def is_orca(f):
    """
    Check if the input file is ORCA file format.

    Parameters
    ----------
    f : str
        User input filename.

    Returns
    -------
    bool : bool
        If file is ORCA output file, return True.

    See Also
    --------
    get_coord_orca :
        Find atomic coordinates of molecule from ORCA file.

    Examples
    --------
    >>> orca.out
    ---------------------------------
    CARTESIAN COORDINATES (ANGSTROEM)
    ---------------------------------
      C      0.009657    0.000000    0.005576
      C      0.009657   -0.000000    1.394424
      C      1.212436   -0.000000    2.088849
      C      2.415214    0.000000    1.394425
      C      2.415214   -0.000000    0.005575
    ...

    """
    orca_file = open(f, 'r')
    nline = orca_file.readlines()
    for i in range(len(nline)):
        if 'CARTESIAN COORDINATES (ANGSTROEM)' in nline[i]:
            return True
    return False
