Analytic gradient techniques for investigating the complex-valued potential energy surfaces of electronic resonances

Electronic resonances are metastable atomic or molecular systems that can decay by electron detachment. They play an important role in biological processes such as DNA fragmentation induced by slow electrons or in interstellar reactions as in the formation of neutral molecules and molecular anions. As opposed to bound states, resonances do not correspond to discrete eigenstates of a Hermitian Hamiltonian, and therefore their theoretical description requires special methods. The complex absorbing potential (CAP) method can be used to calculate both the energy and the lifetime of a resonance as a discrete eigenstate in a non-Hermitian time-independent framework. The CAP method allows for applying well-known bound-state electronic structure methods to resonances as well. In this work, the applicability of CAP-augmented equation-of-motion coupled-cluster (CAP-EOM-CC) methods is extended for locating equilibrium structures and crossings on complex-valued potential energy surfaces of electronic resonances by introducing analytic energy gradients. The structure and energy of these points are needed for, e.g., estimating the importance of a specific dissociation route or deactivation process. The accuracy of structural parameters, vertical and adiabatic electron affinities, and resonance widths obtained with approximate methods and various diffuse basis sets is investigated. Applications of optimization methods are also presented for systems that are relevant in interstellar or biological processes. Properties of the complex-valued potential energy surfaces of anionic resonances of acrylonitrile and methacrylonitrile are connected to experimental observations. Dissociative electron attachment to chlorosubstituted ethylenes is also investigated. This can help in understanding detoxification processes of these compounds and might facilitate the exploration of DEA pathways for other halogenated molecules as well

Analytic gradient techniques for investigating the complex-valued potential energy surfaces of electronic resonances

Electronic resonances are metastable atomic or molecular systems that can decay by electron detachment. They play an important role in biological processes such as DNA fragmentation induced by slow electrons or in interstellar reactions as in the formation of neutral molecules and molecular anions. As opposed to bound states, resonances do not correspond to discrete eigenstates of a Hermitian Hamiltonian, and therefore their theoretical description requires special methods. The complex absorbing potential (CAP) method can be used to calculate both the energy and the lifetime of a resonance as a discrete eigenstate in a non-Hermitian time-independent framework. The CAP method allows for applying well-known bound-state electronic structure methods to resonances as well. In this work, the applicability of CAP-augmented equation-of-motion coupled-cluster (CAP-EOM-CC) methods is extended for locating equilibrium structures and crossings on complex-valued potential energy surfaces of electronic resonances by introducing analytic energy gradients. The structure and energy of these points are needed for, e.g., estimating the importance of a specific dissociation route or deactivation process. The accuracy of structural parameters, vertical and adiabatic electron affinities, and resonance widths obtained with approximate methods and various diffuse basis sets is investigated. Applications of optimization methods are also presented for systems that are relevant in interstellar or biological processes. Properties of the complex-valued potential energy surfaces of anionic resonances of acrylonitrile and methacrylonitrile are connected to experimental observations. Dissociative electron attachment to chlorosubstituted ethylenes is also investigated. This can help in understanding detoxification processes of these compounds and might facilitate the exploration of DEA pathways for other halogenated molecules as well

Molecular Vibration Explorer: an Online Database and Toolbox for Surface-Enhanced Frequency Conversion and Infrared and Raman Spectroscopy

We present Molecular Vibration Explorer, a freely accessible online database and interactive tool for exploring vibrational spectra and tensorial light-vibration coupling strengths of a large collection of thiolated molecules. The "Gold" version of the database gathers the results from density functional theory calculations on 2800 commercially available thiol compounds linked to a gold atom, with the main motivation to screen the best molecules for THz and mid-infrared to visible upconversion. Additionally, the "Thiol" version of the database contains results for 1900 unbound thiolated compounds. They both provide access to a comprehensive set of computed spectroscopic parameters for all vibrational modes of all molecules in the database. The user can simultaneously investigate infrared absorption, Raman scattering, and vibrational sum- and difference-frequency generation cross sections. Molecules can be screened for various parameters in custom frequency ranges, such as a large Raman cross-section under a specific molecular orientation, or a large orientation-averaged sum-frequency generation (SFG) efficiency. The user can select polarization vectors for the electromagnetic fields, set the orientation of the molecule, and customize parameters for plotting the corresponding IR, Raman, and sum-frequency spectra. We illustrate the capabilities of this tool with selected applications in the field of surface-enhanced spectroscopy

Analytic evaluation of non-adiabatic couplings within the complex absorbing potential equation-of-motion coupled-cluster method

We present the theory for the evaluation of non-adiabatic couplings (NACs) involving resonance states within the complex absorbing potential equation-of-motion coupled-cluster (CAP-EOM-CC) framework implemented within the singles and doubles approximation. Resonance states are embedded in the continuum and undergo rapid decay through autodetachment. In addition, nuclear motions can facilitate transitions between different resonances and between resonances and bound states. These non-adiabatic transitions affect the chemical fate of resonances and have distinct spectroscopic signatures. The NAC vector is a central quantity needed to model such effects. In the CAP-EOM-CC framework, resonance states are treated on the same footing as bound states. Using the example of fumaronitrile, which supports a bound radical anion and several anionic resonances, we analyze the non-adiabatic coupling between bound states and pseudocontinuum states, between bound states and resonances and between two resonances. We find that the NAC between a bound state and a resonance is nearly independent of the CAP strength and thus straightforward to evaluate whereas the NAC between two resonance states or between a bound state and a pseudocontinuum state is more difficult to evaluate

Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design

Direct Calculation of Electron Transfer Rates with the Binless Dynamic Histogram Analysis Method

Umbrella sampling molecular dynamics simulations are widely used to enhance sampling along the reaction coordinates of chemical reactions. The effect of the artificial bias can be removed using methods such as the dynamic weighted histogram analysis method (DHAM), which in addition to the global free energy profile also provides kinetic information about barrier-crossing rates directly from the Markov matrix. Here we present a binless formulation of DHAM that extends DHAM to high-dimensional and Hamiltonian-based biasing to allow the study of electron transfer (ET) processes, for which enhanced sampling is usually not possible based on simple geometric grounds. We show the capabilities of binless DHAM on examples such as aqueous ferrous-ferric ET and intramolecular ET in the radical anion of benzoquinone–tetrathiafulvalene–benzoquinone (Q-TTF-Q)−. From classical Hamiltonian-based umbrella sampling simulations and electronic coupling values from quantum chemistry calculations, binless DHAM provides ET rates for adiabatic and nonadiabatic ET reactions alike in excellent agreement with experimental results

Direct Calculation of Electron Transfer Rates with the Binless Dynamic Histogram Analysis Method

Umbrella sampling molecular dynamics simulations are widely used to enhance sampling along the reaction coordinate of chemical reactions. The effect of the artificial bias can be removed using methods such as the Dynamic Weighted Histogram Analysis Method (DHAM), which in addition to the global free energy profile also provides kinetic information on barrier-crossing rates directly from the Markov matrix. Here we present a binless formulation of DHAM, which extends DHAM to high-dimensional and Hamiltonian-based biasing, allowing the study of electron transfer (ET) processes, where enhanced sampling is usually not possible based on simple geometric grounds. We show the capabilities of binless DHAM on examples such as aqueous ferrous-ferric ET and intramolecular ET in the radical anion of benzoquinone–tetrathiafulvalene–benzoquinone (Q-TTF-Q)–. From classical Hamiltonian-based umbrella sampling simulations and electronic coupling values from quantum chemistry calculations, binless DHAM provides ET rates for adiabatic and nonadiabatic ET reactions alike, in excellent agreement with experiment

Direct Calculation of Electron Transfer Rates with the Binless Dynamic Histogram Analysis Method

Umbrella sampling molecular dynamics simulations are widely used to enhance sampling along the reaction coordinates of chemical reactions. The effect of the artificial bias can be removed using methods such as the dynamic weighted histogram analysis method (DHAM), which in addition to the global free energy profile also provides kinetic information about barrier-crossing rates directly from the Markov matrix. Here we present a binless formulation of DHAM that extends DHAM to high-dimensional and Hamiltonian-based biasing to allow the study of electron transfer (ET) processes, for which enhanced sampling is usually not possible based on simple geometric grounds. We show the capabilities of binless DHAM on examples such as aqueous ferrous-ferric ET and intramolecular ET in the radical anion of benzoquinone-tetrathiafulvalene-benzoquinone (Q-TTF-Q)-. From classical Hamiltonian-based umbrella sampling simulations and electronic coupling values from quantum chemistry calculations, binless DHAM provides ET rates for adiabatic and nonadiabatic ET reactions alike in excellent agreement with experimental results

Spectroscopic Evidence for Peptide-Bond-Selective Ultraviolet Photodissociation

We study the photodissociation induced by ultraviolet excitation of amide bonds in gas-phase protonated peptides. Jointly, mass spectrometry and cold ion spectroscopy provide evidence for a selective non-statistical dissociation of specific peptide bonds in the spectral region of the formally forbidden n→π* transition of amide groups. Structural analysis reveals that the activation of this transition, peaked at 226 nm, originates from the non-planar geometry of the bond. In contrast, the statistical dissociation in the electronic ground state appears to be the main outcome of the π→π* excitation of the peptide bonds at 193 nm. We propose a tentative model that explains the difference in the fragmentation mechanisms by the difference in localization of the electronic transitions and the higher amount of vibrational energy released in the electronic excited state upon absorption at 193 nm

Research data supporting "Breaking the Selection Rules of Spin-Forbidden Molecular Absorption in Plasmonic Nanocavities"

The experimental data were taken in the NanoPhotonics Group at the Cavendish Laboratory (University of Cambridge). The dataset is for the journal article," Breaking the Selection Rules of Spin-Forbidden Molecular Absorption in Plasmonic Nanocavities". The photoluminescent spectra were taken with an electron multiplying charge-coupled detected (EMCCD)(Andor iXon) and darkfield scattering spectra by OceanOptics spectrometer. For further description see the Supplementary description of the manuscript