def El2EstVol(Elements):
    """Estimate volume from molecular formula; assumes atom volume = 10A^3
    
    :param dict Elements: elements in molecular formula; 
        each must contain 
        Num: number of atoms in formula
    
    :returns: float volume: estimate of molecular volume in A^3
    
    """
    vol = 0
    for El in Elements:
        vol += 10.0 * Elements[El]['Num']
    return vol
