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Nuclear spin-induced circular dichroism (NSCD) is a molecular effect of differential absorption of left- and right-circularly polarized light due to nuclear spins in the molecule. In this work, new tools for its calculation are presented. Specifically, analytic expressions for the computation of the  K term of NSCD have been derived and implemented for the second-order coupled cluster singles and doubles (CC2) model. NSCD results obtained thereby for three derivatives of azobenzenes have been compared with results from time-dependent density functional theory (TD-DFT). The complementary information that could be obtained from NSCD measurements compared to NMR for these three species is discussed.

This work employs the correlated wavefunction-based methods ADC(2) and CC2 in combination with the implicit solvent model COSMO to calculate the UV/Vis absorption and fluorescence emission energies of particularly strong hydroxypyrene photoacids in acetone. According to the Förster cycle, the electronic transition energies are first used to compute , i.e. , the p K a change upon excitation and then the excited-state p K a (labeled ) with ground-state p K a values based on COSMO-RS as additional inputs. Furthermore, for the strongest photoacid of that class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate, the need to go beyond implicit solvation and to account for explicit solvent effects on the electronic transition energies and the resulting Δp K a is investigated in the solvents acetone, dimethyl sulfoxide (DMSO), and water. For this, a hybrid implicit-explicit approach is followed by comparing micro-solvated structures that are generated based on Kamlet-Taft considerations. While implicit solvent effects are mostly sufficient for the aprotic solvent acetone, one explicit solvent molecule seems relevant for DMSO due to its stronger hydrogen-bond (HB) acceptance and hence larger interaction with the photoacid OH group as a HB donor. For the protic solvent water, the situation is more complicated, involving at least one water molecule at the OH group and up to three water molecules at the O - group of the corresponding base. Finally, these results are used to rationalize the experimentally observed spectral evolution of the photoacid absorption band in acetone-water solvent mixtures.

Iron and cobalt-based oxides crystallizing in the spinel structure are efficient and affordable catalysts for the oxidation of organics, yet, the detailed understanding of their surface structure and reactivity is limited. To fill this gap, we have investigated the (001) surfaces of cobalt ferrite, CoFe 2 O 4 , with the A- and B-layer terminations using density functional theory (DFT/PBE0) and an embedded cluster model. We have considered the five-fold coordinated Co 2+/3+ (O h ), two-fold coordinated Fe 2+ (T d ), and an oxygen vacancy, as active sites for the adsorption of water and short-chain alcohols: methanol, ethanol, and 2-propanol, in the low coverage regime. The adsorbates dissociate upon adsorption on the Fe sites whereas the adsorption is mainly molecular on Co. At oxygen vacancies, the adsorbates always dissociate, fill the vacancy and form (partially) hydroxylated surfaces. The computed vibrational spectra for the most stable configurations are compared with results from diffuse reflectance infrared Fourier transform spectroscopy.

We investigate how the absorption and fluorescence of halogenated imidazolium compounds in acetonitrile solution is influenced by the presence of counterions and the ability to act as halogen-bond donors. Experimental measurements and quantum chemical calculations with correlated wavefunction methods are applied to study three monodentate halogen-bond complexes of iodo-imidazolium, iodo-benzimidazolium and bromo-benzimidazolium cations with triflate counterions, and a bidentate complex of bis(iodo-benzimidazolium) dications with chloride as counterion. The three monodentate complexes with triflate counterions relax after photoexcitation to minima on the S 1 potential energy surface where the C-I bond and the IO halogen bond are partially broken. For the bidentate complex with the smaller chloride counterion the halogen-bond interaction stays intact in the S 1 minimum that is reached by relaxation from the Franck-Condon point. In a complementing experimental approach, stationary absorption and emission as well as transient fluorescence spectra are recorded for iodo- and bromo-benzimidazolium in acetonitrile. Variation of the counterion, substitution of the iodine by bromine, hydrogen, or methyl, and the comparison to theory allows the identification of spectroscopic signatures and photoinduced dynamics associated with ion-pairing.

Pd nanoparticles deposited on nitrogen-doped mesoporous carbon are promising catalysts for highly selective and effective catalytic hydrogenation reactions. To design and utilize these novel catalysts, it is essential to understand the effect of N doping on the metal-support interactions. A combined experimental (X-ray photoelectron spectroscopy) and computational (density functional theory) approach is used to identify preferential adsorption sites and to give detailed explanations of the corresponding metal-support interactions. Pyridinic N atoms turned out to be the preferential adsorption sites for Pd nanoparticles on nitrogen-doped mesoporous carbon, interacting through their lone pairs (LPs) with the Pd atoms via N-LP - Pd dσ and N-LP - Pd s and Pd dπ - π* charge transfer, which leads to a change in the Pd oxidation state. Our results evidence the existence of bifunctional palladium nanoparticles containing Pd0 and Pd2+ centers.

A key step for achieving better insight into catalytic hydrogenation reactions is to understand in detail the process of hydrogen adsorption on the catalyst. The present article focuses on hydrogen adsorption on carbon-supported palladium clusters, which are nowadays one of the most common catalysts in industrial applications. Density functional theory is applied to study Pd 6 and Pd 21 clusters to reveal the influence of the carbon support material on the properties of the catalyst as well as on the mechanisms and energetics of the hydrogen adsorption. In general, a stepwise hydrogen adsorption process is observed consisting of molecular adsorption followed by dissociative chemisorption. The carbon support material does not noticeably affect the reaction mechanisms, but has a large influence on energy barriers and preferential adsorption sites. Our comparison of Pd 6 and Pd 21 systems reveals that small clusters, such as Pd 6 , are able to model some but not all important properties of palladium nanoparticles and, therefore, it is essential to also study larger cluster sizes.

Analytic second nuclear derivatives for excited electronic state energies have been implemented for the resolution-of-the-identity accelerated CC2, CIS(D ∞ ) and ADC(2) models. Our efficient implementation with O(N 2 ) memory demands enables the treatment of medium sized molecules with large basis sets and high numerical precision and thereby paves the way for semi-numerical evaluation of the higher-order derivatives required for anharmonic corrections to excited state vibrational frequencies. We compare CC2 harmonic and anharmonic excited state frequencies with experimental values for para-difluorobenzene, toluene and catechol. Basis set problems occur for out-of-plane bending vibrations due to intramolecular basis set superposition error. For non-planar molecules and in plane modes of planar molecules, the agreement between theory and experiment is better than 30 cm -1 on average and we reassign a number of experimental bands on the basis of the ab initio predictions.

Understanding deactivation mechanisms of functional groups is a key step to design novel photo-active devices and molecular imaging agents. Here, we elucidate the photochemistry of linear triazenes, an extended analogue of the photo-switchable azo group, exemplarily for the widely used DNA-minor-groove binder berenil. Combining ultrafast spectroscopy and ab initio calculations unveils that the E-azo,s-trans structure of berenil predominates in the gas phase and in aqueous solution, and ADC(2) intrinsic reaction coordinate calculations disclose that the excited-state relaxation to the S1 minima/conical intersections follows a two-step mechanism: N[double bond, length as m-dash]N bond stretching followed by a bicycle-pedal rotation in the triazene bridge. Furthermore, studying the ground-state pathways shows that a fraction of the molecules relaxes back to the E-azo,s-trans isomer while the other part photoisomerizes to the Z-azo,s-trans via a hula-twist motion, as evidenced by experimental quantum yields of Φ ≈ 0.5 found for berenil in water, ethylene glycol, or bound to β-trypsin. Moreover, our studies show that while the excited-state relaxation is insensitive to the environment, the ground-state dynamics depend on biomolecular binding partners.

We report the first calculations of rotational strengths for circularly polarised phosphorescence at the coupled cluster level. The rotational strengths for circular dichroism (CD), circularly polarised fluorescence (CPF) and circularly polarised phosphorescence (CPP) were calculated for β,γ-enones and ketones with conjugated double bonds using the CC2 model. To compute spin-forbidden CPP, spin-orbit coupling with perturbation theory is employed within a response theory framework (SOC-PT-CC2). For closed-shell molecules containing only light elements, a spin-free formalism is used to treat the singlet-triplet transitions. It is verified that the simplification obtained in the spin-free formulation from the Wigner-Eckart theorem for spin-forbidden oscillator strengths is also valid for the rotational strengths. Our implementation utilises the resolution of identity (RI) approximation for two-electron integrals which facilitates applications to larger molecules. In the current study, the rotational strength for spin-forbidden circularly-polarised phosphorescence was calculated for a chiral aromatic system, S-DMBDA, containing 44 atoms.

We present an implementation of analytic gradients for electronically excited states for the algebraic-diagrammatic construction through second order, ADC(2), in combination with the conductor-like screening model (COSMO) as an implicit solvent model. The implementation uses a post-SCF reaction field scheme for the coupling between the environment and the quantum system which retains the computational efficiency of the gas-phase RI-ADC(2) calculations. Applying this approach, we computed solvatochromic shifts for UV absorption and fluorescence transitions of 4-(N,N-dimethylamino)benzonitrile using equilibrium geometries for the ground and the first excited states optimized in the presence of acetonitrile as solvent. Furthermore, we investigated the excited state energies and geometries of the 2-iodobenzimidazolium·triflate ion pair in aqueous solution as an example where solvent effects have a large influence on the structure and the UV spectrum.

We present the first origin-independent approach for the treatment of two-photon circular dichroism (TPCD) using coupled cluster methods. The approach is assessed concerning its behavior on the choice of the basis set and different coupled cluster methods. We also provide a comparison of results from CC2 with those from density functional theory using the CAM-B3LYP functional. Concerning the basis set we note that in most cases an augmented triple zeta basis or a doubly augmented double zeta basis is needed for reasonably converged results. In the comparison of different coupled cluster methods results from CCSD, CC3 and CC2 have been found to be quite similar in most cases, while CCS results differ remarkably from the results at the higher levels. However, this proof-of-principle study also shows that further benchmarking of DFT and CC2 against accurate coupled cluster reference values (e.g. CCSD or CC3) is needed.

We present our current progress on the combination of explicit electron correlation with the pair natural orbital (PNO) representation. In particular we show cubic scaling PNO-MP2-F12, and PNO-CCSD[F12] implementations. The PNOs are constructed using a hybrid scheme, where the PNOs are generated in a truncated doubles space, spanned by orbital specific virtuals obtained using an iterative eigenvector algorithm. We demonstrate the performance of our implementation through calculations on a series of glycine chains. The accuracy of the local approximations is assessed using the S66 benchmark set, and we report for the first time explicitly correlated CCSD results for the whole set and improved estimates for the CCSD/CBS limits. For several dimers the PNO-CCSD[F12] calculations are more accurate than the current reference values. Additionally, we present pilot applications of our PNO-CCSD[F12] code to host-guest interactions in a cluster model for zeolite H-ZSM-5 and in a calix[4]arene-water complex.

We investigate the performance of the approximate coupled cluster singles- and doubles model CC2 in the prediction of optical rotations of organic molecules. For this purpose we employ a combination of two test sets from the literature which include small and medium-sized rigid organic molecules and a series of helicenes. CC2 calculations on molecules as large as 11-helicene became possible through a recent implementation of frequency-dependent second-order properties for CC2 which makes use of the resolution-of-the-identity approximation for the electron repulsion integrals. The results are assessed with respect to the accuracy of the absolute values of the optical rotation and the prediction of the correct sign, which is crucial for the determination of absolute configurations. The performance of CC2 is compared with that of density functional theory at the B3LYP and CAM-B3LYP levels. Furthermore we investigated the influence of the molecular geometry and the one-electron basis set and tested to which extent spin-component scaling changes the results.

Optogenetics is by now a well-established field within neuroscience where neuro-response is controlled at the molecular level using the photochemical properties of channelrhodopsin (ChR). In this study the recently published X-ray structure of retinal inside the ChR binding pocket serves as the basis for conducting high-level polarizable embedding quantum mechanical/molecular mechanical (QM/MM) mutation studies with the aim of providing insight into the tuning mechanisms of this remarkable protein. The levels of theory applied are the recently developed PERI-CC2 and PE-DFT approaches. Their computational efficiency makes it possible to rapidly carry out a large number of spectral calculations. This is exploited to construct in silico mutated ChR variants which are characterized in terms of the location of the relevant excitation energy and the magnitude of the two-photon absorption cross section. In turn, this allows us to pinpoint the amino acids that have the largest electrostatic effect on the studied excited state properties. We show that a single/double site mutation strategy in ChR does not perturb the electronic properties of retinal to a degree that satisfies the experimental demand for a significant red-shift. With respect to non-linear absorption we conjecture that the recently synthesized ChETA variant possesses an even larger two-photon cross section than the C1C2 variant and it is thus an ideal candidate for further studies on the two-photon activation of ChR.

In the present study a benchmark set of medium-sized and large aromatic organic molecules with 10-78 atoms is presented. For this test set 0-0 transition energies measured in supersonic jets are compared to those calculated with DFT and the B3LYP functional, ADC(2), CC2 and the spin-scaled CC2 variants SOS-CC2 and SCS-CC2. Geometries of the ground and excited states have been optimized with these methods in polarized triple zeta basis sets. Zero-point vibrational corrections have been calculated with the same methods and basis sets. In addition the energies have been corrected by single point calculations with a triple zeta basis augmented with diffuse functions, aug-cc-pVTZ. The deviations of the theoretical results from experimental electronic origins, which have all been measured in the gas phase with high-resolution techniques, were evaluated. The accuracy of SOS-CC2 is comparable to that of unscaled CC2, whereas ADC(2) has slightly larger errors. The lowest errors were found for SCS-CC2. All correlated wave function methods provide significantly better results than DFT with the B3LYP functional. The effects of the energy corrections from the augmented basis set and the method-consistent calculation of the zero-point vibrational corrections are small. With this benchmark set reliable reference data for 0-0 transition energies for larger organic chromophores are available that can be used to benchmark the accuracy of other quantum chemical methods such as new DFT functionals or semi-empirical methods for excitation energies and structures and thereby augments available benchmark sets augments present benchmark sets which include mainly smaller molecules.

In this work we combined computational density functional theory with experimental infrared spectroscopy to determine the adsorbate structure of NO and its reaction products N(2)O(2), N(2)O, and NO(2) on rutile TiO(2). These reactions are important for the photo-catalytic reduction of NO in exhaust gas, but yet little is known about the mechanisms or the intermediates involved. The combination of high-quality ultrahigh vacuum FTIRS data with large scale embedded cluster calculations using an accurate hybrid density functional rendered it possible to identify and assign unambiguously vibrational frequencies for nine species which are formed upon adsorption and reaction of NO on rutile TiO(2). Some of them have been observed for the first time. As a result of the quantum chemical calculations we can report for all adsorbates accurate structures and binding energies.

We investigate defects in bulk zinc oxide by performing embedded cluster calculations within the density functional theory using the PBE functional. We obtain and discuss equilibrium structures and vibrational spectra of interstitial hydrogen species and of hydrogen complexes at zinc vacancies and at substitutional atoms like copper, beryllium, manganese and calcium in zinc positions. All of these defects differ in their vibrational frequencies which can be assigned to existing experimental data with an agreement that surpasses the results of earlier calculations within the local (spin) density approximation. This leads to a better understanding of these bulk defects and helps to answer some open questions, as e.g. the assignment of a signal observed in infra red spectra at 3326 cm(-1). Furthermore, the computed vibrational spectra may serve as a benchmark and assist in the interpretation of future experimental results.

We report optimised auxiliary basis sets for the resolution-of-the-identity (or density-fitting) approximation of two-electron integrals in second-order Møller-Plesset perturbation theory (MP2) and similar electronic structure calculations with correlation-consistent basis sets for the post-d elements Ga-Kr, In-Xe, and Tl-Rn. The auxiliary basis sets are optimised such that the density-fitting error is negligible compared to the one-electron basis set error. To check to which extent this criterion is fulfilled we estimated for a test set of 80 molecules the basis set limit of the correlation energy at the MP2 level and evaluated the remaining density-fitting and the one-electron basis set errors. The resulting auxiliary basis sets are only 2-6 times larger than the corresponding one-electron basis sets and lead in MP2 calculations to speed-ups of the integral evaluation by one to three orders of magnitude. The density-fitting errors in the correlation energy are at least hundred times smaller than the one-electron basis set error, i.e. in the order of only 1-100 μH per atom., (This journal is © the Owner Societies 2012)

An implementation of two-photon absorption matrix elements using the approximate second-order coupled-cluster singles and doubles model CC2 is presented. In this implementation we use the resolution-of-the-identity approximation for the two-electron repulsion integrals to reduce the computational cost. To avoid storage of large arrays we introduce in addition a numerical Laplace transformation of orbital energy denominators for the response of the doubles amplitudes. The error due to the numerical Laplace transformation is found to be negligible. Using this new implementation, we performed a series of benchmark calculations on substituted benzene and azobenzene derivatives to get reference values for TD-DFT results. We show that results obtained with the Coulomb-attenuated B3LYP functional are in reasonable agreement with the coupled-cluster results, whereas other density functionals which do not have a long-range correction give considerably less accurate results. Applications to the AF240 dye molecule and a weakly bound molecular tweezer complex demonstrate that this new RI-CC2 implementation allows for the first time to compute two-photon absorption cross sections with a correlated wave function method for molecules with more than 70 atoms and to apply this method for benchmarking TD-DFT calculations on molecules which are of particular relevance for experimental studies of two-photon absorption.

A generalization of the spin-component scaling and scaled opposite-spin modifications of second-order Møller-Plesset perturbation theory to the approximate coupled-cluster singles-and-doubles model CC2 (termed SCS-CC2 and SOS-CC2) is discussed and a preliminary implementation of ground and excited state energies and analytic gradients is reported. The computational results for bond distances, harmonic frequencies, adiabatic and 0-0 excitation energies are compared with experimental results to benchmark their performance. It is found that both variants of the spin-scaling increase the robustness of CC2 against strong correlation effects and lead for this method even to somewhat larger improvements than those observed for second-order Møller-Plesset perturbation theory. The spin-component scaling also enhances systematically the accuracy of CC2 for 0-0 excitation energies for pi --> pi* and n --> pi* transitions, if geometries are determined at the same level.

The explicitly-correlated coupled-cluster method CCSD(T)(R12) is extended to include F12 geminal basis functions that decay exponentially with the interelectronic distance and reproduce the form of the average Coulomb hole more accurately than linear-r(12). Equations derived using the Ansatz 2 strong orthogonality projector are presented. The convergence of the correlation energy with orbital basis set for the new CCSD(T)(F12) method is studied and found to be rapid, 98% of the basis set limit correlation energy is typically recovered using triple-zeta orbital basis sets. The performance for reaction enthalpies is assessed via a test set of 15 reactions involving 23 molecules. The title statement is found to hold equally true for total and relative correlation energies.

We present a parallel implementation of second-order Møller-Plesset perturbation theory with the resolution-of-the-identity approximation (RI-MP2). The implementation is based on a recent improved sequential implementation of RI-MP2 within the Turbomole program package and employs the message passing interface (MPI) standard for communication between distributed memory nodes. The parallel implementation extends the applicability of canonical MP2 to considerably larger systems. Examples are presented for full geometry optimizations with up to 60 atoms and 3300 basis functions and MP2 energy calculations with more than 200 atoms and 7000 basis functions.