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!
! Copyright (C) 2017 Quantum ESPRESSO Foundation
! 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 volume (alat, a1, a2, a3, omega)
!---------------------------------------------------------------------
!
! Compute the volume of the unit cell defined by 3 vectors
! a1, a2, a3, given in units of "alat" (alat may be 1):
! omega = alat^3 * [ a1 . (a2 x a3) ]
! ( . = scalar product, x = vector product )
!
USE kinds, ONLY: dp
IMPLICIT NONE
!
REAL(dp), INTENT(IN) :: alat, a1(3), a2(3), a3(3)
REAL(dp), INTENT(OUT) :: omega
!
omega = a1(1) * ( a2(2)*a3(3)-a2(3)*a3(2) ) - &
a1(2) * ( a2(1)*a3(3)-a2(3)*a3(1) ) + &
a1(3) * ( a2(1)*a3(2)-a2(2)*a3(1) )
!
IF ( omega < 0.0_dp) THEN
call infomsg('volume','axis vectors are left-handed')
omega = ABS (omega)
END IF
!
IF ( alat < 1.0_dp) call infomsg('volume','strange lattice parameter')
omega = omega * alat**3
!
RETURN
!
END SUBROUTINE volume
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