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We study the ultrafast time evolution of cyclobutanone excited to the singlet n → Rydberg state through non-adiabatic surface-hopping simulationsperformed at extended multi-state complete active space second-order perturbation (XMS-CASPT2) level of theory. These dynamics predict relaxation to the ground-state with a timescale of 822 ± 45 fs with minimal involvement of the triplets. The major relaxation path to the ground-state involves a three-state degeneracy region and leads to a variety of fragmented photoproducts. We simulate the resulting time-resolved electron-diffraction spectra, which track the relaxation of the excited state and the formation of various photoproducts in the ground state. [ABSTRACT FROM AUTHOR]

A computational protocol based on the complete and restricted active space self-consistent field (CASSCF/RASSCF) methods and their second-order perturbation theory extensions (CASPT2/RASPT2) is employed to benchmark the highly excited-state manifold of the DNA/RNA canonical purine nucleobase guanine in vacuo. Several RASPT2 schemes are tested, displaying a steady convergence of electronic transition energies and dipole moments upon active space enlargement toward the reference values. The outcome allows calibrating and optimizing computational efforts by considering cheaper and more approximate RAS schemes that could enable the characterization of the excited-state manifolds of multi-chromophoric systems, such as DNA/RNA nucleobase dimers or multimers. Simulations of two-dimensional electronic spectra show similar trends to those observed on the other purine nucleobase adenine, deviating from this and other pyrimidine nucleobases in featuring its main excited-state absorption signal, embodied by sizable double HOMO to LUMO excitation contributions, in the UV probing window.

Azobenzene is a prototype and a building block of a class of molecules of extreme technological interest as molecular photo-switches. We present a joint experimental and theoretical study of its response to irradiation with light across the UV to x-ray spectrum. The study of valence and inner shell photo-ionization and excitation processes combined with measurement of valence photoelectron-photoion coincidence and mass spectra across the core thresholds provides a detailed insight into the site- and state-selected photo-induced processes. Photo-ionization and excitation measurements are interpreted via the multi-configurational restricted active space self-consistent field method corrected by second order perturbation theory. Using static modeling, we demonstrate that the carbon and nitrogen K edges of azobenzene are suitable candidates for exploring its photoinduced dynamics thanks to the transient signals appearing in background-free regions of the NEXAFS and XPS. [ABSTRACT FROM AUTHOR]

The electronic transitions of indole, the aromatic UV chromophore of the amino acid tryptophan, are characterized by using state-of-the-art multiconfigurational methods in gas-phase and aqueous solution, revealing the electronic spectrum up to 10eV. Bidimensional near-ultraviolet (2D-NUV) electronic spectra of indole are simulated using the sum-over-states approach, based on ab initio calculations, accounting for different experimental set-ups, including rephasing (KI = - k1 + k2 + k3), quasi-absorptive (PP) and double quantum coherence (KIII = k1 + k2 - k3) signals, and both one-color (2D-NUV) and two-colors (2D-NUV/Vis) regimes. In order to obtain accurate energies of high lying excited states and reliable 2DES spectra, extravalence virtual orbitals have been included using the restricted active space technique. The 2D-NUV spectrum of indole shows off-diagonal signals due to the correlation of the GS→Lb and GS→La transitions, an indole "fingerprint" in the NUV region that differentiate it from other aromatic chromophores in proteins. Further indole-specific transitions are resolved in the whole Vis-NUV range. A background-free region bellow the ionization potential, which shows no absorption signals in the monomer, can be used to resolve charge transfer states in coupled chromophore aggregates with a two-color 2D-NUV/Vis experimental set-up. Fundamental information for design of the 2DES experiments of indole is provided, including possible experimental pulse configurations that improve spectral resolution, revealing anharmonicities and selecting transitions. The proposed 2DES experiments could provide unprecedented level of detail for tracking indole electronic transitions in proteins, laying the groundwork for the use of nonlinear ultrafast optical spectroscopy for the study of protein structure and dynamics in solution. © 2014 Elsevier B.V.

Two-dimensional electronic spectroscopy (2DES) is a cutting-edge technique for investigating with high temporal resolution energy transfer, structure, and dynamics in a wide range of systems in physical chemistry, energy sciences, biophysics, and biocatalysis. However, the interpretation of 2DES is challenging and requires computational modeling. This perspective provides a roadmap for the development of computational tools that could be routinely applied to simulate 2DES spectra of multichromophoric systems active in the UV region (2DUV) using state-of-the-art ab initio electronic structure methods within a quatum mechanics/molecular mechanics (QM/MM) scheme and the sum-over-states (SOS) approach (here called SOS//QM/MM). Multiconfigurational and multireference perturbative methods, such as the complete active space self-consistent field and second-order multireference perturbation theory (CASPT2) techniques, can be applied to reliably calculate the electronic properties of multichromophoric systems. Hybrid QM/MM method and molecular dynamics techniques can be used to assess environmental and conformational effects, respectively, that shape the 2D electronic spectra. DNA and proteins are important biological targets containing UV chromophores. We report ab initio simulation of 2DUV spectra of a cyclic tetrapeptide containing two interacting aromatic side chains, a model system for the study of protein structure and dynamics by means of 2DUV spectroscopy. © 2013 Wiley Periodicals, Inc. Two-dimensional electronic spectroscopy (2DES) is a cutting-edge technique for investigations with high temporal resolution energy transfer, structure, and dynamics in a wide range of systems. The interpretation of 2DES signals is challenging and requires computational modeling. This perspective provides a roadmap for the development of computational tools that could be routinely applied to simulate 2DES spectra of multichromophoric systems active in the UV region (2DUV) using state-of-the-art ab initio electronic structure methods. Copyright © 2013 Wiley Periodicals, Inc.

© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. he ability of nonlinear electronic spectroscopy to track folding/unfolding processes of proteins in solution by monitoring aromatic interactions is investigated by first-principles simulations of two-dimensional (2D) electronic spectra of a model peptide. A dominant reaction pathway approach is employed to determine the unfolding pathway of a tetrapeptide, which connects the initial folded configuration with stacked aromatic side chains and the final unfolded state with distant noninteracting aromatic residues. The p-stacking and excitonic coupling effects are included through ab initio simulations based on multiconfigurational methods within a hybrid quantum mechanics/molecular mechanics scheme. It is shown that linear absorption spectroscopy in the ultraviolet (UV) region is unable to resolve the unstacking dynamics characterized by the three-step process: T-shaped!twisted offset stacking! unstacking. Conversely, pump-probe spectroscopy can be used to resolve aromatic interactions by probing in the visible region, the excited-state absorptions (ESAs) that involve charge-transfer states. 2DUV spectroscopy offers the highest sensitivity to the unfolding process, by providing the disentanglement of ESA signals belonging to different aromatic chromophores and high correlation between the conformational dynamics and the quartic splitting.

We present an extrapolation scheme for the correlation energy in many-body theory that requires only a relatively small fraction of the orbitals spanning the virtual space but recovers nearly the whole dynamic correlation energy, independently of the single- or multi-reference nature of the problem. Applications to both ground-state chemistry and photochemistry are discussed that clearly show how such an extrapolation scheme can be used to overcome the scalingwalls in ab initio quantum chemistry arising from the sheer number of molecular orbitals needed for very accurate calculations. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

MC-SCF and multireference Moller-Plesset methods were used to compute the minimum energy path for photoisomerization of the minimal retinal protonated Shiff base model tZt-penta-3,5-dieniminium cation (cis-Csub4Hsub6NHsub2+). The molecule was shown to undergo a barrierless relaxation upon excitation to the spectroscopic state, where ground states are conically intersecting, providing an efficient nonadiabatic cis-trans isomerization. This indicates that cis-Csub5Hsub6NHsub2+ is a suitable ab initio model for the rationalization of the observed ultrafast isomerization of the retinal chromophore in rhodopsin.

This work investigates the capability of time-dependent density functional response theory to describe excited state potential energy surfaces of conjugated organic molecules. Applications to linear polyenes, aromatic systems, and the protonated Schiff base of retinal demonstrate the scope of currently used exchange-correlation functionals as local, adiabatic approximations to time-dependent Kohn–Sham theory. The results are compared to experimental and ab initio data of various kinds to attain a critical analysis of common problems concerning charge transfer and long range (nondynamic) correlation effects. This analysis goes beyond a local investigation of electronic properties and incorporates a global view of the excited state potential energy surfaces. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ab initio multistate second-order perturbation theory calculations were used to map the reaction path for the ultrafast photochemical electrocyclic ring opening of cyclohexa-1, 3-diene. It resulted in the deactivation of S1/S0 conical intersection.