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!
! Copyright (C) 2001 PWSCF group
! This file is distributed under the terms of the
! GNU General Public License. See the file `License'
! in the root directory of the present distribution,
! or http://www.gnu.org/copyleft/gpl.txt .
!
!
!---------------------------------------------------------------------
subroutine recips (a1, a2, a3, b1, b2, b3)
!---------------------------------------------------------------------
!
! This routine generates the reciprocal lattice vectors b1,b2,b3
! given the real space vectors a1,a2,a3. The b's are units of 2 pi/a.
!
! first the input variables
!
use kinds, ONLY: DP
implicit none
real(DP) :: a1 (3), a2 (3), a3 (3), b1 (3), b2 (3), b3 (3)
! input: first direct lattice vector
! input: second direct lattice vector
! input: third direct lattice vector
! output: first reciprocal lattice vector
! output: second reciprocal lattice vector
! output: third reciprocal lattice vector
!
! then the local variables
!
real(DP) :: den, s
! the denominator
! the sign of the permutations
integer :: iperm, i, j, k, l, ipol
! counter on the permutations
!\
! Auxiliary variables
!/
!
! Counter on the polarizations
!
! first we compute the denominator
!
den = 0
i = 1
j = 2
k = 3
s = 1.d0
100 do iperm = 1, 3
den = den + s * a1 (i) * a2 (j) * a3 (k)
l = i
i = j
j = k
k = l
enddo
i = 2
j = 1
k = 3
s = - s
if (s.lt.0.d0) goto 100
!
! here we compute the reciprocal vectors
!
i = 1
j = 2
k = 3
do ipol = 1, 3
b1 (ipol) = (a2 (j) * a3 (k) - a2 (k) * a3 (j) ) / den
b2 (ipol) = (a3 (j) * a1 (k) - a3 (k) * a1 (j) ) / den
b3 (ipol) = (a1 (j) * a2 (k) - a1 (k) * a2 (j) ) / den
l = i
i = j
j = k
k = l
enddo
return
end subroutine recips
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