def energy_string_to_float( string ):
    """
    Convert a string of a calculation energy, e.g. '-1.2345 eV' to a float.

    Args:
        string (str): The string to convert.
  
    Return
        (float) 
    """
    energy_re = re.compile( "(-?\d+\.\d+)" )
    return float( energy_re.match( string ).group(0) )