Nonadiabatic events, in which the Born-Oppenheimer approximation breaks down, are ubiquitous in chemistry and biology. It is now widely accepted that they are facilitated by conical intersections (CIs), actual degeneracies between electronic states. We review the basic theory of CIs and how they can be studied using modern quantum chemistry and nuclear dynamics. We highlight their importance by presenting their role in radiationless decay pathways present in the building blocks of DNA and proteins. The presence of CIs may contribute to the photostability of these important biomolecules.

We report simulations of laser-driven many-electron dynamics by means of the time-dependent configuration interaction singles (doubles) approach. The method accounts for the correlation of ground and excited states, is capable of describing explicitly time-dependent, nonlinear phenomena, and is systematically improvable. Lithium cyanide serves as a molecular test system in which the charge distribution and hence the dipole moment are shown to be switchable, in a controlled fashion, by (a series of) laser pulses which induce selective, state-to-state electronic transitions. One focus of our time-dependent calculations is the question of how fast the transition from the ionic ground state to a specific excited state that is embedded in a multitude of other states can be made, without creating an electronic wave packet.

Ionization rates of molecules have been modeled with time-dependent configuration interaction simulations using atom centered basis sets and a complex absorbing potential. The simulations agree with accurate grid-based calculations for the ionization of hydrogen atom as a function of field strength and for charge resonance enhanced ionization of H2(+) as the bond is elongated. Unlike grid-based methods, the present approach can be applied to simulate electron dynamics and ionization in multi-electron polyatomic molecules. Calculations on HCl(+) and HCO(+) demonstrate that these systems also show charge resonance enhanced ionization as the bonds are stretched.

In this paper we report the calculation of molecular electric response properties with the help of explicitly time-dependent configuration interaction (TD-CI) methods. These methods have the advantage of being applicable (within the limitations of the time-dependent Schrodinger equation) to time-dependent perturbations of arbitrary shape and strength. Three variants are used to solve the time-dependent electronic Schrodinger equation, namely, the TD-CIS (inclusion of single excitations only), TD-CISD (inclusion of single and double excitations), and TD-CIS(D) (single excitations and perturbative treatment of double excitations) methods and applied for illustration to small molecules, H(2) and H(2)O. In the calculation, slowly varying off-resonant electric fields are applied to the molecules and linear (polarizabilities) and nonlinear (hyperpolarizabilities, harmonic generation) response properties are determined from the time-dependent dipole moments.

Selective bond breaking may be achieved in two steps. First, an intense ultrashort, i.e. few-cycle infrared (IR) laser pulse creates momentum along the bond to be broken. Another ultrashort few-cycle ultraviolet (UV) laser pulse then induces a Franck-Condon (FC)-type transition from the electronic ground to an excited state. The initial bond selective momentum is approximately conserved during this FC-type transition, thus causing a stretch and finally a break in the specific bond. Bond selectivity via few-cycle IR + UV laser pulses can be achieved even if the forces of the excited molecule are not bond selective in the domain of the FC-type transition. The mechanism is demonstrated by means of quantum simulations of IR + UV laser driven wavepackets of the model system, HOD* ! H + OD versus HO + D.