def _position_autocorrelation_fitting_eqn(t, Gamma, AngTrapFreq):
    """
    The value of the fitting equation:
    exp(-t*Gamma/2) * (cos(t* sqrt(Omega**2 - Gamma**2 /4)) + Gamma* sin(t* sqrt(Omega**2-Gamma**2 /4))/(2* sqrt(Omega**2 - Gamma**2 /4)))
    [eqn 4.20 taken from DOI: DOI: 10.1007/978-1-4614-6031-2]
    to be fit to the autocorrelation-exponential decay

    Parameters
    ----------
    t : float
        time 
    Gamma : float
        Big Gamma (in radians), i.e. damping 
    AngTrapFreq : float
        Angular Trapping Frequency in Radians

    Returns
    -------
    Value : float
        The value of the fitting equation
    """
    return _np.exp(-t*Gamma/2)* ( _np.cos(t* _np.sqrt(AngTrapFreq**2-Gamma**2/4)) + Gamma* _np.sin(t* _np.sqrt(AngTrapFreq**2-Gamma**2/4))/(2* _np.sqrt(AngTrapFreq**2-Gamma**2/4)) )