def str2l(orbit):
    """
    Transform orbital type into orbital qauntum number: 's'->0, 'p'->1, ...
    
    Parameters
    ----------
    orbit : string 
        Orbital type as a string. For example `orbit` = 's', 'p', 'd', '5d', 
        'f', '7f'
    
    Returns
    ----------
    l_orbit : integer
        Orbital quantum number. For the same ordering of `orbit` orbital 
        quantum numbers will be: 0, 1, 2, 2, 3, 3 
    """
    
    if orbit=='s':
        return 0
    elif orbit=='p':
        return 1
    elif orbit=='d' or orbit=='5d':
        return 2
    elif orbit=='f' or orbit=='7f':
        return 3