Frozen-density embedding theory with average solvent charge densities from explicit atomistic simulations.

Besides molecular electron densities obtained within the Born-Oppenheimer approximation (ρB(r)) to represent the environment, the ensemble averaged density (〈ρB〉(r)) is also admissible in frozen-density embedding theory (FDET) [Wesolowski, Phys. Rev. A, 2008, 77, 11444]. This makes it possible to introduce an approximation in the evaluation of the solvent effect on quantum mechanical observables consisting of replacing the ensemble averaged observable by the observable evaluated at ensemble averaged ρB(r). This approximation is shown to affect negligibly the solvatochromic shift in the absorption of hydrated acetone. The proposed model provides a continuum type of representation of the solvent, which reflects nevertheless its local structure, and it is to be applied as a post-simulation analysis tool in atomistic level simulations.

Pushing the frontiers of first-principles based computer simulations of chemical and biological systems.

The Laboratory of Computational Chemistry and Biochemistry is active in the development and application of first-principles based simulations of complex chemical and biochemical phenomena. Here, we review some of our recent efforts in extending these methods to larger systems, longer time scales and increased accuracies. Their versatility is illustrated with a diverse range of applications, ranging from the determination of the gas phase structure of the cyclic decapeptide gramicidin S, to the study of G protein coupled receptors, the interaction of transition metal based anti-cancer agents with protein targets, the mechanism of action of DNA repair enzymes, the role of metal ions in neurodegenerative diseases and the computational design of dye-sensitized solar cells. Many of these projects are done in collaboration with experimental groups from the Institute of Chemical Sciences and Engineering (ISIC) at the EPFL.

Mixed quantum mechanical/molecular mechanical (QM/MM) simulations of adiabatic and nonadiabatic ultrafast phenomena.

A thorough theoretical description of ultrafast phenomena that occur in complex systems constitutes a formidable challenge. It not only necessitates the use of quantum mechanical methods that can describe ground and possibly even electronically excited state potential energy surfaces with sufficient accuracy but also calls for approaches that can take the real-time dynamics of a system and the coupling between its electronic and nuclear degrees of freedom fully into account. Over the last years, our group has been active in the development of mixed quantum mechanical/molecular mechanical (QM/MM) methods for the in situ simulations of dynamical phenomena in ground and excited states within the adiabatic (Born-Oppenheimer) approximation. Recently, we have extended our theoretical tools with the explicit inclusion of nonadiabatic effects in the framework of Ehrenfest dynamics and Tully's fewest switches surface hopping. These extensions allow the theoretical description of nonadiabatic ultrafast phenomena in the gas phase as well as in solution, and complex biological environments.

Nonadiabatic coupling vectors for excited states within time-dependent density functional theory in the Tamm-Dancoff approximation and beyond.

Recently, we have proposed a scheme for the calculation of nonadiabatic couplings and nonadiabatic coupling vectors within linear response time-dependent density functional theory using a set of auxiliary many-electron wavefunctions [I. Tavernelli, E. Tapavicza, and U. Rothlisberger, J. Chem. Phys. 130, 124107 (2009)]. As demonstrated in a later work [I. Tavernelli, B. F. E. Curchod, and U. Rothlisberger, J. Chem. Phys. 131, 196101 (2009)], this approach is rigorous in the case of the calculation of nonadiabatic couplings between the ground state and any excited state. In this work, we extend this formalism to the case of coupling between pairs of singly excited states with the same spin multiplicity. After proving the correctness of our formalism using the electronic oscillator approach by Mukamel and co-workers [S. Tretiak and S. Mukamel, Chem. Rev. (Washington, D.C.) 102, 3171 (2002)], we tested the method on a model system, namely, protonated formaldimine, for which we computed S(1)/S(2) nonadiabatic coupling vectors and compared them with results from high level (MR-CISD) electronic structure calculations.