def atom_information(xyz_file):
    """
    extract information about system: number of atoms and atom numbers
    """
    atom_numbers = []
    with open(xyz_file, 'r') as _file:
        line = _file.readline()
        n_atoms = int(line.split()[0])
        _file.readline()
        for i in range(n_atoms):
            line = _file.readline().split()
            atom_number = line[0]
            atom_numbers.append(atom_number)

    return n_atoms, atom_numbers