Compute the minimisation target, not normalised.

def _tm(self, theta, phi, psi, dx, dy, dz):
        """
        Compute the minimisation target, not normalised.
        """
        matrix = self.get_matrix(theta, phi, psi, dx, dy, dz)
        coord = matrix.dot(self.coord2)
        dist = coord - self.coord1

        d_i2 = (dist * dist).sum(axis=0)

        tm = -(1 / (1 + (d_i2 / self.d02)))

        return tm