def coords2dr(coord):
    """
    Convert xyz coordinates of axis curve to delta-r vectors.

    Parameters
    ----------
    coord : ndarray, shape (N,3)
        The axis curve xyz coordinates in unit of Å (r vectors).

    Returns
    -------
    dr : ndarray, shape (N,3)
        Delta-r vectors (dr[i] = r[i+1] - r[i]).

    See Also
    --------
    dr2coords :
        Convert delta-r vectors to coordinates.
    """
    dr = coord[1:] - coord[:-1]
    return dr
