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Quantum dynamics simulations are becoming a powerful tool for understanding photo-excited molecules. Their poor scaling, however, means that it is hard to study molecules with more than a few atoms accurately, and a major challenge at the moment is the inclusion of the molecular environment. Here, we present a proof of principle for a way to break the two bottlenecks preventing large but accurate simulations. First, the problem of providing the potential energy surfaces for a general system is addressed by parameterizing a standard force field to reproduce the potential surfaces of the molecule's excited-states, including the all-important vibronic coupling. While not shown here, this would trivially enable the use of an explicit solvent. Second, to help the scaling of the nuclear dynamics propagation, a hierarchy of approximations is introduced to the variational multi-configurational Gaussian method that retains the variational quantum wavepacket description of the key quantum degrees of freedom and uses classical trajectories for the remaining in a quantum mechanics/molecular mechanics like approach. The method is referred to as force field quantum dynamics (FF-QD), and a two-state ππ*/nπ* model of uracil, excited to its lowest bright ππ* state, is used as a test case. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) vibronic spectroscopy of molecular trimers is studied theoretically. The solution of the time-dependent Schrödinger equation is carried out with the multi-configurational time-dependent Hartree (MCTDH) method which allows for an efficient propagation of the multicomponent wave functions. 2D-spectra are calculated for H- and J-type aggregates incorporating one or two vibrational modes for each monomer. In performing calculations for monomer, dimer, and trimer systems, it is documented how the vibronic structure of the 2D-spectrum changes upon aggregation. This is of importance for the characterization of aggregation behavior being influenced by experimental conditions such as temperature or concentration. [ABSTRACT FROM AUTHOR]

The first band in the electronic spectrum of pyrrole is calculated from wavepacket propagations performed using the MCTDH method. To do so, two model Hamiltonians are constructed to describe seven low-lying excited electronic states of pyrrole. These Hamiltonians are based on the vibronic coupling model, and are parameterised via fitting to extensive CASPT2 and EOM-CCSD calculations. A detailed analysis of the structure of pyrrole's electronic spectrum in the range 5.5 to 6.5 eV is made. The role of intensity borrowing from transitions to ππ* states by lower-lying 3s and 3p Rydberg states is assessed, and reassignments of much of the spectrum are subsequently made which indicate that most of the states in the spectrum are predominantly Rydberg in character. The resulting conclusions drawn serve to highlight the limitations of assignments based on the matching of calculated vertical excitation energies and the positions of peak maxima observed in electronic spectra. [ABSTRACT FROM AUTHOR]

Time-resolved photoelectron spectroscopy can obtain detailed information about the dynamics of a chemical process on the femtosecond timescale. The resulting signal from such detailed experiments is often difficult to analyze and therefore theoretical calculations are important in providing support. In this paper we continue our work on the competing pathways in the photophysics and photochemistry of benzene after excitation into the 'channel 3' region [R. S. Minns, D. S. N. Parker, T. J. Penfold, G. A. Worth, and H. H. Fielding, Phys. Chem. Chem. Phys. 12, 15607 (2010)] with details of the calculations shown previously, building on a vibronic coupling Hamiltonian [T. J. Penfold and G. A. Worth, J. Chem. Phys. 131, 064303 (2009)] to include the triplet manifold. New experimental data are also presented suggesting that an oscillatory signal is due to a hot band excitation. The experiments show that signals are obtained from three regions of the potential surfaces, three open channels, which are assigned with the help of simulations showing that following excitation into vibrationally excited-states of S1 the wavepacket not only crosses through the prefulvenoid conical intersection back to the singlet ground state, but also undergoes ultrafast intersystem crossing to low lying triplet states. The model is, however, not detailed enough to capture the full details of the oscillatory signal due to the hot band. [ABSTRACT FROM AUTHOR]

The Gaussian-based multiconfiguration time-dependent Hartree (G-MCTDH) method is applied to calculate the S2(ππ*) absorption spectrum of the pyrazine molecule, whose diffuse structure results from the ultrafast nonadiabatic dynamics at the S2-S1 conical intersection. The 24-mode second-order vibronic-coupling model of Raab et al. [J. Chem. Phys. 110, 936 (1999)] is employed, along with 4-mode and 10-mode reduced-dimensional variants of this model. G-MCTDH can be used either as an all-Gaussian approach or else as a hybrid method using a partitioning into primary modes, treated by conventional MCTDH basis functions, and secondary modes described by Gaussian particles. Comparison with standard MCTDH calculations shows that the method converges to the exact result. The variational, nonclassical evolution of the moving Gaussian basis is a key element in obtaining convergence. For high-dimensional systems, convergence is significantly accelerated if the method is employed as a hybrid scheme. [ABSTRACT FROM AUTHOR]

The ground state and the first excited electronic states of the octa-atomic butatriene cation (C[sub 4]H[sub 4][sup +]) exhibit a multidimensional conical intersection. This intersection is energetically low lying and is located in the vicinity of the Franck–Condon region of the neutral molecule’s ground state. The conical intersection thus dominates the nuclear dynamics in ionization processes of this molecule. This is a particularly interesting example of vibronic coupling, introducing what appears to be a new, structured band into the energy spectrum. In this work, the potential energy surfaces and their intersection are investigated by ab initio methods. A diabatic model Hamiltonian including all possible linear, quadratic and bilinear vibronic coupling terms is introduced, with coupling constants determined by a fit to the ab initio data. The nuclear dynamics of all 18 vibrational modes is then evaluated by propagating the wave packet using the multi-configuration time-dependent Hartree method. Finally, the photoelectron spectrum of butatriene is computed and compared with the experimental one. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The absorption spectrum of pyrazine to the S2 electronic state can be usefully described by a 4-mode system interacting with a 20-mode bath. In this paper wave packet propagation techniques, using the multiconfiguration time-dependent Hartree approach, are used to study this problem. The investigation was made in stages so as to study the nature of the wave function needed to correctly describe various properties of this multimode problem: the absorption spectrum; the energy exchange between the system and the bath; and the rate of inter-state crossing. It was found that, despite the relatively weak system–bath coupling, a multiconfigurational wave function was necessary to describe the interaction between the two parts of the problem. While it was not possible to treat the full 24-mode problem with such a wave function, the spectrum for a 14-mode system, which includes all the important bath modes, has been calculated in this way. The results, in agreement with the path integral calculations of Krempl et al. [J. Chem. Phys. 100, 926 (1994)], show that the effect of a model bath linearly coupled to the system is to reduce the vibrational structure of the spectrum, so as to produce a broad envelope analogous to that observed experimentally. The details of the spectrum are however different for the two methods. The effect of introducing anharmonicity to the bath was also studied, with the result that this leads to a yet broader spectrum. [ABSTRACT FROM AUTHOR]