def retr_sites(xtalstr,ini0):
    """
    Retrieve the atomic positions and elements in fractional coordinates.

    The atomic positions and elements are extracted from the structures
    instance. They are added to the dictionary passed in. The updated dictionary
    is returned.
    """
    ini0["natom"]  = xtalstr.natom
    # print(xtalstr.frac_coords)    
    ini0["a_coeff"] = xtalstr.frac_coords[0,:]
    ini0["b_coeff"] = xtalstr.frac_coords[1,:]
    ini0["c_coeff"] = xtalstr.frac_coords[2,:]
    # print(ini0["a_coeff"])
    # print(ini0["b_coeff"])
    # print(ini0["c_coeff"])
    ini0["islist"] = xtalstr.equivalent_atoms
    ini0["symbol"] = xtalstr.species
    return ini0