def closestsites(struct_blk, struct_def, pos):
    """
    Returns closest site to the input position
    for both bulk and defect structures
    Args:
        struct_blk: Bulk structure
        struct_def: Defect structure
        pos: Position 
    Return: (site object, dist, index)
    """
    blk_close_sites = struct_blk.get_sites_in_sphere(pos, 5, include_index=True)
    blk_close_sites.sort(key=lambda x:x[1])
    def_close_sites = struct_def.get_sites_in_sphere(pos, 5, include_index=True)
    def_close_sites.sort(key=lambda x:x[1])

    return blk_close_sites[0], def_close_sites[0]