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Studying the effects of UV and VUV radiation on non‐canonical DNA/RNA nucleobases allows us to compare how they release excess energy following absorption with respect to their canonical counterparts. This has attracted much research attention in recent years because of its likely influence on the origin of our genetic lexicon in prebiotic times. Here we present a CASSCF and XMS‐CASPT2 theoretical study of the photoionisation of non‐canonical pyrimidine nucleobase isocytosine in both its keto and enol tautomeric forms. We analyse their lowest energy cationic excited states including 2π+ , 2nO+ and 2nN+ and compare these to the corresponding electronic states in cytosine. Investigating lower‐energy decay pathways we find – unexpectedly ‐ that keto‐isocytosine+ presents a sizeable energy barrier potentially inhibiting decay to its cationic ground state, whereas enol‐isocytosine+ features a barrierless and consequently ultrafast pathway analogous to the one previously found for the canonical (keto) form of cytosine+. Dynamic electron correlation reduces the energy barrier in the keto form substantially (by ∼1 eV) but it is nevertheless still present. We additionally compute the UV/Vis absorption signals of the structures encountered along these decay channels to provide spectroscopic fingerprints to assist future experiments in monitoring these intricate photo‐processes., The photostability of isocytosine upon ionising radiation exposure is assessed theoretically in both keto and enol tautomeric forms. This non‐canonical base, which features analogous UV excited state reactivity to DNA nucleobase cytosine, unexpectedly displays sizeable energy barriers along the cationic excited state in its keto form that hamper the decay. Simulations suggest isocytosine is less photostable against photoionisation, a factor that might contribute to selecting the more resilient cytosine under prebiotic VUV radiation exposure.

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.

We present a computational protocol devised to test the suitability of newly proposed chromophores for singlet fission. The protocol includes two main steps: testing the basic energetic requirements by ab initio calculations and simulating the photodynamics to determine singlet fission quantum yields. We applied this protocol to 2,3-diamino-1,4-benzoquinone (DAPBQ), recently proposed as a possible chromophore for singlet fission. We determined the excitation energies of DAPBQ by second order perturbation CASPT2 and NEVPT2 calculations. Then, we optimized a possible crystal structure of DAPBQ, we identified the most favourable pair of molecules within it and we ran a simulation of the excited state dynamics for that dimer embedded in the crystal at a semiempirical QM/MM level. The results indicate that DAPBQ in the crystalline phase does not undergo singlet fission and the initially prepared singlet excited state rapidly decays to the ground state.

The theory of maximum probability domains (MPDs) is formulated for the Hubbard model in terms of projection operators and generating functions for both exact eigenstates as well as Slater determinants. A fast MPD analysis procedure is proposed, which is subsequently used to analyse numerical results for the Hubbard model. It is shown that the essential physics behind the considered Hubbard models can be exposed using MPDs. Furthermore, the MPDs appear to be in line with what is expected from Valence Bond (VB) Theory-based knowledge.

Multiconfigurational wave functions are calculated for a series of Fe complexes. We find a linear correlation between the experimental Fe-57 Mossbauer isomer shift and the calculated electron density at the Fe nucleus. However, the analysis of the wave function in valence bond terms shows that there is no straightforward relation between the density at the nucleus and the Fe charge. The analysis of the CASSCF wave function expressed in localized orbitals shows that the isomer shift is very sensitive to the weight of charge transfer configurations and hence to the covalency, rather than to the absolute charge. (c) 2008 Elsevier B. V. All rights reserved.

The origin of the features in the Ni 3s X-ray photoelectron spectrum of NiO is investigated using a non-orthogonal configuration interaction approach for an embedded [NiO6] Cluster. We study the interplay of inter-atomic screening with the metal core hole and intra-atomic exchange and electron correlation effects. We show that the spectrum can be described in terms of only few key configurations, provided that orbital relaxation effects are explicitly taken into account for the excited charge transfer configurations. The strength of this approach has been demonstrated earlier for those final states that have a high-spin coupling. In the present contribution the analysis is extended to include low-spin coupled 3s-hole states. The effects of enlarging the embedded cluster and of an improved representation of the nearest cluster surroundings were studied for the high-spin final states. We found only minor effects on the computed peak separations. (c) 2006 Elsevier B.V. All rights reserved.

Magnetic interactions in ladder vanadates are determined with quantum chemical computational schemes using the embedded cluster model approach to represent the material. The available experimental data for calcium vanadate is accurately reproduced and the nature of the interladder interaction is established to be ferromagnetic. An analysis of the main contributions to the magnetic couplings is presented and the role of the covalently bonded apex oxygen is elucidated. In the sodium vanadate, the ground state configuration of the rungs is V-3d(1)-O-2p(5)-V-3d(1). We show that with this configuration good intrachain coupling constants are obtained for the high-temperature phase. The interchain coupling in NaV2O5 is predicted to be approximate to34 K, ferromagnetic in nature. (C) 2004 American Institute of Physics.

Early theoretical studies of magnetic interactions between paramagnetic centres in solids and molecules are briefly reviewed as an introduction to the main theme of this paper: nonorthogonal CI approaches for the prediction and interpretation of magnetic couplings. In a non-orthogonal CI approach, the wavefunctions are linear combinations of configuration state functions, each of which is expressed in its own optimized orbital basis set. The NOCI approach allows for an adequate treatment of near-degeneracy correlation effects using a compact, transparent wavefunction. This facilitates straightforward analysis of the physical effects involved. A closely related method is state interaction, where the final wavefunctions are linear combinations of multi-configuration functions. Comparisons are made with the use of conventional configuration interaction and perturbation theory methods. The compound La(2)CuO(4) is selected as an illustrative example.

The absorption edge in Mn K-edge X-ray absorption spectra of manganese oxide compounds shows a shift of several electronvolts in going from MnO through LaMnO3 to CaMnO3 .O nthe other hand, in X-ray photoelectron spectra much smaller shifts are observed. To identify the mechanisms that cause the observed chemical shifts, 1s ionization as well as 1s 3 "4p" transition energies have been determined by electronic structure calculations on embedded Mn ions and embedded MnO6 clusters. Systematic variation of the cluster geometry and the cluster embedding showed that the chemical shifts are predominantly determined by two effects: the changes in the Mn 3d occupation and the changes in the Madelung potential. The large chemical shift in the 1s 3 4p transition energies between different materials occurs because the two effects do not compensate each other. The chemical shifts obtained for the embedded MnO6 clusters agree reasonably with the experimental shifts. The small sensitivity to the material observed for the Mn 1s ionization energies is explained by the near cancellation of the effects of the Madelung potential and the 3d occupation of the Mn ion. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 91: 57- 61, 2003

In benzophenone, intersystem crossing occurs efficiently between the S-1(n pi(star)) state and the T-1 state of dominant n pi(star) character, leading to excited triplet states after photoexcitation. The transition mechanism between S-1(n pi(star)) and T-1 is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S-1 -> T-1 and indirect S-1 -> T-2(pi pi(star)) -> T-1 ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths and conical intersections. It is found that the T-1 state has a mixed n pi(star)/pi pi(star) character and that the T-2(pi pi(star)) state acts as an intermediate state between the S-1 and T-1 states. This result is in line with recent experiments, which suggested a two-step kinetic model to populate the phosphorescent state after photoexcitation [Aloise et al., J. Phys. Chem. A, 2008, 112, 224-231].

A theoretical investigation of the crystal field excitations in CoO is presented. Special attention is given to the excitation energy of the 4 A 2 g state. In recent experimental and theoretical studies an excitation energy around 3.1 eV was reported. This is in disagreement with the 2.1 eV deduced from optical spectroscopy data. After analyzing electron correlation effects, spin–orbit interactions and the material model to represent the CoO crystal, we can confirm the interpretation of the optical data, not only for the 4 A 2 g state but also for all other low-lying crystal field excitations. Electron correlation effects are found to have a significant differential effect on the excitation energies, ranging from +0.3 to −0.6 eV. Spin–orbit interactions are less important, affecting the excitation energies by at most 0.05 eV. Finally, we discuss the effect of the Pauli repulsion between the cluster ions and the first shell of ions around the cluster. This affects the excitation energies by a small, but significant, amount.

A series of embedded cluster calculations have been performed to study the dependence of the calculated ionization and excitation energies in CuCl and NiO on the representation of the direct cluster surrounding. Different embedding schemes have been applied. First, a cluster of one TM ion plus its nearest counterions has been embedded in point charges only. Next, model potentials (AIEMPs) have been used to represent the first layer of ions around the basic cluster, and finally, we embedded the cluster in the charge distribution of frozen ions. The calculations show that the ionization and excitation energies change considerably when the Pauli repulsion of the cluster atoms with the direct surrounding is accounted for. (C) 1999 Elsevier Science B.V. All rights reserved.

The spectral transitions and the character of the low-lying excited states of the copper halides, CuX (X = Cl, Br, I) are studied by means of two different relativistic computational approaches. One is based on the CASSCF/CASPT2 approach with operators accounting for scalar relativistic effects evaluated as a first order correction to the CASSCF energy. The other is a fully relativistic four component SCF-CI treatment based on the Dirac-Coulomb Hamiltonian and hence accounts intrinsically for spin-orbit coupling as well as for scalar effects. The lowest excited states ((1,3)Sigma(+), (1,3)Pi, (1,3)Delta) are all closely related to the formal ionic configuration Cu+(3d(9)4s(1)) X-(ns(2)np(6)). The agreement between calculated and measured transition energies and transition dipoles and their trends in the series strengthens recent assignments of the observed bands. Unobserved 'neutral' states, dominated by the configuration Cu(3d(10)4s(1)) X(ns(2)np(5)), are situated mostly far above the 'ionic' states. Particular attention was given to the mixing of these states, i.e. to the importance of charge transfer effects in the description of the observed states. These seem to be of significance only for the (1) Sigma(+) states, judging from the weights of the charge transfer configurations in the total wave functions and the character of the open shell orbitals. The calculated increase in charge transfer on going from Cl to I in the series goes together with an increase in the calculated transition dipoles for the (1) Sigma(+) states. This is consistent with the observed decrease of the lifetimes. The magnitudes of the spin-orbit splittings in the ionic states are governed by the splitting in Cu+ (2000 cm(-1)) as expected.

A crystalline sample of TiOBr is probed at room temperature by a combination of electron spectroscopies and the results are compared to theoretical embedded-cluster calculations. Resonant photoemission of the valence band confirms that the lowest binding energy feature arises from the singly occupied Ti 3d orbital. The polarization dependence of this orbital in nonresonant photoemission is consistent with the expected dominant d(y)(2)-z(2) character. The analysis of the Ti L(2,3) x-ray absorption spectra confirms the complete splitting of the Ti 3d shell. X-ray absorption and resonant photoemission at the O 1s edge provide direct evidence for hybridization between the transition metal orbitals and the O 2p levels, which leads to superexchange interactions between the Ti ions. The existence of a mixing of O and Ti states and of strong superexchange interactions is supported by calculations of the ground-state electronic and magnetic properties. The calculated superexchange interchain interaction is one fifth in strength of the total magnetic coupling along the chain, and is antiferromagnetic in character. This O-mediated interchain interaction is frustrated in the room temperature phase of TiOBr and thus couples strongly to distortions of the soft lattice. The competition between the interchain magnetoelastic coupling and the spin-Peierls interaction might be at the origin of the complex TiOX phase diagram.

A simple scheme is proposed to analyze the N-electron wave function obtained in embedded cluster calculations in valence bond terms such as ligand-to-metal charge transfer and non-charge-transfer determinants. The analysis is based on a unitary transformation of pairs of natural orbitals to optimal atomiclike orbitals. The procedure is applied to compare the degree of ionicity in NiO and MnO and to explain the existence or absence of Jahn-Teller distortions in LaMnO3, CaMnO3, and CaFeO3. The authors find that the ground state of LaMnO3 is dominated by non-charge-transfer configurations, whereas the charge transfer configurations dominate the ground state wave function in the other two perovskites. (c) 2007 American Institute of Physics.

Ab initio calculations have been performed to clarify the character of the electronic ground state of the high-temperature phase of CaFeO3 at different external pressures. The analysis of the correlated N-electron wave function of properly embedded FeO6 clusters in terms of optimal atomic orbitals clearly establishes the character of the ground state as being dominated by charge transfer configurations. For all pressures, the number of Fe 3d electrons is around 5 and iron should be considered as a Fe3+ ion. We find an S = 2 to S = 1 transition around 25 GPa in the CaFeO3 crystal.

Ab initio calculations have been performed to clarify the character of the electronic ground state of the high-temperature phase of CaFeO3 at different external pressures. The analysis of the correlated N-electron wave function of properly embedded FeO6 clusters in terms of optimal atomic orbitals clearly establishes the character of the ground state as being dominated by charge transfer configurations. For all pressures, the number of Fe 3d electrons is around 5 and iron should be considered as a Fe3+ ion. We find an S = 2 to S = 1 transition around 25 GPa in the CaFeO3 crystal.

The character of the ground and optically excited states was investigated by quantum chemical calculations. We propose a rung ground state with V 3d1xy-O 2p1y-V 3d1xy character, instead of the conventional picture of one unpaired electron shared by 2 V ions. The unpaired electron on O is low-spin coupled to the V d electrons and spin density is predicted to be localized on vanadium. The absorption peak at 0.9 eV is assigned to a state with similar orbital occupations but a different spin coupling scheme, resulting in spin density localized on the bridging oxygen.

The character of the ground and optically excited states was investigated by quantum chemical calculations. We propose a rung ground state with V 3d1xy-O 2p1y-V 3d1xy character, instead of the conventional picture of one unpaired electron shared by 2 V ions. The unpaired electron on O is low-spin coupled to the V d electrons and spin density is predicted to be localized on vanadium. The absorption peak at 0.9 eV is assigned to a state with similar orbital occupations but a different spin coupling scheme, resulting in spin density localized on the bridging oxygen.

The two lowest singlet excitation energies of 26 2-imidazolyl-2-thiazolylazo compounds have been investigated by ab initio methods within the second-order polarization propagator approximation (SOPPA). Various combinations of 4- and 5-substituents at both the imidazole and thiazole units have been studied for obtaining pi --> pi* excitation energies corresponding to an absorption maximum in the red or infrared regions. In many cases, pi --> pi excitation energies below 2 eV (corresponding to wavelengths longer than 600 run) are found for molecules that possibly may be synthesized. For the molecules included in this study, the longest wavelength was calculated to be 1049 nm. (C) 2001 Elsevier Science B.V. All rights reserved.

We present ab initio quantum chemical embedded cluster calculations of Mn core-valence and d−d transitions in LaMnO3. The results are also important for the analysis of recent x-ray absorption and x-ray scattering experiments at the Mn K edge in LaMnO3. We find that the first two peaks of the pre-edge region correspond to majority-spin and minority-spin eg(3d) states on adjacent Mn ions. For on-site 1s to 4p transitions we find that the ordering of the p components is due to the Jahn-Teller distortion. We also show that the 1→s4p transitions are split due to 3d−4p exchange interactions.

We present ab initio quantum chemical embedded cluster calculations of Mn core-valence and d−d transitions in LaMnO3. The results are also important for the analysis of recent x-ray absorption and x-ray scattering experiments at the Mn K edge in LaMnO3. We find that the first two peaks of the pre-edge region correspond to majority-spin and minority-spin eg(3d) states on adjacent Mn ions. For on-site 1s to 4p transitions we find that the ordering of the p components is due to the Jahn-Teller distortion. We also show that the 1→s4p transitions are split due to 3d−4p exchange interactions.

Electronic excitation energies of 16 azobenzene dyes have been calculated by ab initio methods within the second-order polarization propagator approximation (SOPPA). Good agreement with expriment is found for the lowest singlet and triplet states for both the trans- and cis-azobenzene molecules. The differences are in the range of +/-0.3 eV, with the exception of the lowest n --> pi* transition in trans-azobenzene, where a deviation of -0.64 eV is found. The lowest,pi --> pi* transition in trans-azobenzene, on the other hand, is particularly well reprensented with a deviation of only -0.15 eV. Furthermore, the experimental singlet pi --> pi* transitions are reproduced for a set of azobenzene dyes with different electron donor and acceptor groups and the correct shifts in excitation energy are obtained for the different substituents. It has also hen demonstrated that ab initio methods can be used to determine suitable candidates for azo components used in materials for data storage.

The superexchange interaction, parameterized by J, of bulk NiO and the NiO[100] surface was investigated using ab initio quantum chemical techniques. J is influenced by two opposing mechanisms; the reduced Madelung potential at the surface leads to a small increase, whereas the change in the nickel coordination from six to five oxygens gives a larger decrease. As a result J at the [100] surface is predicted to decrease by about 20% with respect to the bulk. This contradicts another recent prediction to the effect that J should increase by about 50%. Experimental data are not yet available.

Ring currents calculated in the ipsocentric CTOCD-DZ formalism are presented for four representative metallabenzenes, compounds in which a benzene CH group is formally replaced by a transition metal atom with ligands. Aromaticity is probed using ring currents computed using non-relativistic and relativistic orbitals (derived with relativistic effective core potentials or ZORA). Maps computed at different levels of relativistic theory turn out to be similar, showing that orbital nodal character is the main determinant of ring current. Diatropic/paratropic global ring currents in these compounds, and also circulations localised on the metal centre, are interpreted in terms of contributions of localised pi-type orbitals and metal d-orbitals, respectively. All four considered metallabenzenes should be regarded as 6 pi electron species, despite the fact that three support diatropic ('aromatic') ring currents and one a paratropic ('anti-aromatic') current. The current-density maps determine the correct way to count electrons in these species: differential occupation of d-orbitals of formal pi-symmetry contributes to circulation on the metal centre, but not around the benzenoid ring. The overall trend from strongly diatropic to weakly paratropic ring currents along the series 1 to 4 is explained by the increasing strength of interaction between formally non-bonding orbitals on the metal centre and C5H5 moiety, which together make up the six-membered ring.

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+O 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, 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 photofragmentation and thus explaining their photostability.

The two lowest singlet excitation energies of 18 azo dyes have been studied by ab initio quantum-chemical methods within the second-order polarization propagator approximation (SOPPA). Various combinations of five-membered rings (furan, thiophene, pyrrole, oxazole, thiazole, and imidazole) have been investigated as diazo components for a potential use in optical das storage materials. It is found that the diazo compounds with two heterocyclic five-membered rings have pi --> pi* excitation energies corresponding to laser wavelengths in the region 450-500 nm whereas one five-membered ring and a phenyl group as diazo components results in wavelengths in the region 400-335 nm. (C) 2000 Published by Elsevier Science B.V.