulane University,
c New Orleans, Louisiana, USA. Email: PY03APF%music.tcs.tulane.edu
c The modifications necessary to fit them to the atom program was
c made by Tomas Holmquist 1993 (so now you know who to blame).
c Email: tomash@fy.chalmers.se
c A more thorough description of the GGA used may be found in
c J.P. Perdew et al, Phys. Rev. B (46) 1992 pp. 6671-6687.
c The GGA package is:
c function deriv
c subroutine grad
c subroutine grabgr
c subroutine laplac
c subroutine spline
c subroutine splint
c subroutine ggaexc
c subroutine exchen
c subroutine ldaec
c subroutine gcor
c subroutine ggaec
c subroutine ggauxc
c subroutine exchpt
c subroutine ldauc
c subroutine ggauc
c The grad, grabgr, and laplace subroutines uses the function deriv
c to calculate different derivatives with respect to the radial
c coordinate r. Spline and splint calculates the cubic spline
c interpolation which is used in function deriv.
c The two main subprograms, ggaexc and ggauxc, calculates
c the exchange-correlation energy and potential respectively.
c The other subroutines are modified versions of Perdew's program.
c*****************************************************************
c*****************************************************************
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