Quantum Mechanical Insights into Biological Processes at the Electronic Level - Computational Modeling of Biological Systems - page 161

Fig. 18 Summary of pathways of excited cytosine. The arrows indicate the passage between
different electronic states, with the observed time lapse and the number of trajectories shown.
The S 1 electronic state passes through regions with three different characters . ; n O ,and
n N ), and electronic-state quenching to S 0 occurs through three different conical intersections.
Adapted with permission from [ 41 ]
popular approach. QM/MM has many different versions. Some of them are static,
like ONIOM, and some include statistical mechanical sampling and dynamics.
QM/MM calculations are instrumental in the assessment of reaction mechanisms
in complex biological molecules in solution. We further considered the treatment of
excited electronic states and electronic detachment energies with chemical accuracy.
Excited states are involved in photoprocesses characteristic of biomolecules. Also,
electronic spectroscopy provides an invaluable means of analysis of biomolecules.
Calculations of this kind require methods such as TD-DFT, EOM-CC, and mul-
tireference methods. Using these techniques allows for the prediction of electronic
spectra within 0.1-0.2 eV from the experimental values. Finally, dynamical sim-
ulations were considered as the ultimate approach to gain mechanistic insights
into biochemical processes. Dynamical simulations may operate on a single PES
(adiabatic dynamics), or on multiple PESs (nonadiabatic dynamics). The techniques