anchor.diff:                                  ! connected_orbs does not contain self-connections 
                                   ! due to the periodic boundary conditions.
                                   call set_orb(connected_orbs(:,i),j)
  -                                call set_orb(connected_orbs(:,i+1),j+1)                      
  +                                if (isystem == 2) call set_orb(connected_orbs(:,i+1),j+1)                      
                                   call set_orb(connected_orbs(:,j),i)
  -                                call set_orb(connected_orbs(:,j+1),i+1)
  +                                if (isystem == 2) call set_orb(connected_orbs(:,j+1),i+1)
                               end if
                           end if
                       end if
  
positive.code: 
          use basis, only: nbasis, bit_lookup, basis_lookup, basis_length, basis_fns, set_orb
          use determinants, only: decode_det
          use system, only: lattice, ndim, box_length
          use bit_utils
          use checking, only: check_allocate
          use errors, only: stop_all
  
          integer :: i, j, k, ierr, pos, ind, ivec, v
          integer :: r(ndim)
  
          integer :: lvecs(ndim, 3**ndim)
  
negative.code:         end select
  
          ! Construct how the lattice is connected.
          do i = 1, nbasis-(isystem-1), isystem
              do j = i, nbasis-(isystem-1), isystem
                  ! Loop only over one spin: the other spin is identical so can be
                  ! filled in automatically.
                  ! All matrix elements between different spins are zero
  