anchor.diff:  
           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 system, only: lattice, ndim, box_length, system_type
           use bit_utils
           use checking, only: check_allocate
           use errors, only: stop_all
   
  -        integer :: i, j, k, ierr, pos, ind, ivec, v
  +        integer :: i, j, k, ierr, pos, ind, ivec, v, isystem
           integer :: r(ndim)
   
           integer :: lvecs(ndim, 3**ndim)
  
positive.code: integer, allocatable :: connected_sites(:,:) ! (2ndim, nbasis)
  
  ! True if any site is its own periodic image.
  ! This is the case if one dimension (or more) has only one site per crystal
  ! cell.  If so then the an orbital can incur a kinetic interaction with itself.
  ! This is the only way that the integral < i | T | i >, where i is a basis
  ! function centred on a lattice site, can be non-zero.
  
negative.code:                                 ! connected_orbs does not contain self-connections 
                                  ! due to the periodic boundary conditions.
                                  call set_orb(connected_orbs(:,i),j)
                                  if (isystem == 2) call set_orb(connected_orbs(:,i+1),j+1)                      
                                  call set_orb(connected_orbs(:,j),i)
                                  if (isystem == 2) call set_orb(connected_orbs(:,j+1),i+1)
                              end if
                          end if
                      end if
  