Spectroscopy from quantum dynamics: a mixed wave function/analytical line shape functions approach

Quantum dynamics is the natural framework in which accurate simulation of spectroscopy of nonadiabatically coupled molecular systems can be obtained. Even if efficient quantum dynamics approaches have been developed, the number of degrees of freedom that need to be considered in realistic systems is typically too high to explicitly account for all of them. Moreover, in open-quantum systems, a quasi-continuum of low-frequency environment modes need to be included to get a proper description of the spectral bands. Here, we describe an approach to account for a large number of modes, based on their partitioning into two sets: a set of dynamically relevant modes (so-called active modes) that are treated explicitly in quantum dynamics, and a set of modes that are only spectroscopically relevant (so-called spectator modes), treated via analytical line shape functions. Linear and nonlinear spectroscopy for a realistic model system is simulated, providing a clear framework and domain of applicability in which the introduced approach is exact, and assessing the error introduced when such a partitioning is only approximate

Time-Resolved X-ray Absorption Spectroscopy: An MCTDH Quantum Dynamics Protocol

Expressions for linear and nonlinear spectroscopy simulation in the X-ray window in which the time evolution of a photoexcited molecular system is treated via quantum dynamics are derived. By leveraging on the peculiar properties of core-excited/ionized states, first- and third-order response functions are recast in the limit of time-scale separation between the extremely short core-state lifetime and the (comparably longer) electronic-state transfer and nuclear vibrational motion. This work is a natural extension of Segatta et al. (J. Chem. Theory Comput. 2023, 19, 2075-2091), in which some of the present authors coupled MCTDH quantum dynamics to spectroscopy simulation at different levels of sophistication. Full quantum dynamics and approximate expressions are compared by simulating X-ray transient absorption spectroscopy at the carbon K-edge in the pyrene molecule

Nonlinear Molecular Electronic Spectroscopy via MCTDH Quantum Dynamics: From Exact to Approximate Expressions

We present an accurate and efficient approach to computing the linear and nonlinear optical spectroscopy of a closed quantum system subject to impulsive interactions with an incident electromagnetic field. It incorporates the effect of ultrafast nonadiabatic dynamics by means of explicit numerical propagation of the nuclear wave packet. The fundamental expressions for the evaluation of first- and higher-order response functions are recast in a general form that can be used with any quantum dynamics code capable of computing the overlap of nuclear wave packets evolving in different states. Here we present the evaluation of these expressions with the multiconfiguration time-dependent Hartree (MCTDH) method. Application is made to pyrene, excited to its lowest bright excited state S2 which exhibits a sub-100-fs nonadiabatic decay to a dark state S1. The system is described by a linear vibronic coupling Hamiltonian, parametrized with multiconfiguration electronic structure methods. We show that the ultrafast nonadiabatic dynamics can have a remarkable effect on the spectral line shapes that goes beyond simple lifetime broadening. Furthermore, a widely employed approximate expression based on the time scale separation of dephasing and population relaxation is recast in the same theoretical framework. Application to pyrene shows the range of validity of such approximations

Ultrafast photochemistry and electron-diffraction spectra in n->(3s) Rydberg excited cyclobutanone resolved at the multireference perturbative level

We study the ultrafast time evolution of cyclobutanone excited to singlet n-->Rydberg state through XMS-CASPT2 nonadiabatic surface-hopping simulations. These dynamics predict relaxation to ground-state with a timescale of 822 +/- 45 fs with minimal involvement of triplets. The major relaxation path to the ground-state involves a three-state degeneracy region and leads to 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

Ultrafast photochemistry and electron-diffraction spectra in n → (3s) Rydberg excited cyclobutanone resolved at the multireference perturbative level

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

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

Photo-Active Biological Molecular Materials: From Photoinduced Dynamics to Transient Electronic Spectroscopies

We present an overview of a methodology for the simulation of the photo-response of biological (macro)molecules, designed around a Quantum Mechanics / Molecular Mechanics (QM/MM) subtractive scheme. The resulting simulation workflow, that goes from the characterization of the photo-active system to the modeling of (transient) electronic spectroscopies is implemented in the software COBRAMM, but is completely general and can be used in the framework of any specific QM/MM implementation. COBRAMM is a smart interface to existing state-of-the-art theoretical chemistry codes, combining different levels of description and different algorithms to realize tailored problem-driven computations. The power of this approach is illustrated by reviewing the studies of two fundamental problems involving biological light-sensitive molecules. First, we will consider the photodynamics of the retinal molecule, the pigment of rhodopsin, a visual receptor protein contained in the rod cells of the retina. Retinal, with its light-induced isomerization, triggers a cascade of events leading to the production of the nerve impulse. Then, we will review some studies focusing on the interaction of DNA systems with ultraviolet (UV) light, a problem that has become one of the benchmark for the development of nonlinear spectroscopy, because of the ultrashort excited state lifetimes that arise from very efficient radiationless excited state decay and consent self-protection of DNA against UV damage

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

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

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