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Recent theoretical investigations claim that tailored laser pulses may selectively steer benzene's aromatic ground state to localized non-aromatic excited states. For instance, it has been shown that electronic wavepackets, involving the two lowest electronic eigenstates, exhibit subfemtosecond charge oscillation between equivalent Kekule resonance structures. In this contribution, we show that such dynamical electron-localization in the molecule-fixed frame contravenes the principle of the indistinguishability of identical particles. This breach stems from a total omission of the nuclear degrees of freedom, giving rise to nonsymmetric electronic wavepackets under nuclear permutations. Enforcement of the latter leads to entanglement between the electronic and nuclear states. To obey quantum statistics, the entangled molecular states should involve compensating nuclear-permutation symmetries. This in turn engenders complete quenching of dynamical electron-localization in the molecule-fixed frame. Indeed, for the (six-fold) equilibrium geometry of benzene, group-theoretic analysis reveals that any electronic wavepacket exhibits a (D-6h) totally symmetric electronic density, at all times. Thus, our results clearly show that the six carbon atoms, and the six C-C bonds, always have equal Mulliken charges, and equal bond orders, respectively. However, electronic wavepackets may display dynamical localization of the electronic density in the space-fixed frame, whenever they involve both even and odd space-inversion (parity) or permutation-inversion symmetry. Dynamical spatial-localization can be probed experimentally in the laboratory frame, but it should not be deemed equivalent to charge oscillation between benzene's identical electronic substructures, such as Kekule resonance structures.
Modern laser technology and ultrafast spectroscopies have pushed the timescales for detecting and manipulating dynamical processes in molecules from the picosecond over femtosecond domains, to the attosecond regime (1 as = 10(-18) s). This way, real-time dynamics of electrons after their photoexcitation can be probed and manipulated. In particular, experiments are moving more and more from atomic and solid state systems to molecules, opening the fields of "molecular electron dynamics" and "attosecond chemistry." Also on the theory side, powerful quantum dynamical tools have been developed to rationalize experiments on ultrafast electron dynamics in molecular species. <br /> In this contribution, we concentrate on theoretical aspects of ultrafast electron dynamics in molecules, mostly driven by lasers. The dynamics will be described with the help of wavefunction-based ab initio methods such as time-dependent configuration interaction (TD-CI) or the multiconfigurational time-dependent Hartree-Fock (MCTDHF) methods. Besides a survey of the methods and their extensions toward, e.g., treatment of ionization, laser pulse optimization, and open quantum systems, two specific examples of applications will be considered: The creation and/or dynamical fate of electronic wavepackets, and the nonlinear optical response to laser pulse excitation in molecules by high harmonic generation (HHG).