minimum of the charge transfer (CT) Guanine–Cytosine stacked dimer in

water solution to the ground electronic state (GS). We adopt the quantum

non-adiabatic theory of electron transfer reactions, where key

ingredients to determine the CR rate are the electronic coupling and the

so-called Franck–Condon density of states (FCWD). In order to compute

the FCWD, we exploited recent developments in the field of the

time-independent and time-dependent simulations of vibronic spectra of

large systems, based on model harmonic potential energy surfaces (PESs).

Both mean-field solvent effects on the PESs and the solvent

reorganization effects on the CR rate were described by implicit

polarizable continuum model. We show that an improper treatment of the

contributions of the inter-base modes to the FCWD results in artefacts

which can change the estimate of the CR rates by orders of magnitude and

we devise a computational protocol in internal coordinates able to

determine “effective normal modes” that separate the stiff (intra-base)

modes and the inter-base ones. The results for the CR rate are

qualitatively consistent with available experimental data. By computing

the CR rate for four different stacking geometries, we show that all the

parameters ruling the CR rate, namely the CT character of the excited

state (ES), the equilibrium position of stiff modes (and therefore the

FCWD), the electronic coupling and the energy gap between the GS and ES

are all strongly dependent on the fluctuation of the dimer structure

along the inter-base modes. On this ground, we discuss some possible

future theoretical developments to achieve a non-phenomenological fully

first-principle estimate of the CR rate that can be directly compared

with experiment.}, keywords={, }, references={}, document_type={, }, affiliation={}, ibbaffiliation={1}, }