Necessity of eigenstate thermalisation for equilibration towards unique expectation values when starting from generic initial states

We investigate dynamical equilibration of expectation values in closed quantum systems for realistic non-equilibrium initial states. Thereby we find that the corresponding long-time expectation values depend on the initial expectation values if eigenstate thermalisation is violated. An analytical expression for the deviation from the expected ensemble value is derived for small displacements from equilibrium. Additional numerics for magnetisation and energy equilibration in an asymmetric anisotropic spin-(1/2)-ladder demonstrate that the analytical predictions persist beyond the limits of the theory. The results suggest eigenstate thermalisation as a physically necessary condition for initial-state–independent equilibration