subroutine form_tau(t1,t2,tau)

! Form tau in CCSD

  implicit none

! Input variables

  double precision,intent(in)   :: t1(spin_occ_num,spin_vir_num)
  double precision,intent(in)   :: t2(spin_occ_num,spin_occ_num,spin_vir_num,spin_vir_num)

! Local variables

  integer                       :: i,j,k,l
  integer                       :: a,b,c,d

! Output variables

  double precision,intent(out)  :: tau(spin_occ_num,spin_occ_num,spin_vir_num,spin_vir_num)

  do b=1,spin_vir_num
    do a=1,spin_vir_num
      do j=1,spin_occ_num
        do i=1,spin_occ_num

          tau(i,j,a,b) = 0.5d0*t2(i,j,a,b) + t1(i,a)*t1(j,b)

        enddo
      enddo
    enddo
  enddo

end subroutine form_tau