init_basis_vecs_chain Subroutine

    subroutine init_basis_vecs_chain(this)
        class(chain) :: this

        integer :: i, j, k

        if (allocated(this%basis_vecs)) deallocate(this%basis_vecs)

        if (t_trans_corr_2body) then
            j = 2
            allocate(this%basis_vecs(5, 3))
        else
            j = 1
            allocate(this%basis_vecs(3, 3))
        end if

        this%basis_vecs = 0

        k = 0
        do i = -j, j
            k = k + 1
            this%basis_vecs(k, 1) = i * this%get_length(1)
        end do

    end subroutine init_basis_vecs_chain