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The CC2 and ADC(2) wave function models and their spin-component scaled modifications are adopted for predicting vertical ionization potentials (VIPs) and electron affinities (VEAs). The ionic solutions are obtained as electronic excitations in the continuum orbital formalism, making possible the use of existing, widespread quantum chemistry codes with minimal modifications, in full consistency with the treatment of charge transfer excitations. The performance of different variants is evaluated via benchmark calculations on various sets from previous works, containing small and medium-sized systems, including the nucleobases. It is shown that with the spin-scaled approximate methods, in particular the scaled opposite-spin variant of the ADC(2) method the accuracy of EOM-CCSD is achievable at a fraction of the computational cost, also outperforming many common electron propagator approaches.

The performance of multilevel quantum chemical approaches, which utilize an atom-based system partitioning scheme to model various electronic excited states, is studied. The considered techniques include the mechanical-embedding (ME) of "our own N-layered integrated molecular orbital and molecular mechanics" (ONIOM) method, the point charge embedding (PCE), the electronic-embedding (EE) of ONIOM, the frozen density-embedding (FDE), the projector-based embedding (PbE), and our local domain-based correlation method. For the investigated multilevel approaches, the second-order algebraic-diagrammatic construction [ADC(2)] approach was utilized as the high-level method, which was embedded in either Hartree-Fock or a density functional environment. The XH-27 test set of Zech et al. [

While Coupled-Cluster methods have been proven to provide an accurate description of excited electronic states, the scaling of the computational costs with the system size limits the degree for which these methods can be applied. In this work different aspects of fragment-based approach are studied on non-covalently bound molecular complexes with interacting chromophores of the fragments (so called Frenkel pairs), such as pi-stacked nucleobases. The interaction of the fragments is considered at two distinct steps. First, the states localized on the fragments are described in the presence of the other fragment(s); for this we test two approaches. One method is founded on QM/MM principles, only including the electrostatic interaction between the fragments in the electronic structure calculation with Pauli repulsion and dispersion effects added separately. The other model, a Projection-based Embedding (PbE) using the Huzinaga equation includes both electrostatic and Pauli repulsion and only needs to be augmented by dispersion interactions. In both schemes the extended Effective Fragment Potential (EFP2) method of Gordon et al. was found to provide an adequate correction for the missing terms. In the second step, the interaction of the localized chromophores is modeled for a proper description of the excitonic coupling. Here the inclusion of purely electrostatic contributions appears to be sufficient: it is found that the Coulomb part of the coupling provides accurate splitting of the energies of interacting chromophores that are separated by more than 4 Angstrom.

An alternative approach for obtaining accurate vertical ionization potentials (VIPs) and electron affinities (VEAs) via coupled-cluster excitation energy calculations is proposed. The concept allows a coherent handling of all ionic states, including ionizations from lower valence orbitals and attachments to higher-lying virtual ones. The use of existing, widespread quantum chemistry codes with minimal modifications makes the application of well-established wave function models possible, in full consistency with the treatment of charge transfer excitations. Among them, the spin-component scaled forms of the CC2 and ADC(2) methods are potent approaches, especially the scaled opposite-spin variants whose efficient implementations allow the handling of larger systems. The performance of several models is evaluated via benchmark calculations on various sets from previous works, containing small and medium-sized systems, including nucleobases. It is shown that with the most effective scaled approximate methods the accuracy of EOM-CCSD is achievable at a fraction of the computational cost, also outperforming many common electron propagator approaches.

The molecular level understanding of electronic transport properties depends on the reliable theoretical description of charge-transfer (CT)-type electronic states. In this paper, the performance of spin-component-scaled variants of the popular CC2 and ADC(2) methods is evaluated for CT states, following benchmark strategies of earlier studies that revealed a compromised accuracy of the unmodified models. In addition to statistics on the accuracy of vertical excitation energies at equilibrium and infinite separation of bimolecular complexes, potential energy surfaces of the ammonia-fluorine complex are also reported. The results show the capability of spin-component-scaled approaches to reduce the large errors of their regular counterparts to a significant extent, outperforming even the coupled-cluster single and double method in many cases. The cost-effective scaled-opposite-spin variants are found to provide a remarkably good agreement with the CCSDT-3 reference data, thereby being recommended methods of choice in the study of charge-transfer states.

The numerous existing publications on benchmarking quantum chemistry methods for excited states rarely include Charge Transfer (CT) states, although many interesting phenomena in, e.g., biochemistry and material physics involve the transfer of electrons between fragments of the system. Therefore, it is timely to test the accuracy of quantum chemical methods for CT states, as well. In this study we first propose a new benchmark set consisting of dimers having low-energy CT states. On this set, the vertical excitation energy has been calculated with Coupled Cluster methods including triple excitations (CC3, CCSDT-3, CCSD(T)(a)*), as well as with methods including full or approximate doubles (CCSD, STEOM-CCSD, CC2, ADC(2), EOM-CCSD(2)). The results show that the popular CC2 and ADC(2) methods are much less accurate for CT states than for valence states. On the other hand, EOM-CCSD seems to have similar systematic overestimation of the excitation energies for both types of states. Among the triples methods the novel EOM-CCSD(T)(a)* method including noniterative triple excitations is found to stand out with its consistently good performance for all types of states, delivering essentially EOM-CCSDT quality results.

The ground state intermolecular potential of bimolecular complexes of N-heterocycles is analyzed for the impact of individual terms in the interaction energy as provided by various, conceptually different theories. Novel combinations with several formulations of the electrostatic, Pauli repulsion, and dispersion contributions are tested at both short- and long-distance sides of the potential energy surface, for various alignments of the pyrrole dimer as well as the cytosine-uracil complex. The integration of a DFT/CCSD density embedding scheme, with dispersion terms from the effective fragment potential (EFP) method is found to provide good agreement with a reference CCSD(T) potential overall; simultaneously, a quantum mechanics/molecular mechanics approach using CHELPG atomic point charges for the electrostatic interaction, augmented by EFP dispersion and Pauli repulsion, comes also close to the reference result. Both schemes have the advantage of not relying on predefined force fields; rather, the interaction parameters can be determined for the system under study, thus being excellent candidates for ab initio modeling.

The overriding theme of John Stanton's prestigious career has been the close collaboration between experiment and theory. This theme began early in John's academic training as a Ph.D. student in th...

The electron transport through the single-molecule junction of benzene-1,4-diamine (BDA) is modelled using ab initio quantum-classical molecular dynamics of electron attached states. Observations on the nature of the process are made by time-resolved analysis of energy differences, non-adiabatic transition probabilities and the spatial distribution of the excess electron. The role of molecular vibrations that facilitate the transport by being responsible for the periodic behaviour of these quantities is shown using normal mode analysis. The results support a mechanism involving the electron's direct hopping between the electrodes, without its presence on the molecule, with the prime importance of the bending vibrations that periodically alter the molecule���electrode interactions. No relevant differences are found between results provided by the ADC(2) and SOS-ADC(2) excited state models. Our approach provides an alternative insight into the role of nuclear motions in the electron transport process, one which is more expressive from the chemical perspective.

Mass-dependent diagonal Born–Oppenheimer corrections (DBOCs) to the ab initio electronic ground state potential energy surface for the main 16O3 isotopologue and for homogeneous isotopic substitutions 17O3 and 18O3 of the ozone molecule are reported for the first time. The system being of strongly multiconfigurational character, multireference configuration interaction wave function ansatz with different complete active spaces was used. The reliable DBOC calculations with the targeted accuracy were possible to carry out up to about half of the dissociation threshold D0. The comparison with the experimental band centers shows a significant improvement of the accuracy with respect to the best Born–Oppenheimer (BO) ab initio calculations reducing the total root-mean-squares (calculated–observed) deviations by about a factor of two. For the set of 16O3 vibrations up to five bending and four stretching quanta, the mean (calculated–observed) deviations drop down from 0.7 cm−1 (BO) to about 0.1 cm−1, with the most pronounced improvement seen for bending states and for mixed bending-stretching polyads. In the case of bending band centers directly observed under high spectral resolutions, the errors are reduced by more than an order of magnitude down to 0.02 cm−1 from the observed levels, approaching nearly experimental accuracy. A similar improvement for heavy isotopologues shows that the reported DBOC corrections almost remove the systematic BO errors in vibrational levels below D0/2, though the scatter increases towards higher energies. The possible reasons for this finding, as well as remaining issues are discussed in detail. The reported results provide an encouraging accuracy validation for the multireference methods of the ab initio theory. New sets of ab initio vibrational states can be used for improving effective spectroscopic models for analyses of the observed high-resolution spectra, particularly in the cases of accidental resonances with “dark” states requiring accurate theoretical predictions.

International audience; 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.

The ground-state rotational spectrum of propene-3-d1, CH2═CHCH2D, was measured by Fourier transform microwave spectroscopy. Transitions were assigned for the two conformers, one with the D atom in the symmetry plane (S) and the other with the D atom out of the plane (A). The energy difference between the two conformers was calculated to be 6.5 cm–1, the S conformer having lower energy. The quadrupole hyperfine structure due to deuterium was resolved and analyzed for both conformers. The experimental quadrupole coupling and the centrifugal distortion constants compared favorably to their ab initio counterparts. Ground-state rotational constants for the S conformer are 40582.157(9), 9067.024(1), and 7766.0165(12) MHz. Ground-state rotational constants for the A conformer are 43403.75(3), 8658.961(2), and 7718.247(2) MHz. For the A conformer, a small tunneling splitting (19 MHz) due to internal rotation was observed and analyzed. Using the new rotational constants of this work as well as those previously det...

There are numerous publications on benchmarking quantum chemistry methods for excited states. These studies rarely include Charge Transfer (CT) states although many interesting phenomena in e.g. biochemistry and material physics involve transfer of electron between fragments of the system. Therefore, it is timely to test the accuracy of quantum chemical methods for CT states, as well. In this study we first suggest a set benchmark systems consisting of dimers having low-energy CT states. On this set, the excitation energy has been calculated with coupled cluster methods including triple excitations (CC3, CCSDT-3, CCSD(T)(a)* ), as well as with methods including full or approximate doubles (CCSD, STEOM-CCSD, CC2, ADC(2), EOM-CCSD(2)). The results show that the popular CC2 and ADC(2) methods are much more inaccurate for CT states than for valence states. On the other hand, CCSD seems to have similar systematic overestimation of the excitation energies for both valence and CT states. Concerning triples methods, the new CCSD(T)(a)* method including non-iterative triple excitations preforms very well for all type of states, delivering essentially CCSDT quality results.

An up-to-date overview of the CFOUR program system is given. After providing a brief outline of the evolution of the program since its inception in 1989, a comprehensive presentation is given of its well-known capabilities for high-level coupled-cluster theory and its application to molecular properties. Subsequent to this generally well-known background information, much of the remaining content focuses on lesser-known capabilities of CFOUR, most of which have become available to the public only recently or will become available in the near future. Each of these new features is illustrated by a representative example, with additional discussion targeted to educating users as to classes of applications that are now enabled by these capabilities. Finally, some speculation about future directions is given, and the mode of distribution and support for CFOUR are outlined. ispartof: JOURNAL OF CHEMICAL PHYSICS vol:152 issue:21 ispartof: location:United States status: published

Benchmark calculations with the Spin-Component-Scaled CC2 variants SCS-CC2 and SOS-CC2 are presented for the electronically excited valence and Rydberg states of small- and medium-sized molecules. Besides the vertical excitation energies and excited state gradients, the potential energy surfaces are also investigated via scans following the forces that act in the Franck-Condon region. The results are compared to the regular CC2 ones, as well as higher level methods CCSD, CCSD(T)(a)*, and CCSDT. The results indicate that a large fraction of the flaws of CC2 revealed by earlier studies disappears if spin-component scaling is employed. This makes these variants attractive alternatives of their unscaled counterparts, offering competitive accuracy of vertical excitation energies of both valence and Rydberg type states and reliable potential energy surfaces, while also maintaining a low-power-scaling computational cost with the system size.

In this paper the potential energy curves of charge transfer (CT) electronic states and their interaction with local ones have been investigated. Besides the global view of these curves, special at...

The validation of the quality of the description of excited electronic states is of special importance in quantum chemistry as the general reliability of ab initio methods shows a much larger variation for these states than for the ground state. In this study, we investigate the quality of excited state energy gradients and potential energy surfaces on selected systems, as provided by the single reference coupled cluster variants CC2, CCSD, CCSD(T)(a)*, and CC3. Gradients and surface plots that follow the Franck–Condon forces are compared to the respective CCSDT reference values, thereby establishing a useful strategy for judging each variant’s accuracy. The results reveal serious flaws of lower order methods - in particular, CC2 - in several situations where they otherwise give accurate vertical excitation energies, as well as excellent accuracy and consistency of the recently proposed CCSD(T)(a)* method.

International audience; Obtaining an understanding of the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in Chemistry, Molecular Physics, Molecular Biology, and Materials Science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed non-experts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications.

The FCNS ⇆ FC(NS) photoisomerization process is a simple model system for molecular switches. Here, we examined the switching processes by experimental and theoretical methods. Prior matrix-isolation IR spectroscopic studies were complemented by matrix-isolation UV spectroscopic measurements to assist the interpretation of the mechanism of the ring closure and opening processes and to verify the accuracy of the computations on the vertical excitation energies. Vertical excitation energies were computed by the EOMEE-CCSD, MCSCF, and MR-CISD methods. Conical intersections were also searched for and three conical intersections along the reaction path FCNS → FC(NS) were located, one conical intersection between the 2A′ and 1A″ state, one between the 1A″ and 1A′ state and one where all three states intersect. The ring opening and closing processes were simulated by non-adiabatic dynamics propagation with the trajectory surface hopping method. The combined computational and experimental results suggest that upon 365 nm irradiation the ring closure FCNS → FC(NS) occurs under participation of all three conical intersections, while 254 nm irradiation causes ring opening FC(NS) → FCNS. Both processes, especially the ring opening, are accompanied by fragmentation into FCN+S.

For the energy emitted in a textbook example of chemiluminescence, the peculiar red light produced by singlet molecular oxygen is about twice that of the spin-forbidden O2(a1Δg) → O2(X3∑g–) transition. Theoretical studies suggest that the O2(a1Δg)–O2(a1Δg) van der Waals interaction is weak, and at room temperature no long-lived complex is formed. Our high-level ab initio calculations show that in the bound domain of the dimer, the oscillator strength is very small, but increases at smaller intermolecular separations, where, however, the interaction is repulsive. We propose that the emission is induced by collisions: it takes place “on-the-fly”, when the collision energy allows the system to access the repulsive part of the potential energy surface where the oscillator strength is relatively large. The contribution of different orientations of the two O2 molecules to the emission has been evaluated with a simple semiclassical model. The position of the emission peak is in accord with the experiment, and th...

In this paper, benchmark results are presented on Coupled Cluster calculation of singlet excitation energies and the corresponding oscillator strength. The test set of Thiel et al. (Schreiber, M.; Silva, M. R. J.; Sauer, S. P. A.; Thiel, W. J. Chem. Phys. 2008, 128, 134110) has been used, and the earlier results have been extended by CC3 oscillator strength for the whole set, CC3 excitation energies for larger molecules, and CCSDT results for some small molecules. Accuracy of the members of the hierarchy CC2-CCSD-CC3-CCSDT has been analyzed. The results show that both CC2 and CCSD are quite accurate and the difference to CC3 excitations energies is typically not larger than 0.2-0.3 eV. While the mean deviation of the CC2 results is close to zero, CCSD systematically overshoots the CC3 results by about 0.2 eV. The standard deviation is, however, somewhat smaller for CCSD, that is, the latter method provides more systematic results. Still, only a few cases could be identified were the absolute value of the error is over 0.3 eV in case of CC2. The results are even better for CCSD, with the exception of uracil, where surprisingly large error of the excitation energies have been found for two of the four lowest n-π* transitions. Both LR (Linear Response) and EOM (Equation of Motion) style oscillator strengths have been calculated. The former is more accurate at both CC2 and CCSD levels, but the difference between them is only 1-2% in case of CCSD. The error of the CC2 oscillator strength are substantially larger than that of CCSD but qualitatively still correct.

Recent developments in Coupled-Cluster (CC) theory of excited states, which allow the application of these expensive methods for nucleobases and even for their complexes, are overviewed. Accuracy of the methods is analyzed and some recent encouraging results summarized. Finally, we speculate about possible directions of future research and how the CC calculations can be extended to the electric properties of DNA, in particular transport properties. © 2013 Wiley Periodicals, Inc.

We present a comprehensive statistical analysis on the accuracy of various excited state Coupled Cluster methods, accentuating the effect of diffuse basis sets on vertical excitation energies of valence and Rydberg-type states. Many popular approximate doubles and triples methods are benchmarked with basis sets up to aug-cc-pVTZ, with high level EOM-CCSDT results used as reference. The results reveal a serious deficiency of CC2 linear response and CIS(D) techniques in the description of Rydberg states, a feature not shown by the EOM-CCSD(2) and EOM-CCSD variants. The CC3 theory proves to be an accurate choice among the iterative approximate triples methods, while the novel perturbation-based CCSD(T)(a)* variant turns out to be the best way to include the effect of triple excitations in a noniterative way.

Describing electronically excited states of molecules accurately poses a challenging problem for theoretical methods. Popular second order techniques like Linear Response CC2 (CC2-LR), Partitioned Equation-of-Motion MBPT(2) (P-EOM-MBPT(2)), or Equation-of-Motion CCSD(2) (EOM-CCSD(2)) often produce results that are controversial and are ill-balanced with their accuracy on valence and Rydberg type states. In this study, we connect the theory of these methods and, to investigate the origin of their different behavior, establish a series of intermediate variants. The accuracy of these on excitation energies of singlet valence and Rydberg electronic states is benchmarked on a large sample against high-accuracy Linear Response CC3 references. The results reveal the role of individual terms of the second order similarity transformed Hamiltonian, and the reason for the bad performance of CC2-LR in the description of Rydberg states. We also clarify the importance of the T1 transformation employed in the CC2 proce...

DNA building blocks consisting of up to four nucleobases are investigated using the EOM-CCSD and CC2-LR methods in two B-DNA-like arrangements of a poly-adenine:poly-thymine (poly-A:poly-T) system. Excitation energies and oscillator strengths are presented and the characteristics of the excited states are discussed. Excited states of single-stranded poly-A systems are highly delocalized, especially the spectroscopically bright states, where delocalization over up to four fragments can be observed. In the case of poly-T systems, the states are somewhat less delocalized, extending to maximally about three fragments. A single A:T Watson–Crick pair has highly localized states, while delocalization over base pairs can be observed for some excited states of the (A)2:(T)2 system, but intrastrand delocalization is more pronounced in this case, as well. As for the characteristics of the simulated UV absorption spectra, a significant decrease of intensity can be observed in the case of single strands with increasing chain length; this is due to the stacking interactions and is in accordance with previous results. On the other hand, the breaking of H-bonds between the two strands does not alter the spectral intensity considerably, it only causes a redshift of the absorption band, thus it is unable to explain the experimentally observed DNA hyperchromism on its own, and stacking interactions need to be considered for the description of this effect as well.

A new algorithm is presented for the calculation of the ladder-type term of the coupled cluster singles and doubles (CCSD) equations using two-electron integrals in atomic orbital (AO) basis. The method is based on an orbital grouping scheme, which results in an optimal partitioning of the AO integral matrix into sparse and dense blocks allowing efficient matrix multiplication. Carefully chosen numerical tests have been performed to analyze the performance of all aspects of the new algorithm. It is shown that the suggested scheme allows an efficient utilization of modern highly parallel architectures and devices in CCSD calculations. Details of the implementation in the development version of CFOUR quantum chemical program package are also presented.

WOS:000287741100002; International audience; Theoretical study of the absorption spectrum of trans-azomethane (AZM) was presented. Coupled-Cluster type methods (EOMEE-CCSD, CC3), ADC(2) as well as TDDFT/B3LYP calculations with different basis sets were used to obtain the vertical excitation energies and transition moments. The absorption spectrum was simulated by the Linear Vibronic Coupling method by Koppel et al. [Adv. Chem. Phys. 57 (1984) 59] and by semi-classical procedure. Our investigations show that complicated vibronic spectra such as the one of AZM can be well simulated and analyzed by these theoretical techniques. (C) 2010 Elsevier B.V. All rights reserved.

The COLUMBUS Program System allows high-level quantum chemical calculations based on the multiconfiguration self-consistent field, multireference configuration interaction with singles and doubles, and the multireference averaged quadratic coupled cluster methods. The latter method includes size-consistency corrections at the multireference level. Nonrelativistic (NR) and spin–orbit calculations are available within multireference configuration interaction (MRCI). A prominent feature of COLUMBUS is the availability of analytic energy gradients and nonadiabatic coupling vectors for NR MRCI. This feature allows efficient optimization of stationary points and surface crossings (minima on the crossing seam). Typical applications are systematic surveys of energy surfaces in ground and excited states including bond breaking. Wave functions of practically any sophistication can be constructed limited primarily by the size of the CI expansion rather than by its complexity. A massively parallel CI step allows state-of-the art calculations with up to several billion configurations. Electrostatic embedding of point charges into the molecular Hamiltonian gives access to quantum mechanical/molecular mechanics calculations for all wave functions available in COLUMBUS. The analytic gradient modules allow on-the-fly nonadiabatic photodynamical simulations of interesting chemical and biological problems. Thus, COLUMBUS provides a wide range of highly sophisticated tools with which a large variety of interesting quantum chemical problems can be studied. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 191-199 DOI: 10.1002/wcms.25

In this paper I summarize our efforts at QTP to construct a proper Multireference Coupled Cluster method. We have not succeeded, however, the popular MR-AQCC method emerged from this project.

A systematic study of the ozone potential energy surface was performed by means of high level ab initio techniques. The methods include icMR-CISD and icMR-AQCC with all electrons correlated using a full valence CAS reference space and basis sets up to sextuple-ζ quality along with extrapolation to the complete basis set limit. We computed a dense 3D grid as well as 1D cuts along stretching and bending coordinates around the open (C(2v)) equilibrium structure as well as along the minimum energy path to dissociation including the transition state and the van der Waals minimum region. The detailed analysis of our results confirms earlier calculations by the Schinke group and assures that these are not biased by deficiencies of the basis set, lack of relativistic corrections, or core correlation effects. Finally, we discuss possible sources of error that may explain the remaining discrepancies compared to experimental findings.

Any structural change in the nucleotide bases is of literally vital importance for DNA, as such a change may completely destroy the hydrogen bond system of the double helix. Two types of changes can be distinguished. In the electronic ground state, tautomerism is a possible cause of genetic point mutations. In the excited states, when UV light is absorbed— leading potentially to photochemical damage—the chromophores responsible for absorption are the nucleotide bases, including cytosine, subject of the present study. Both experimental and theoretical studies emphasize that the first excited states of nucleotide bases have short lifetimes which ensure that DNA is well protected from photodamage as there is no time for nuclear rearrangements. [1] The ultrafast decay is normally attributed to conical intersection of two states. The latter is the subject of a broader project of ours which aims at investigating all important tautomers of cytosine. When starting this project, however, we soon realized that the very first step, interpretation of the experimental UV spectrum of cytosine (in the ‘canonical’ amino-keto form) already brings up questions. We will argue that the spectrum contains only two, rather than four electronic transitions.

In this short paper a new version of the multireference (MR) averaged coupled pair functional (ACPF) and the MR averaged quadratic coupled-cluster (AQCC) methods is presented which corrects the failure of the original formulation in case of close-lying states described with small reference space. This new version is based on a functional which also includes the reference energy, i.e. instead of optimizing the correlation energy, the total energy will be made stationary with respect the wave function parameters. It is demonstrated that this method gives smooth potential energy surfaces in case of avoided crossings.

We compare experimental Re values with computed Re values for 20 molecules using three multireference electronic structure methods, MCSCF, MR-SDCI, and MR-AQCC. Three correlation-consistent orbital basis sets are used, along with complete basis set extrapolations, for all of the molecules. These data complement those computed previously with single-reference methods. Several trends are observed. The SCF Re values tend to be shorter than the experimental values, and the MCSCF values tend to be longer than the experimental values. We attribute these trends to the ionic contamination of the SCF wave function and to the corresponding systematic distortion of the potential energy curve. For the individual bonds, the MR-SDCI Re values tend to be shorter than the MR-AQCC values, which in turn tend to be shorter than the MCSCF values. Compared to the previous single-reference results, the MCSCF values are roughly comparable to the MP4 and CCSD methods, which are more accurate than might be expected due to the fact that these MCSCF wave functions include no extra-valence electron correlation effects. This suggests that static valence correlation effects, such as near-degeneracies and the ability to dissociate correctly to neutral fragments, play an important role in determining the shape of the potential energy surface, even near equilibrium structures. The MR-SDCI and MR-AQCC methods predict Re values with an accuracy comparable to, or better than, the best single-reference methods (MP4, CCSD, and CCSD(T)), despite the fact that triple and higher excitations into the extra-valence orbital space are included in the single-reference methods but are absent in the multireference wave functions. The computed Re values using the multireference methods tend to be smooth and monotonic with basis set improvement. The molecular structures are optimized using analytic energy gradients, and the timings for these calculations show the practical advantage of using variational wave functions for which the Hellmann–Feynman theorem can be exploited.

This collection of papers presents a special cross section of recent advances in Theoretical Chemistry. It gives a fi ngerprint of the scientifi c interest of Peter R. Surjan, contributors having either interacted with him and/or working on topics closely related to his expertise.

Theoretical modeling of the charge transport in organic materials in the diabatic representation requires an accurate evaluation of the charge transfer integrals. In this paper, we show that the coupled cluster and MBPT(2) approaches are the methods of choice for performing the benchmark calculations of this quantity, in contrast to some recently published results. We demonstrate that a proper treatment of the involved ionized states, achieved by applying the continuum-orbital strategy, reduces the error of the transfer integrals by one order of magnitude, which in the case of the CC2 method corresponds to a lowering of the mean relative unsigned error (MRUE) from 39.9 to 3.8%. Moreover, we extend the application of the continuum-orbital strategy to the TDDFT method, and show that it leads to a dramatic improvement of the description of ionized states compared to the conventional TDDFT approach, characterized by lowering of MRUE from 209.0 to 24.5%.

Analytical forces have been derived in the Lagrangian framework for several random phase approximation (RPA) correlated total energy methods based on the range separated hybrid (RSH) approach, which combines a short-range density functional approximation for the short-range exchange-correlation energy with a Hartree-Fock-type long-range exchange and RPA long-range correlation. The RPA correlation energy has been expressed as a ring coupled cluster doubles (rCCD) theory. The resulting analytical gradients have been implemented and tested for geometry optimization of simple molecules and intermolecular charge transfer complexes, where intermolecular interactions are expected to have a non-negligible effect even on geometrical parameters of the monomers., 18 two-column pages + 4 tables and 6 figures

A detailed quantum chemical investigation was undertaken to obtain the structure and energetics of cytosine hydrates Cyt·nH2O, with n = 1 to 7. The MP2(fc)/aug-cc-pVDZ level was used as the standard, with some DFT (B3LYP) and coupled cluster calculations, as well as calculations with the aug-cc-pVTZ basis set added for comparison. In a systematic search for microhydrated forms of cytosine, we have found that several structures have not yet been reported in the literature. The energies of different isomers, as well as binding energies are compared. When predicting the stability of a complex, we suggest using a scheme where the water molecules are extracted from a finite model of bulk water. Finally, based on energetic data, we suggest a rational definition of the first hydration shell; with this definition, it contains just six water molecules.

International audience; Accurate ab initio full-dimensional dipole moment surfaces of ethylene are computed at 82 542 nuclear configurations using coupled-cluster approach and its explicitly correlated counterpart CCSD(T)-F12 combined respectively with ccpVQZ and cc-pVTZ-F12 basis sets. Their analytical representations are provided through 4-th order normal mode expansions. First-principles predictions of line intensities in rotationally resolved spectra using variational method up to J = 30 are in excellent agreement with experimental data in the range 0-3200 cm−1. Errors of 0.25 - 6.75% in integrated intensities for fundamental bands are comparable with experimental uncertainties. Overall calculated C2H4 opacity in 600-3300 cm−1 range agrees with experimental determination better than to 0.5%. The improved accuracy permitted to resolve some controversial issues related to the qualitative behavior of intensity patterns.

The charge transfer integral is a key parameter required by various theoretical models to describe charge transport properties, e.g., in organic semiconductors. The accuracy of this important property depends on several factors, which include the level of electronic structure theory and internal simplifications of the applied formalism. The goal of this paper is to identify the performance of various approximate approaches of the latter category, while using the high level equation-of-motion coupled cluster theory for the electronic structure. The calculations have been performed on the ethylene dimer as one of the simplest model systems. By studying different spatial perturbations, it was shown that while both energy split in dimer and fragment charge difference methods are equivalent with the exact formulation for symmetrical displacements, they are less efficient when describing transfer integral along the asymmetric alteration coordinate. Since the “exact” scheme was found computationally expensive, we examine the possibility to obtain the asymmetric fluctuation of the transfer integral by a Taylor expansion along the coordinate space. By exploring the efficiency of this novel approach, we show that the Taylor expansion scheme represents an attractive alternative to the “exact” calculations due to a substantial reduction of computational costs, when a considerably large region of the potential energy surface is of interest. Moreover, we show that the Taylor expansion scheme, irrespective of the dimer symmetry, is very accurate for the entire range of geometry fluctuations that cover the space the molecule accesses at room temperature.