Invariance correction to Grad’s equations: where to go beyond approximations?

We review some recent developments of Grad’s approach to solving the Boltzmann equation and creating a reduced description. The method of the invariant manifold is put forward as a unified principle to establish corrections to Grad’s equations. A consistent derivation of regularized Grad’s equations in the framework of the method of the invariant manifold is given. A new class of kinetic models to lift the finite-moment description to a kinetic theory in the whole space is established. Relations of Grad’s approach to modern mesoscopic integrators such as the entropic lattice Boltzmann method are also discussed.