Accurate high-harmonic spectra from time-dependent two-particle reduced   density matrix theory
  The accurate description of the non-linear response of many-electron systems to strong-laser fields remains a major challenge. Methods that bypass the unfavorable exponential scaling with particle number are required to address larger systems. In this paper we present a fully three-dimensional implementation of the time-dependent two-particle reduced density matrix (TD-2RDM) method for many-electron atoms. We benchmark this approach by a comparison with multi-configurational time-dependent Hartree-Fock (MCTDHF) results for the harmonic spectra of beryllium and neon. We show that the TD-2RDM is very well-suited to describe the non-linear atomic response and to reveal the influence of electron-correlation effects. 
Fabian Lackner, Iva B\v{r}ezinov\'a, Takeshi Sato, Kenichi L.   Ishikawa, Joachim Burgd\"orfer
physics.atom-ph