Computation of the S1 ← S0 Vibronic Absorption Spectrum of Formaldehyde by Variational Gaussian Wavepacket and Semiclassical IVR Methods.

The vibronic absorption spectrum of the electric dipole forbidden and vibronically allowed S1(1 A2) ← S0(1 A1) transition of formaldehyde is calculated by Gaussian wavepacket and semiclassical methods, along with numerically exact reference calculations, using the potential energy surface of Fu, Shepler, and Bowman ( J. Am. Chem. Soc. 2011, 133, 7957). Specifically, the variational multiconfigurational Gaussian (vMCG) approach and the Herman-Kluk semiclassical initial value representation (HK-SCIVR) are compared to assess the accuracy and convergence of these methods, benchmarked against numerically exact time-dependent wavepacket propagation (TDWP) on the reference potential energy surface. The vMCG calculation is shown to converge quite well with about 100 variationally evolving Gaussian functions and using a local cubic expansion instead of the conventional local harmonic approximation. By contrast, the HK-SCIVR approach with ∼105 trajectories reproduces the vibrationally structured spectral envelope correctly but yields a strongly broadened spectrum. The comparison of the computed absorption spectrum with experiment shows that the relevant vibronic progressions are reasonably reproduced by all computations, but deviations of the order of 10-100 cm-1 occur, underscoring that both electronic structure calculations and dynamical approaches remain challenging in the calculation of typical small-molecule excited-state spectra by trajectory-based methods.

Multi-configurational Ehrenfest simulations of ultrafast nonadiabatic dynamics in a charge-transfer complex.

Multi-configurational Ehrenfest (MCE) approaches, which are intended to remedy the lack of correlations in the standard mean-field Ehrenfest method, have been proposed as coherent-state based ansätze for quantum propagation [D. V. Shalashilin, J. Chem. Phys. 130, 244101 (2009)] and also as the classical limit of the variational Gaussian-based multiconfiguration time dependent Hartree (G-MCTDH) method [S. Römer and I. Burghardt, Mol. Phys. 111, 3618 (2013)]. In the present paper, we establish the formal connection between these schemes and assess the performance of MCE for a coherent-state representation of the classical-limit subsystem. As a representative model system, we address the ultrafast, coherent charge transfer dynamics in an oligothiophene-fullerene donor-acceptor complex described by a two-state linear vibronic coupling model. MCE calculations are compared with reference calculations performed with the MCTDH method, for 10-40 vibrational modes. Beyond a dimensionality of 10 modes, it is shown that the correct representation of electronic coherence depends crucially on the sampling of initially unoccupied Gaussians.

Controlling charge separation and recombination by chemical design in donor-acceptor dyads.

Conjugated donor-acceptor block co-oligomers that self-organize into D-A mesomorphic arrays have raised increasing interest due to their potential applications in organic solar cells. We report here a combined experimental and computational study of charge transfer (CT) state formation and recombination in isolated donor-spacer-acceptor oligomers based on bisthiophene-fluorene (D) and perylene diimide (A), which have recently shown to self-organize to give a mesomorphic lamellar structure at room temperature. Using femtosecond transient absorption spectroscopy and Time-Dependent Density Functional Theory in combination with the Marcus-Jortner formalism, the observed increase of the CT lifetimes is rationalized in terms of a reduced electronic coupling between D and A brought about by the chemical design of the donor moiety. A marked dependence of the CT lifetime on solvent polarity is observed, underscoring the importance of electrostatic effects and those of the environment at large. The present investigation therefore calls for a more comprehensive design approach including the effects of molecular packing.

Molecular Packing Determines Charge Separation in a Liquid Crystalline Bisthiophene-Perylene Diimide Donor-Acceptor Material.

Combined electronic structure and quantum dynamical calculations are employed to investigate charge separation in a novel class of covalently bound bisthiophene-perylene diimide type donor-acceptor (DA) co-oligomer aggregates. In an earlier spectroscopic study of this DA system in a smectic liquid crystalline (LC) film, efficient and ultrafast (subpicosecond) initial charge separation was found to be followed by rapid recombination. By comparison, the same DA system in solution exhibits ultrafast resonant energy transfer followed by slower (picosecond scale) charge separation. The present first-principles study explains these contrasting observations, highlighting the role of an efficient intermolecular charge-transfer pathway that results from the molecular packing in the LC phase. Despite the efficiency of this primary charge-transfer step, long-range charge separation is impeded by a comparatively high Coulomb barrier in conjunction with small electron- and hole-transfer integrals. Quantum dynamical calculations are carried out for a fragment-based model Hamiltonian, parametrized by ab initio second-order Algebraic Diagrammatic Construction (ADC(2)) and Time-Dependent Density Functional Theory (TDDFT) electronic structure calculations. Simulations of coherent vibronic quantum dynamics for up to 156 electronic states and 48 modes are performed using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method. Excellent agreement with experimentally determined charge separation time scales is obtained, and the spatially coherent nature of the dynamics is analyzed.

Structure determination from powder data without prior indexing, using a similarity measure based on cross-correlation functions.

A method to refine organic crystal structures from powder diffraction data with incorrect lattice parameters has been developed. The method is based on the similarity measure developed by de Gelder et al. [J. Comput. Chem. (2001), 22, 273-289], using the cross- and auto-correlation functions of a simulated and an experimental powder pattern. The lattice parameters, molecular position, molecular orientation and selected intramolecular degrees of freedom are optimized until the similarity measure reaches a maximum; subsequently, a Rietveld refinement is carried out. The program FIDEL (FIt with DEviating Lattice parameters) implements this method. The procedure is also suitable for unindexed powder data, powder diagrams of very low quality and powder diagrams of non-phase-pure samples. Various applications are shown, including structure determinations from powder data using crystal structure predictions by standard force-field methods. Other useful applications include the automatic structure determination from powder data starting from the crystal structures of isostructural compounds (e.g. a solvate, hydrate or chemical derivative), or from crystal data measured at a different temperature or pressure.

Simulation of photoelectron spectra using the reflection principle in combination with unrestricted excitation ADC2 to assess the accuracy of excited-state calculations.

The gas-phase photoelectron spectra of ethene, formaldehyde, formic acid and difluoromethane are simulated using the reflection principle and the unrestricted second-order algebraic diagrammatic construction [UADC(2)] scheme of the polarization propagator for the computation of the vertical-excited states of the cations at the equilibrium geometry of the parent neutral molecule. Comparison is made with experimental spectra and the established highly accurate ionization IP-ADC(3) theory to gain insight into the accuracy and applicability of recently developed excitation UADC schemes. Within UADC(2), we distinguish between the strict and extended schemes UADC(2)-s and UADC(2)-x. While the latter approach is found to slightly underestimate the experimental photoelectron spectra by 0.3 eV and can thus be regarded as a reliable scheme within the limits of the applied reflection principle and the underlying approximations, the UADC(2)-s scheme tends to overestimate the excitation energies by about 0.5 eV. Time-dependent density functional theory is also applied in combination with the standard B3LYP xc functional and turns out to be a useful computational tool for the simulation of the photoelectron spectra of the studied species.