!
! Copyright (C) 2001-2007 Quantum ESPRESSO 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 stres_knl( sigmanlc, sigmakin )
  !-----------------------------------------------------------------------
  !! Computes the kinetic + nonlocal contribuition to the stress
  !
  USE kinds,                ONLY: DP
  USE constants,            ONLY: pi, e2
  USE cell_base,            ONLY: omega, alat, at, bg, tpiba
  USE gvect,                ONLY: g
  USE gvecw,                ONLY: qcutz, ecfixed, q2sigma
  USE klist,                ONLY: nks, xk, ngk, igk_k
  USE io_files,             ONLY: iunwfc, nwordwfc
  USE buffers,              ONLY: get_buffer
  USE symme,                ONLY: symmatrix
  USE wvfct,                ONLY: npwx, nbnd, wg
  USE control_flags,        ONLY: gamma_only
  USE noncollin_module,     ONLY: noncolin, npol
  USE wavefunctions,        ONLY: evc
  USE mp_pools,             ONLY: inter_pool_comm
  USE mp_bands,             ONLY: intra_bgrp_comm
  USE mp,                   ONLY: mp_sum
  !
  IMPLICIT NONE
  !
  REAL(DP) :: sigmanlc (3, 3)
  !! non-local contribution to stress
  REAL(DP) :: sigmakin (3, 3)
  !! kinetic contribution to stress
  !
  ! ... local variables
  !
  REAL(DP), ALLOCATABLE :: gk (:,:), kfac (:)
  REAL(DP) :: twobysqrtpi, gk2, arg
  INTEGER  :: npw, ik, l, m, i, ibnd, is
  !
  ALLOCATE( gk(npwx,3) )
  ALLOCATE( kfac(npwx) )
  !
  sigmanlc(:,:) = 0.d0
  sigmakin(:,:) = 0.d0
  twobysqrtpi   = 2.d0/SQRT(pi)
  !
  kfac(:) = 1.d0
  !
  DO ik = 1, nks
     IF ( nks > 1 ) CALL get_buffer( evc, nwordwfc, iunwfc, ik )
     npw = ngk(ik)
     DO i = 1, npw
        gk(i,1) = ( xk(1,ik) + g(1,igk_k(i,ik)) ) * tpiba
        gk(i,2) = ( xk(2,ik) + g(2,igk_k(i,ik)) ) * tpiba
        gk(i,3) = ( xk(3,ik) + g(3,igk_k(i,ik)) ) * tpiba
        IF (qcutz > 0.d0) THEN
           gk2 = gk(i,1)**2 + gk(i,2)**2 + gk(i,3)**2
           arg = ( (gk2-ecfixed)/q2sigma )**2
           kfac(i) = 1.d0 + qcutz / q2sigma * twobysqrtpi * EXP(-arg)
        ENDIF
     ENDDO
     !
     ! ... kinetic contribution
     !
     DO l = 1, 3
        DO m = 1, l
           DO ibnd = 1, nbnd
              DO i = 1, npw
                 IF (noncolin) THEN
                    sigmakin(l,m) = sigmakin(l,m) + wg(ibnd,ik) * &
                     gk(i,l) * gk(i, m) * kfac(i) * &
                     ( DBLE (CONJG(evc(   i  ,ibnd))*evc(   i  ,ibnd)) + &
                       DBLE (CONJG(evc(i+npwx,ibnd))*evc(i+npwx,ibnd)))
                 ELSE
                    sigmakin(l,m) = sigmakin(l,m) + wg(ibnd,ik) * &
                        gk(i,l) * gk(i, m) * kfac(i) * &
                          DBLE (CONJG(evc(i, ibnd) ) * evc(i, ibnd) )
                 ENDIF
              ENDDO
           ENDDO
        ENDDO
        !
     ENDDO
     !
     !  ... contribution from the  nonlocal part
     !
     CALL stres_us( ik, gk, sigmanlc )
     !
  ENDDO
  !
  DEALLOCATE( kfac )
  DEALLOCATE(  gk  )
  !
  ! ... the kinetic term must be summed over PW's and over k-points
  !
  CALL mp_sum( sigmakin, intra_bgrp_comm )
  CALL mp_sum( sigmakin, inter_pool_comm )
  !
  ! ... the nonlocal term is summed here only over k-points, because we add
  ! ... to it the US term from augmentation charge derivatives
  !
  CALL mp_sum( sigmanlc, inter_pool_comm )
  !
  ! ... add US term from augmentation charge derivatives, sum result over PW's
  !
  CALL addusstress( sigmanlc )
  CALL mp_sum( sigmanlc, intra_bgrp_comm )
  !
  DO l = 1, 3
     DO m = 1, l-1
        sigmanlc(m,l) = sigmanlc(l,m)
        sigmakin(m,l) = sigmakin(l,m)
     ENDDO
  ENDDO
  !
  IF ( gamma_only ) THEN
     sigmakin(:,:) = 2.d0 * e2 / omega * sigmakin(:,:)
  ELSE
     sigmakin(:,:) = e2 / omega * sigmakin(:,:)
  ENDIF
  sigmanlc(:,:) = -1.d0 / omega * sigmanlc(:,:)
  !
  ! ... symmetrize stress
  !
  CALL symmatrix( sigmakin )
  CALL symmatrix( sigmanlc )
  !
  RETURN
  !
END SUBROUTINE stres_knl

