box_ahelmh Subroutine

  subroutine box_ahelmh(mg, id, nc, i_out)
    type(mg_t), intent(inout) :: mg
    integer, intent(in)       :: id
    integer, intent(in)       :: nc
    integer, intent(in)       :: i_out
    !< Index of variable to store Helmholtz in
    integer                   :: i
    real(dp)                  :: idr2(2*1), a0(2*1), u0, u(2*1), a(2*1)

    ! Duplicate 1/dr^2 array to multiply neighbor values
    idr2(1:2*1:2) = 1/mg%dr(:, mg%boxes(id)%lvl)**2
    idr2(2:2*1:2) = idr2(1:2*1:2)

    associate (cc => mg%boxes(id)%cc, n => mg_iphi, i_eps1 => mg_iveps1)
      do i = 1, nc
         a0(1:2)     = cc(i, i_eps1)
         a(1:2) = cc(i-1:i+1:2, i_eps1)
         u0     = cc(i, n)
         u(1:2) = cc(i-1:i+1:2, n)

         cc(i, i_out) = sum(2 * idr2 * a0(:)*a(:)/(a0(:) + a(:)) * (u(:) - &
            u0)) - ahelmholtz_lambda * u0
      end do
    end associate
  end subroutine box_ahelmh