def half_period_uncoupled(t, x, par):
    """
    Returns the event function 
    
    Zero of this function is used as the event criteria to stop integration along a trajectory. For symmetric periodic orbits this acts as the half period event when the momentum coordinate is zero.

    Parameters
    ----------
    t : float
        time instant

    x : float (list of size 4)
        phase space coordinates at time instant t

    parameters : float (list)
        model parameters

    Returns
    -------
    float 
        event function evaluated at the phase space coordinate, x, and time instant, t.

    """
    
    return x[3]