• Surface Hopping
• Quantum Mechanics
• QM/MM
• Molecular Dynamics
• Exciton Model

The detailed knowledge of the processes following photoexcitation is
extremely important, but the understanding of the underlying mechanisms, as
well as the proper description of the corresponding non-adiabatic dynamics
are still real challenges for theoretical chemistry. This is true especially for
biologically relevant systems, where not only the chromophores, but also
the complex environment need to be included in the simulation in order to
obtain a correct picture. The environment, in fact, can play an important
role in the mechanisms of the photoinduced processes by tuning both the
electronic and structural properties of the chromophore(s). In this thesis, we
present novel computational multiscale models, based on a hybrid quantum
mechanical/molecular mechanical (QM/MM) description of the system able
to study such processes.
First, the theoretical formulation and implementation of excited states
gradients of a polarizable QM/MM method using an induced dipole scheme
are introduced within the framework of time-dependent density functional
theory. Secondly, the development and the implementation of a novel exciton
approach are presented and discussed, where the environment is included
via an electrostatic or mechanical embedding scheme. Consistent hybrid
QM/MM energies, gradients and non-adiabatic couplings were developed,
allowing ab initio non-adiabatic dynamics simulations in multichromophoric
systems, using the surface hopping approach.
The methods are finally applied to the study of (i) the effects of a DNA
pocket on the excitation process and the corresponding excited state properties
of an organic dye, and (ii) the excitation energy transfer process in an
orthogonal molecular dyad.