We investigate the formation of electronic coherences in pyrazine following excitation to the S2 1B2u (tctc*) state (D2h) and relaxation via a conical intersection (CI) to the S1 1B3u (n pi*) state. In our Quantum Ehrenfest (Qu-Eh) simulations, Gaussian wavepackets (GWPs) are started in the positive and negative directions of all the selected normal modes. The GWPs moving along the derivative coupling vector cross the CI and generate coherences from time zero. The effect nevertheless cancels in the total wavefunction because the wavepackets have opposite geometric phases. Thus, our results support the theoretical conjecture which states that coherences should not be observable if the intersecting states have different Abelian point group symmetries at the Franck-Condon (FC) point. However, if initial conditions start with a small admixture of the second coupled state, in this case the S1 1B3u(ntt*) state, rather than a pure S2 state, one generates an initial gradient along either the positive or negative direction of the derivative coupling vector, thus biasing the motion of the wavepacket so that the coherence becomes non-zero.

The benzene isomer fulvene has established itself as a computational benchmark for characterizing conical intersections of potential energy surfaces and modeling radiationless decay. However, in contrast to other benchmark systems such as benzene itself or the DNA/RNA nucleobases, there is as yet no time-resolved experimental data for fulvene to compare computational predictions with. This article has three main aims. The first is to thoroughly characterize the S 1/S 0 conical intersection seam of fulvene mediating ultrafast decay along its main reaction coordinates. The second is to understand how radiationless decay is shaped by the electron correlation included in the model and is carried out by comparing XMS-CASPT2 with CASSCF results and extracting how dynamic correlation effects impact differently the critical points encountered along the seam. The third and final aim is to compare fulvene against 6-methyl-fulvene (6MFulv) and 6,6-dimethyl-fulvene (DMF), potential surrogate systems available experimentally. We find methylation does not alter the energetics along the majority of the conical intersection seam itself, while single methylation (6MFulv) enables easier access to the planar region of the seam through the C1-C6 stretching coordinate compared to the parent fulvene or DMF. We thus suggest DMF as an appropriate surrogate system accessible by current experimental setups and where predictions on fulvene excited state reactivity can be tested.

The benzene isomer fulvene has established itself as a computational benchmark for characterizing conical intersections of potential energy surfaces and modeling radiationless decay. However, in contrast to other benchmark systems such as benzene itself or the DNA/RNA nucleobases, there is as yet no time-resolved experimental data for fulvene to compare computational predictions with. This article has three main aims. The first is to thoroughly characterize the S1/S0 conical intersection seam of fulvene mediating ultrafast decay along its main reaction coordinates. The second is to understand how radiationless decay is shaped by the electron correlation included in the model and is carried out by comparing XMS-CASPT2 with CASSCF results and extracting how dynamic correlation effects impact differently the critical points encountered along the seam. The third and final aim is to compare fulvene against 6-methyl-fulvene (6MFulv) and 6,6-dimethyl-fulvene (DMF), potential surrogate systems available experimentally. We find methylation does not alter the energetics along the majority of the conical intersection seam itself, while single methylation (6MFulv) enables easier access to the planar region of the seam through the C1-C6 stretching coordinate compared to the parent fulvene or DMF. We thus suggest DMF as an appropriate surrogate system accessible by current experimental setups and where predictions on fulvene excited state reactivity can be tested.

We present a theoreticalstudy of intersystem crossing (ISC) inacrolein and ketene with the Ehrenfest method that can describe asuperposition of singlet and triplet states. Our simulations illustratea new mechanistic effect of ISC, namely, that a superposition of singletsand triplets yields nonadiabatic dynamics characteristic of that superpositionrather than the constituent state potential energy surfaces. Thiseffect is particularly significant in ketene, where mixing of singletand triplet states along the approach to a singlet/singlet conicalintersection occurs, with the spin-orbit coupling (SOC) remainingsmall throughout. In both cases, the effects require many recrossingsof the singlet/triplet state crossing seam, consistent with the textbooktreatment of ISC.

In this work, we have studied the nuclear and electron dynamics in the glycine cation starting from localized hole states using the quantum Ehrenfest method. The nuclear dynamics is controlled both by the initial gradient and by the instantaneous gradient that results from the oscillatory electron dynamics (charge migration). We have used the Fourier transform (FT) of the spin densities to identify the "normal modes" of the electron dynamics. We observe an isomorphic relationship between the electron dynamics normal modes and the nuclear dynamics, seen in the vibrational normal modes. The FT spectra obtained this way show bands that are characteristic of the energy differences between the adiabatic hole states. These bands contain individual peaks that are in one-to-one correspondence with atom pair (+·) ↔ (·+) resonances, which, in turn, stimulate nuclear motion involving the atom pair. With such understanding, we anticipate "designer" coherent superpositions that can drive nuclear motion in a particular direction.

CHAMP is a high performance computing (HPC) and metadata portal which provides an easy to use workflow for FAIR (Findable, Accessible, Interoperable, and Reusabe) data generation and publication. It provides a web based interface allowing submission of HPC workloads and subsequent one-click publication of the results to data repositories such as Zenodo. Depositions support rich metadata to repositories that include a full implemention of the Subject property of the DataCite metadata schema (DataCite Metadata Working Group, 2021). Users submit jobs simply by choosing from pre-configured software and computing resource specifications (see Figure 1).

UV and VUV-induced processes in DNA/RNA nucleobases are central to understand photo-damaging and photo-protecting mechanisms in our genetic material. Here we model the events following photoionisation and electronic excitation in uracil, methylated in the 1 ' and 3 ' positions, using the correlated XMS-CASPT2 method. We compare our results against those for uracil and 5-methyl-uracil (thymine) previously published. We find 3-methylation, an epigenetic modification in non-negligible amounts, shows the largest differences in photoionised decay of all three derivatives studied compared to uracil itself. At the S-0 minimum, 3-methyl-uracil (3mUra) shows almost degenerate excited cation states. Upon populating the cation manifold, a crossing is predicted featuring different topography compared to other methylated uracil species in this study. We find an effective 3-state conical intersection accessible for 3mUra(+), which points towards an additional pathway for radiationless decay. 3-Methylation reduces the potential energy barrier mediating decay to the cation ground state, making it vanish and leading to a pathway that we expect will contribute to the fastest radiationless decay amongst all methylated uracil species studied to date. 1- and 5-methylation, on the other hand, give differences from uracil in detail only: ionisation potentials are slightly red-shifted and the potential energy barrier mediating decay to the cation ground state is small but almost unchanged. By comparing against CASSCF calculations, we establish XMS-CASPT2 is essential to correctly describe conical intersections for 3mUra(+). Our calculations show how a chemical modification that seems relatively small electronically can nevertheless have a significant impact on the behaviour of electronic excited states: a single methylation in the 3 ' position alters the behaviour of the RNA base uracil and appears to open an additional pathway for radiationless decay following ionisation and electronic excitation.

Ab initio electronic excited state calculations are necessary for the quantitative study of photochemical reactions, but their accurate computation on classical computers is plagued by prohibitive resource scaling. The Variational Quantum Deflation (VQD) is an extension of the quantum-classical Variational Quantum Eigensolver (VQE) algorithm for calculating electronic excited state energies, and has the potential to address some of these scaling challenges using quantum computers. However, quantum computers available in the near term can only support a limited number of quantum circuit operations, so reducing the quantum computational cost in VQD methods is critical to their realisation. In this work, we investigate the use of adaptive quantum circuit growth (ADAPT-VQE) in excited state VQD calculations, a strategy that has been successful previously in reducing the resources required for ground state energy VQE calculations. We also invoke spin restrictions to separate the recovery of eigenstates with different spin symmetry to reduce the number of calculations and accumulation of errors in computing excited states. We created a quantum eigensolver emulation package - Quantum Eigensolver Building on Achievements of Both quantum computing and quantum chemistry (QEBAB) - for testing the proposed adaptive procedure against two existing VQD methods that use fixed-length quantum circuits: UCCGSD-VQD and k-UpCCGSD-VQD. For a lithium hydride test case we found that the spin-restricted adaptive growth variant of VQD uses the most compact circuits out of the tested methods by far, consistently recovers adequate electron correlation energy for different nuclear geometries and eigenstates while isolating the singlet and triplet manifold. This work is a further step towards developing techniques which improve the efficiency of hybrid quantum algorithms for excited state quantum chemistry, opening up the possibility of exploiting real quantum computers for electronic excited state calculations sooner than previously anticipated.

The study of radiation effects in DNA is a multidisciplinary endeavour, connecting the physical, chemical and biological sciences. Despite being mostly filtered by the ozone layer, sunlight radiation is still expected to (photo)ionise DNA in sizeable yields, triggering an electron removal process and the formation of potentially reactive cationic species. In this manuscript, photoionisation decay channels of important DNA tautomeric derivatives, 7H-adenine and 7H-guanine, are characterised with accurate CASSCF/XMS-CASPT2 theoretical methods. These simulation techniques place the onset of ionisation for 7H-adenine and 7H-guanine on average at 8.98 and 8.43 eV, in line with recorded experimental evidence when available. Cationic excited state decays are analysed next, uncovering effective barrierless deactivation routes for both species that are expected to decay to their (cationic) ground state on ultrafast timescales. Conical intersection topographies reveal that these photoionisation processes are facilitated by sloped single-path crossings, known to foster photostability, and which are predicted to enable the (VUV) photo-protection mechanisms present in these DNA tautomeric species.

The possibility of suddenly ionized molecules undergoing extremely fast electron hole dynamics prior to significant structural change was first recognized more than 20 years ago and termed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specific hole within the molecule. We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump/x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment. We record the signature of transient hole dynamics and make the first observation of dynamics driven by frustrated Auger-Meitner transitions. We verify that the hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localized in space and time in molecules and thus to charge transfer phenomena that are fundamental in chemical and material physics.

The front cover artwork is provided by Dr. Javier Segarra-Martí (University of Valencia, Spain) and Prof. Michael J. Bearpark (Imperial College London, UK). The image shows the ultrafast photoionisation of DNA canonical nucleobase cytosine and the slower ionization process in non-canonical base isocytosine embedded within a DNA backbone. Read the full text of the Article at 10.1002/cphc.202100402.

The Front Cover shows the ultrafast photoionisation of DNA canonical nucleobase cytosine and the slower ionization process in non-canonical base isocytosine embedded within a DNA backbone. More information can be found in the Article by Javier Segarra-Martí and Michael J. Bearpark.

Reaction intermediates in the green-to-red photoconversion of the photochromic fluorescent protein EosFP have been observed using high-intensity continuous blue illumination. An intermediate was identified through light-induced accumulation that continues to convert the green form in subsequent darkness, putatively containing a tyrosyl radical, albeit with anomalously shifted features in both the electronic and FTIR spectra. Lowering the pH to 5.5 significantly delays the decay of this tyrosyl intermediate, which is accompanied by Stark-shifted features in the electronic spectra of reactants and products. Vibrational mode assignments for the high-frequency and fingerprint FTIR spectral regions of the reaction intermediates support a proposed sequence of events where the newly formed Cα═Cβ ethylenic bond precedes modifications on the His-62 imidazole ring and confirms a C═O(NH2) product group on Phe-61. We propose a reaction mechanism that involves tyrosyl generation via singlet excited-state-mediated oxidation which subsequently triggers the covalent reactions by oxidation of the green chromophore.

Unrestricted DFT (UDFT), time-dependent DFT (TDDFT) and spin-flip TDDFT (SF-TDDFT) were used to investigate the potential energy surfaces of the ground and first two electronic excited states of the naphthalene, pyrene and perylene radical cations. In particular, conical intersections (which play a central role in the photophysics of these cations) were located with these DFT-based methods. The results are consistent with accurate multiconfigurational wavefunction-based ab initio methods. These show that naphthalene and pyrene cations can quickly relax nonradiatively from their excited states back down to the original ground state species through easily accessible conical intersections, but the perylene cation cannot do so, due to the absence of any accessible funnels between the lowest excited state and the ground state, leaving radiative decay as the most probable photophysical pathway. This study paves the way for using computationally efficient density functional theory (DFT)-based methods in future investigations of the photophysics of much larger polycyclic aromatic hydrocarbons, for which multiconfigurational wavefunction-based methods become prohibitively expensive.

In this discussion we present a methodology to describe spectral lineshape from first principles, providing insight into the solvent-solute molecular interactions in terms of static and dynamic disorder and how these shape the signals recorded experimentally in linear and nonlinear optical spectroscopies, including two-dimensional electronic spectroscopy (2DES). Two different strategies for simulating the lineshape are compared: both rely on the same evaluation of the coupling between the electronic states and the intra-molecular vibrations, while they differ in describing the influence exerted by the diverse water configurations attained along a molecular dynamics (MD) simulation. The first method accounts for such water arrangements as first order perturbations on the adenine energies computed for a single reference (gas phase) quantum calculation. The second method requires computation of the manifold of excited states explicitly at each simulation snapshot, employing a hybrid quantum mechanics/molecular mechanics (QM/MM) scheme. Both approaches are applied to a large number of states of the adenine singlet excited manifold (chosen because of its biological role), and compared with available experimental data. They give comparable results but the first approach is two orders of magnitude faster. We show how the various contributions (static/dynamic disorder, intra-/inter-molecular interactions) sum up to build the total broadening observed in experiments.

RASSCF calculations of vertical excitation energies were carried out on a benchmark set of 19 organic molecules studied by Thiel and co-workers [ J. Chem. Phys. 2008 , 128 , 134110 ]. The best results, in comparison with the MS-CASPT2 results of Thiel, were obtained using a RASSCF space that contains at most one hole and one particle in the RAS1 and RAS3 spaces, respectively, which we denote as RAS[1,1]. This subset of configurations recovers mainly the effect of polarization and semi-internal electronic correlation that is only included in CASSCF in an averaged way. Adding all-external correlation by allowing double excitations from RAS1 and RAS2 into RAS3 did not improve the results, and indeed, they were slightly worse. The accuracy of the first-order RASSCF computations is demonstrated to be a function of whether the state of interest can be classified as covalent or ionic in the space of configurations built from orbitals localized onto atomic sites. For covalent states, polarization and semi-internal correlation effects are negligible (RAS[1,1]), while for ionic states, these effects are large (because of inherent diffusiveness of these states compared to the covalent states) and, thus, an acceptable agreement with MS-CASPT2 can be obtained using first-order RASSCF with the extra basis set involving 3p orbitals in most cases. However, for those ionic states that are quasi-degenerate with a Rydberg state or for nonlocal nπ* states, there remains a significant error resulting from all external correlation effects.

In this article we characterise the radiationless decay of the first few electronic excited states of the cations of DNA/RNA nucleobases uracil and thymine, including the effects of dynamic electron correlation on energies and geometries (optimised with XMS-CASPT2). In both systems, we find that one state of 2n and another two of 2π+ character can be populated following photoionisation, and their different minima and interstate crossings are located. We find strong similarities between uracil and thymine cations: with accessible conical intersections suggesting that depopulation of their electronic excited states takes place on ultrafast timescales in both systems, suggesting that they are photostable in agreement with previous theoretical (uracil+) evidence. We find that dynamic electron correlation separates the energy levels of the "3-state" conical intersection (D2/D1/D0)CI previously located with CASSCF for uracil+, which will therefore have a different geometry and higher energy. Simulating the electronic and vibrational absorptions allows us to characterise spectral fingerprints that could be used to monitor these cation photo-processes experimentally.

Photoionisation in DNA, i. e. the process of photoinduced electron removal from the chromophoric species - the nucleobases - leading to their cationic form, has been scarcely studied despite being considered to be responsible for significant damaging instances in our genetic material. In this contribution we theoretically characterise the electronic ground and excited state decay pathways of cationic DNA nucleobase cytosine(+) and its epigenetic derivative 5-methyl-cytosine(+), including the effects of dynamic electron correlation on energies and geometries of minima and conical intersections. We do this by comparing the results of XMS-CASPT2 calculations with CASSCF estimates and we find some significant differences between the results of these two methods. In particular, including dynamic electron correlation is found to significantly reduce the barrier to access the (D1/D0) conical intersection. We find notable similarities in both cytosine and 5-methyl-cytosine cations, and accessible conical intersections in the vicinity of the Franck-Condon region are found. This points towards an ultrafast depopulation of their electronic excited states. Moreover, the shape of the ground state potential energy surface strongly directs the decaying excited state population towards the cationic ground state minimum on ultrafast timescales, preventing photo-fragmentation and thus explaining their photostability. To better compare our calculations with the available experimental data we compute the UV (ground and excited state) and IR absorptions.

We present an implementation of the spin-dependent unitary group approach to calculate spin densities for configuration interaction calculations in a basis of spin symmetry-adapted functions. Using S-2 eigenfunctions helps to reduce the size of configuration space and is beneficial in studies of the systems where selection of states of specific spin symmetry is crucial. To achieve this, we combine the method to calculate U(n) generator matrix elements developed by Downward and Robb (Theor. Chim. Acta 1977, 46, 129) with the approach of Battle and Gould to calculate U(2n) generator matrix elements (Chem. Phys. Lett. 1993, 201, 284). We also compare and contrast the spin density formulated in terms of the spin-independent unitary generators arising from the group theory formalism and equivalent formulation of the spin density representation in terms of the one- and two-electron charge densities.