init_hop_cache_bounds Subroutine

    subroutine init_hop_cache_bounds(this, r_min, r_max)
        ! initialize the lower and upper bounds of the cached
        ! hopping-transcorr factor, indexed by the involved r-vector
        ! maybe move this function to lattice_mod?!
        class(lattice) :: this
        integer, intent(out), optional :: r_min(3), r_max(3)
        integer :: n_sites, i, j, ri(3), rj(3), r_diff(3)

        if (.not. (all(this%r_min == this%r_max) .and. all(this%r_min == 0))) then

            if (present(r_min) .and. present(r_max)) then
                r_min = this%r_min
                r_max = this%r_max
            end if

            return
        end if

        n_sites = this%get_nsites()

        do i = 1, n_sites
            ri = this%get_r_vec(i)
            do j = 1, n_sites
                rj = this%get_r_vec(j)

                r_diff = ri - rj

                where (r_diff < this%r_min) this%r_min = r_diff
                where (r_diff > this%r_max) this%r_max = r_diff

            end do
        end do

        if (present(r_min) .and. present(r_max)) then
            r_min = this%r_min
            r_max = this%r_max
        end if

    end subroutine init_hop_cache_bounds