  Recently introduced time-dependent renormalized natural orbital theory
(TDRNOT) is tested on non-sequential double ionization (NSDI) of a numerically
exactly solvable one-dimensional model He atom subject to few-cycle, 800-nm
laser pulses. NSDI of atoms in strong laser fields is a prime example of
non-perturbative, highly correlated electron dynamics. As such, NSDI is an
important "worst-case" benchmark for any time-dependent few and many-body
technique beyond linear response. It is found that TDRNOT reproduces the
celebrated NSDI "knee," i.e., a many-order-of-magnitude enhancement of the
double ionization yield (as compared to purely sequential ionization) with only
the ten most significant natural orbitals (NOs) per spin. Correlated
photoelectron spectra - as "more differential" observables - require more NOs.
