def parse_property_string(prop_str):
    """
    Generate valid property string for extended xyz files.
    (ref. https://libatoms.github.io/QUIP/io.html#extendedxyz)

    Args:
        prop_str (str): Valid property string, or appendix of property string

    Returns:
        valid property string
    """
    if prop_str.startswith("Properties="):
        return prop_str
    return "Properties=species:S:1:pos:R:3:" + prop_str