def _getLaplaceCovar(self):
        """
        Internal function for estimating parameter uncertainty
        Returns:
            the
        """
        assert self.init,        'GP not initialised'
        assert self.fast==False, 'Not supported for fast implementation'

        if self.cache['Sigma'] is None:
            self.cache['Sigma'] = sp.linalg.inv(self._getHessian())
        return self.cache['Sigma']