def derivative_local_R(df_ep, df_ki, fvirg):
    """ Calculates the derivative of the local strain-to-kinetic function.

    Args:
        df_ep (:obj:`numpy.array`): Elastic potential energy derivative.
        df_ki (:obj:`numpy.array`): Kinetic energy derivative.
        fvirg (:obj:`float`): Local strain-to-kinetic function.

    Returns:
        Derivative of the local strain-to-kinetic function function.
    """
    #fvirg = (ep,ki)
    return df_ep * (1/fvirg[1]) - (fvirg[0]/fvirg[1]**2)*df_ki