init_spin_free_tmat Subroutine

    subroutine init_spin_free_tmat(lat)
        ! also construct a spin-free form of the hopping matrix
        class(lattice), optional :: lat
        character(*), parameter :: this_routine = "init_spin_free_tmat"

        integer :: i, ind

        if (present(lat)) then
            if (associated(spin_free_tmat)) deallocate(spin_free_tmat)
            allocate(spin_free_tmat(nBasis / 2, nBasis / 2), source=h_cast(0.0_dp))

            do i = 1, lat%get_nsites()
                ind = lat%get_site_index(i)

                ASSERT(lat%get_nsites() == nBasis / 2)
                ASSERT(ind > 0)
                ASSERT(ind <= nBasis / 2)

                associate(next => lat%get_neighbors(ind))

                    ASSERT(all(next > 0))
                    ASSERT(all(next <= nBasis / 2))

                    spin_free_tmat(ind, next) = bhub

                end associate
            end do
        else
            call stop_all(this_routine, "not yet implemented!")
        end if

    end subroutine init_spin_free_tmat