Accurate reference data for electronically excited states are essential for assessing multireference electronic-structure methods. We benchmark state-averaged completeactive-space self-consistent field (SA-CASSCF), exchange-scaled CASSCF (XS-CASSCF), fully uncontracted multireference configuration interaction with single and double excitations (MR-CISD), and four a posteriori size-consistency corrections against the theoretical best estimates of QUESTDB. The dataset comprises 262 valence excitations of 41 small and medium-sized molecules, including singlet and triplet n*, *, and doubly excited states. All calculations used a consistent COLUMBUS protocol with molecule-specific active spaces and the aug-cc-pVDZ basis set. For the 232 safe transitions, SA-CASSCF and MR-CISD yield mean absolute errors of 0.57 and 0.31 eV, respectively, with predominantly positive biases. The Davidson–Silver/Siegbahn (+DV3) and Pople (+P) corrections reduce the mean absolute errors to 0.10 and 0.11 eV and the root-mean-square errors to 0.13 eV. They also provide a more balanced description across spin multiplicities and excitation characters. Both XSCASSCF improve singlet * states, particularly those with pronounced ionic character, but degrade n* states and do not improve the overall statistics relative to SA-CASSCF. For 36 selected unsafe or doubly excited transitions, MR-CISD+DV3 and MR-CISD+P retain mean absolute errors of 0.16 eV. Their agreement with exFCI-based reference values and discrepancies from several CASPT3- and PC-NEVPT2-based estimates identify states whose current best estimates merit independent re-examination. Overall, size-consistencycorrected MR-CISD provides highly accurate and broadly balanced valence excitation energies.

The COLUMBUS program system provides the tools for performing high-level multireference (MR) computations, including the multireference configuration interaction (MRCI) method and its multireference averaged quadratic coupled cluster (MR-AQCC) extension, allowing computations on a wide range of fascinating atomic and molecular systems, including the treatment of open-shells and complicated excited state phenomena. The inclusion of spin-orbit coupling (SOC) directly within the MRCI step enables the description of systems containing heavy elements, such as lanthanides and actinides, whose properties are strongly influenced by SOC. Analytic energy gradients and nonadiabatic couplings at the correlated MRCI level provide the foundation for a variety of dynamics studies, giving insight into ultrafast photochemistry. New and ongoing method developments in COLUMBUS include the computation of spin densities, improved descriptions of ionic states, enhancements to the AQCC method, and the porting of COLUMBUS to graphical processing units (GPUs). New external interfaces enable an enhanced description of electronic resonances and molecules in strong laser fields. This work highlights these new developments while providing a detailed account of the diverse applications of COLUMBUS in recent years.

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

Quantum computers promise to impact industrial applications, for which quantum chemical calculations are required, by virtue of their high accuracy. This perspective explores the challenges and opportunities of applying quantum computers to drug design, discusses where they could transform industrial research and elaborates on what is needed to reach this goal.

Recent developments in quantum computing are highly promising, particularly in the realm of quantum chemistry. Due to the noisy nature of currently available quantum hardware, hybrid quantum-classical algorithms have emerged as a reliable option for near-term simulations. Mixed quantum-classical dynamics methods effectively capture nonadiabatic effects by integrating classical nuclear dynamics with quantum chemical computations of the electronic properties. However, these methods face challenges due to the high computational cost of the quantum chemistry part. To mitigate the computational demand, we propose a method where the required electronic properties are computed through a hybrid quantum-classical approach that combines classical and quantum hardware. This framework employs the variational quantum eigensolver and variational quantum deflation algorithms to obtain ground and excited state energies, gradients, nonadiabatic coupling vectors, and transition dipole moments. These quantities are used to propagate the nonadiabatic molecular dynamics using the Tully's fewest switches surface hopping method, although the implementation is also compatible with other molecular dynamics approaches. The approach, implemented by integrating the molecular dynamics program package SHARC with the TEQUILA quantum computing framework, is validated by studying the cis-trans photoisomerization of methanimine and the electronic relaxation of ethylene. The results show qualitatively accurate molecular dynamics that align with experimental findings and other computational studies. This work is expected to mark a significant step towards achieving a "quantum advantage" for realistic chemical simulations.

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.

In this work, we report the synthesis of O-doped naphthalene-based electrochromes. Exploiting the CuO-mediated Pummerer oxidative cycloetherification reaction, a series of 1,4- and 1,5-disubstituted naphthalene-cored dipyrano, difurano, and furano-pyrano polycyclic aromatic hydrocarbons (PAHs) have been prepared. Steady-state UV-Vis absorption and emission investigations showed that the spectroscopic profile strongly depends on the O-doping topology, with the dipyrano and the difurano derivatives demonstrating the most red-shifted and blue-shifted electronic transition, respectively. Computational investigations revealed that the cycloetherification reaction raises the HOMO energy level (while the LUMO remains largely unaffected), with the dipyrano derivatives displaying the highest values. Spectroelectrochemical measurements showed that, depending on the O-topology and the type of O-ring, different electrochromic responses could be obtained with colour transitions featuring high contrasts involving yellow, pink, orange or blue colours.

The Cover Feature shows the color changes achieved, upon oxidation, by newly synthesized electrochromic oxygen-doped small molecules. The crucial moieties are furano and pyrano. Incorporation of these groups into other organic molecules can be used to greatly expand the library of electrochromic small molecules, giving very good control over color transition observed. Such controlled color changes could take electrochromism from the niche applications it currently occupies, to the wider world. Cover design by Adriana Lopez-Biagi. More information can be found in the Full Paper by D. Bonifazi et al.

Nachrichten aus der ChemieVolume 70, Issue 2 p. 82-83 GÖCH GÖCH-Arbeitsgruppen stellen sich vor: AG „Computational Chemistry“ Markus Oppel, Markus OppelSearch for more papers by this authorLeticia González, Leticia GonzálezSearch for more papers by this author Markus Oppel, Markus OppelSearch for more papers by this authorLeticia González, Leticia GonzálezSearch for more papers by this author First published: 01 February 2022 https://doi.org/10.1002/nadc.20224122720Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume70, Issue2Februar 2022Pages 82-83 RelatedInformation

A density matrix renormalization group-self consistent field (DMRG-SCF) study has been carried out to calculate the low-lying excited states of CpMo(CO)(2)NO, a molybdenum complex containing NO and CO ligands. In order to automatically select an appropriate active space, a novel procedure employing the maximum single-orbital entropy for several states has been introduced and shown to be efficient and easy-to-implement when several electronic states are simultaneously considered. The analysis of the resulting natural transition orbitals and charge-transfer numbers shows that the lowest five excited electronic states are excitation into metal-NO antibonding orbitals, which offer the possibility for nitric oxide (NO) photorelease after excitation with visible light. Higher excited states are metal-centered excitations with contributions of metal-CO antibonding orbitals, which may serve as a gateway for carbon monoxide (CO) delivery. Time-dependent density functional theory calculations done for comparison, show that the state characters agree remarkably well with those from DMRG-SCF, while excitation energies are 0.4-1.0 eV red-shifted with respect to the DMRG-SCF ones.

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with post calculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory and new electronic and muonic basis sets.

The possibility to discriminate the ortho and para nuclear spin isomers of 1,1-difluoroethylene via their excited state dynamics is studied using wavepacket propagations including non-adiabatic couplings. The two nuclear spin isomers are connected via a torsional motion around the double bond. Photo-excitation induces a different interference pattern, with different torsional periods -a fact that can be exploited to distinguish, and eventually separate the nuclear spin isomers using femtosecond pump-probe experiments. The inclusion of non-adiabatic couplings results in a slowdown of the wavepacket, increasing the torsional periods of the nuclear spin isomers. (C) 2017 Elsevier B. V. All rights reserved.

Stark control of chemical reactions uses intense laser pulses to distort the potential energy surfaces of a molecule, thus opening new chemical pathways. We use the concept of Stark shifts to convert a local minimum into a local maximum of the potential energy surface, triggering constructive and destructive wave-packet interferences, which then induce different dynamics on nuclear spin isomers in the electronically excited state of a quinodimethane derivative. Model quantum-dynamical simulations on reduced dimensionality using optimized ultrashort laser pulses demonstrate a difference of the excited-state dynamics of two sets of nuclear spin isomers, which ultimately can be used to discriminate between these isomers.

A new algorithm for the computation of the overlap between many-electron wave functions is described. This algorithm allows for the extensive use of recurring intermediates and thus provides high computational efficiency. Because of the general formalism employed, overlaps can be computed for varying wave function types, molecular orbitals, basis sets, and molecular geometries. This paves the way for efficiently computing nonadiabatic interaction terms for dynamics simulations. In addition, other application areas can be envisaged, such as the comparison of wave functions constructed at different levels of theory. Aside from explaining the algorithm and evaluating the performance, a detailed analysis of the numerical stability of wave function overlaps is carried out, and strategies for overcoming potential severe pitfalls due to displaced atoms and truncated wave functions are presented.

Phenothiazinium dyes are used as photosensitizers in photodynamic therapy. Their mode of action is related to the generation of triplet excited states by intersystem crossing. Therefore, rationalizing the factors that influence intersystem crossing is crucial to improve the efficacy of photodynamic therapy. Here we employ quantum mechanics/molecular mechanics calculations to investigate the effect of aqueous and nucleic acid environments on the intersystem crossing mechanism in methylene blue. We find that the mechanism by which the triplet states are generated depends strongly on the environment. While intersystem crossing in water is mediated exclusively by vibronic spin-orbit coupling, it is enhanced in DNA due to a second pathway driven by electronic spin-orbit coupling. Competing charge-transfer processes, which are also possible in the presence of DNA, can therefore be suppressed by a suitable structural functionalization, thereby increasing the efficacy of photodynamic therapy.

AbstractPhenothiazinfarbstoffe werden als Photosensibilisatoren in der photodynamischen Therapie eingesetzt. Ihr Wirkmodus ist eng an die Erzeugung von angeregten Triplettzuständen durch Interkombination geknüpft. Das Verständnis der Faktoren, welche die Interkombination beeinflussen, ist daher essenziell zur Verbesserung der Wirksamkeit der photodynamischen Therapie. Wir setzen eine Kombination aus quantenmechanischen und molekülmechanischen Rechenverfahren ein, um die Effekte einer wässrigen und einer DNA‐Umgebung auf den Mechanismus der Interkombination in Methylenblau zu untersuchen. Dabei stellt sich heraus, dass die Erzeugung von Triplettzuständen stark von der Umgebung beeinflusst wird. Während die Interkombination in Wasser ausschließlich durch vibronische Spin‐Bahn‐Kopplungen ausgelöst wird, findet sich in DNA‐Umgebung ein weiterer, durch elektronische Spin‐Bahn‐Kopplung vermittelter Weg. Konkurrierende, in DNA‐Umgebungen ebenfalls mögliche Ladungstransferprozesse können daher unterdrückt werden, um die Wirksamkeit der photodynamischen Therapie zu erhöhen.

The concept of nuclear spin isomers was already introduced in the early days of quantum mechanics. Despite its importance, not much work has been done to separate them experimentally by pushing the ratio away from its equilibrium value. We propose to use ultrashort laser pulses in a pump-dump-like experiment to enhance the ratio between different nuclear spin isomers. Exemplary wave packet simulations with optimized femtosecond pump and dump laser pulses are shown on a quinodimethane derivative to illustrate that the ratio between two different groups of nuclear spin isomers is enhanced.

Despite the concept of nuclear spin isomers (NSIs) exists since the early days of quantum mechanics, only few approaches have been suggested to separate different NSIs. Here, a method is proposed to discriminate different NSIs of a quinodimethane derivative using its electronic excited state dynamics. After electronic excitation by a laser field with femtosecond time duration, a difference in the behavior of several quantum mechanical operators can be observed. A pump-probe experimental approach for separating these different NSIs is then proposed.

We consider a class of molecules with C-2 symmetry axis and three segments A, B, C which can rotate independently about that axis, corresponding to two independent torsions (B vs. A and C vs. B). The torsions may be feasible either in the electronic ground or in the excited states. We determine the corresponding molecular symmetry group, i.e. the Abelian group G(16)(A) representing 16 feasible permutations and permutation-inversions, and its permutation subgroup with eight permutations, together with their properties, e.g. their character tables and the corresponding 16 or 8 irreducible representations (IREPs), respectively. Accordingly, the molecules which belong to this class have at most eight different nuclear spin isomers (NSIs). A subset of them "survives" at low temperature, T -> 0. The corresponding NSI selective wavefunctions contain products of torsional times nuclear wavefunctions with specific IREPs. The NSIs are characterized by these IREPs. As an example, we determine the molecular symmetry adapted torsional wavefunctions of the model 2-[4-(cyclopenta-2,4-dien-1-ylidene)cyclohexa-2,5-dien-1-ylidene]-2H-1,3-dioxole, abbreviated as CCD. In order to demonstrate the principles of the derivations, we employ a simple model, with the C-2 symmetry axis oriented along the laboratory Z-axis, and with all degrees of freedom frozen in the equilibrium structure of CCD, except the two torsional degrees of freedom. The resulting torsional wavefunctions represent different NSIs of CCD, ready for subsequent applications, e.g. for demonstrations of NSI selective dynamics.