Modeling Photoinduced Events and Non-linear Spectroscopy in Complex Multichromophoric Systems

What are the processes activated by light when it impinges on a sample of light- sensitive molecules in mutual interaction? How can this information be accessed from both the experimental and theoretical sides? This work is aimed at answering at these specific questions. In order to accomplish this goal, we use state-of-the-art computational methods and develop novel theoretical approaches for investigating static and dynamical properties of networks of interacting molecular organic chromophores, and compute their spectroscopy. We focus in particular on the simulation of non-linear time-resolved techniques, such as the pump-probe and the two dimensional electronic spectroscopy. These approaches have been proven to be fundamental tools to track the system photoinduced dynamics with extremely high time and spectral resolution and disentangle contributions from different system components. We conclude that the synergic combination of “independent” results from accurate quantum chemical calculations and detailed spectroscopic experiments is the way to reach a reliable map of the activated energy transfer processes and to gain new physical insights into the system properties

Near-ultraviolet circular dichroism and two-dimensional spectroscopy of polypeptides

A fully quantitative theory of the relationship between protein conformation and optical spectroscopy would facilitate deeper insights into biophysical and simulation studies of protein dynamics and folding. In contrast to intense bands in the far-ultraviolet, near-UV bands are much weaker and have been challenging to compute theoretically. We report some advances in the accuracy of calculations in the near-UV, which were realised through the consideration of the vibrational structure of the electronic transitions of aromatic side chains

The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry

The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations

Predicting Charge Mobility of Organic Semiconductors with Complex Morphology

We introduce a mesoscopic model to predict the charge mobility of organic semiconductors characterized by a coexistence of crystalline and amorphous phases. First, we validate our scheme by reproducing the trends in charge mobility observed in thin films of poly(3-hexylthiophene) (P3HT) polymers. Next, we address the problem of predicting the morphologies that lead to the highest mobility. Our main finding is the identification of a region of the model's multidimensional parameter space, in which the charge mobility effectively depends on a single morphological feature: the average intercrystallite distance. This scaling behavior provides insight into the main physical mechanism limiting charge mobility in organic semiconductors. Our proposed framework can be adapted to study a wide class of polymeric systems and used to guide the manufacturing of new, high-performing organic semiconductor materials

Soft X-ray Spectroscopy Simulations with Multiconfigurational Wave Function Theory: Spectrum Completeness, Sub-eV Accuracy, and Quantitative Reproduction of Line Shapes.

Multireference methods are known for their ability to accurately treat states of very different nature in many molecular systems, facilitating high-quality simulations of a large variety of spectroscopic techniques. Here, we couple the multiconfigurational restricted active space self-consistent field RASSCF/RASPT2 method (of the CASSCF/CASPT2 methods family) to the displaced harmonic oscillator (DHO) model, to simulate soft X-ray spectroscopy. We applied such an RASSCF/RASPT2+DHO approach at the K-edges of various second-row elements for a set of small organic molecules that have been recently investigated at other levels of theory. X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) are simulated with a sub-eV accuracy and a correct description of the spectral line shapes. The method is extremely sensitive to the observed spectral shifts on a series of differently fluorinated ethylene systems, provides spectral fingerprints to distinguish between stable conformers of the glycine molecule, and accurately captures the vibrationally resolved carbon K-edge spectrum of formaldehyde. Differences with other theoretical methods are demonstrated, which show the advantages of employing a multireference/multiconfigurational approach. A protocol to systematically increase the number of core-excited states considered while maintaining a contained computational cost is presented. Insight is eventually provided for the effects caused by removing core-electrons from a given atom in terms of bond rearrangement and influence on the resulting spectral shapes within a unitary orbital-based framework for both XPS and XANES spectra

Modeling Nonperturbative Field-Driven Vibronic Dynamics: Selective State Preparation and Nonlinear Spectroscopy

The partially linearized density matrix formalism for nonadiabatic dynamics is adapted to incorporate a classical external electromagentic field into the system Hamiltonian. This advancement encompasses the possibility of describing field-driven dynamics and computing a variety of linear and nonlinear spectroscopic signals beyond the perturbative limit. The capabilities of the developed approach are demonstrated on a simple two-state vibronic model coupled to a bath, for which we (a) perform an exhaustive search in the field parameter space for optimal state preparation and (b) compute time-resolved transient absorption spectroscopy to monitor the effect of different pulse shapes on measurable experimental signals. While no restrictions on the form of the field have to be assumed, we focus here on Gaussian shaped (linearly) chirped pulses