def einsteinA(S: float, frequency: float):
    """
    Calculate the Einstein A coefficient for a transition with
    specified transition frequency and intrinsic linestrength.
    
    Parameters
    ----------
    S : float
        Intrinsic linestrength; unitless
    frequency : float
        Transition frequency in MHz
    
    Returns
    -------
    float
        Einstein A coefficient in units of per second
    """
    # Prefactor is given in the PGopher Intensity formulae
    # http://pgopher.chm.bris.ac.uk/Help/intensityformulae.htm
    # Units of the prefactor are s^-1 MHz^-3 D^-2
    # Units of Einstein A coefficient should be in s^-1
    prefactor = 1.163965505e-20
    return prefactor * frequency ** 3.0 * S