def file_gen(fil_name):
    """
    Function opens the input file.

    Parameters
    ------------
    fil_name : str
        name of the input lammps dump file
    Returns
    ----------
    fiw:
        the opened lammps dump file
    fil_name: str
        name of the input lammps dump file
    """
    fiw = open(fil_name, 'w')
    return fiw, fil_name