Prediction Of Excitation Energies Research Articles

Theoretical study of excited states of DNA base dimers and tetramers using optimally tuned range-separated density functional theory.

Excited states of various DNA base dimers and tetramers including Watson-Crick H-bonding and stacking interactions have been investigated by time-dependent density functional theory using nonempirically tuned range-separated exchange (RSE) functionals. Significant improvements are found in the prediction of excitation energies and oscillator strengths, with results comparable to those of high-level coupled-cluster (CC) models (RI-CC2 and EOM-CCSD(T)). The optimally-tuned RSE functional significantly outperforms its non-tuned (default) version and widely-used B3LYP functional. Compared to those high-level CC benchmarks, the large mean absolute deviations of conventional functionals can be attributed to their inappropriate amount of exact exchange and large delocalization errors which can be greatly eliminated by tuning approach. Furthermore, the impacts of H-bonding and π-stacking interactions in various DNA dimers and tetramers are analyzed through peak shift of simulated absorption spectra as well as corresponding change of absorption intensity. The result indicates the stacking interaction in DNA tetramers mainly contributes to the hypochromicity effect. The present work provides an efficient theoretical tool for accurate prediction of optical properties and excited states of nucleobase and other biological systems. © 2015 Wiley Periodicals, Inc.

Multiconfiguration Pair-Density Functional Theory Outperforms Kohn-Sham Density Functional Theory and Multireference Perturbation Theory for Ground-State and Excited-State Charge Transfer.

The correct description of charge transfer in ground and excited states is very important for molecular interactions, photochemistry, electrochemistry, and charge transport, but it is very challenging for Kohn-Sham (KS) density functional theory (DFT). KS-DFT exchange-correlation functionals without nonlocal exchange fail to describe both ground- and excited-state charge transfer properly. We have recently proposed a theory called multiconfiguration pair-density functional theory (MC-PDFT), which is based on a combination of multiconfiguration wave function theory with a new type of density functional called an on-top density functional. Here we have used MC-PDFT to study challenging ground- and excited-state charge-transfer processes by using on-top density functionals obtained by translating KS exchange-correlation functionals. For ground-state charge transfer, MC-PDFT performs better than either the PBE exchange-correlation functional or CASPT2 wave function theory. For excited-state charge transfer, MC-PDFT (unlike KS-DFT) shows qualitatively correct behavior at long-range with great improvement in predicted excitation energies.

Influence of Force Fields and Quantum Chemistry Approach on Spectral Densities of BChl a in Solution and in FMO Proteins.

Studies on light-harvesting (LH) systems have attracted much attention after the finding of long-lived quantum coherences in the exciton dynamics of the Fenna-Matthews-Olson (FMO) complex. In this complex, excitation energy transfer occurs between the bacteriochlorophyll a (BChl a) pigments. Two quantum mechanics/molecular mechanics (QM/MM) studies, each with a different force-field and quantum chemistry approach, reported different excitation energy distributions for the FMO complex. To understand the reasons for these differences in the predicted excitation energies, we have carried out a comparative study between the simulations using the CHARMM and AMBER force field and the Zerner intermediate neglect of differential orbital (ZINDO)/S and time-dependent density functional theory (TDDFT) quantum chemistry methods. The calculations using the CHARMM force field together with ZINDO/S or TDDFT always show a wider spread in the energy distribution compared to those using the AMBER force field. High- or low-energy tails in these energy distributions result in larger values for the spectral density at low frequencies. A detailed study on individual BChl a molecules in solution shows that without the environment, the density of states is the same for both force field sets. Including the environmental point charges, however, the excitation energy distribution gets broader and, depending on the applied methods, also asymmetric. The excitation energy distribution predicted using TDDFT together with the AMBER force field shows a symmetric, Gaussian-like distribution.

Why do TD-DFT excitation energies of BODIPY/Aza-BODIPY families largely deviate from experiment? Answers from electron correlated and multireference methods.

The vertical excitation energies of 17 boron-dipyrromethene (BODIPY) core structures with a variety of substituents and ring sizes are benchmarked using time-dependent density functional theory (TD-DFT) with nine different functionals combined with the cc-pVTZ basis set. When compared to experimental measurements, all functionals provide mean absolute errors (mean AEs) greater than 0.3 eV, larger than the 0.1-0.3 eV differences typically expected from TD-DFT. Due to the high linear correlation of TD-DFT results with experiment, most functionals can be used to predict excitation energies if corrected empirically. Using the CAM-B3LYP functional, 0-0 transition energies are determined, and while the absolute difference is improved (mean AE = 0.478 eV compared to 0.579 eV), the correlation diminishes substantially (R(2) = 0.961 to 0.862). Two very recently introduced charge transfer (CT) indices, q(CT) and d(CT), and electron density difference (EDD) plots demonstrate that CT does not play a significant role for most of the BODIPYs examined and, thus, cannot be the source of error in TD-DFT. To assess TD-DFT methods, vertical excitation energies are determined utilizing TD-HF, configuration interaction CIS and CIS(D), equation of motion EOM-CCSD, SAC-CI, and Laplace-transform based local coupled-cluster singles and approximate doubles LCC2* methods. Moreover, multireference CASSCF and CASPT2 vertical excitation energies were also obtained for all species (except CASPT2 was not feasible for the four largest systems). The SAC-CI/cc-pVDZ, LCC2*/cc-pVDZ, and CASPT2/cc-pVDZ approaches are shown to have the smallest mean AEs of 0.154, 0.109, and 0.100 eV, respectively; the utility of the LCC2* approach is demonstrated for eight extended BODIPYs and aza-BODIPYs. We found that the problems with TD-DFT arise from difficulties in dealing with the differential electron correlation (as assessed by comparing CCS, CC2, LR-CCSD, CCSDR(T), and CCSDR(3) vertical excitation energies for five compounds) and from contributions of multireference character and double excitations (from analysis of the CASSCF wave functions).

Tuning the near-IR band energy and redox potentials of magnesium tetra(ferrocenyl)tetraazaporphyrins

Magnesium 2(3),7(8),12(13),17(18)-tetraferrocenyl-5,10,15,20-tetraazaporphyrin (3), and magnesium 2(3),7(8),12(13),17(18)-tetracyano-3(2),8(7),13(12),18(17)-tetraferrocenyl-5,10,15,20-tetraazaporphyrin (4) complexes were prepared using template condensation reaction between magnesium butoxide and (Z)-dicyanovinylferrocene (1) and tricyanovinylferrocene (2), respectively. Redox and unusual optical properties of macrocyclic compounds 3 and 4 were investigated in details using UV-vis, MCD, electro-, and spectroelectrochemical methods as well as DFT and TDDFT calculations. It was shown that the HOMO in both compounds resembles Goutermans' a1u orbital (D4h symmetry), which is different from meso-ferrocenyl containing porphyrins. In both macrocycles 3 and 4 overlapping oxidation waves for ferrocene substituents were observed in electrochemical and spectroelectrochemical experiments. Electrochemically observed oxidation waves are rather broad because of the presence of positional isomers and span only ~300 mV range in DCM/0.05 M TBAF system. Based on the excellent agreement between experimental UV-vis spectra and TDDFT predicted excitation energies, it was suggested that the low-energy NIR and visible regions are predominantly composed by the MLCT transitions with a significant π–π* mixing.

Erratum: “How accurate is the strongly orthogonal geminal theory in predicting excitation energies? Comparison of the extended random phase approximation and the linear response theory approaches” [J. Chem. Phys. 140, 014101 (2014)

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The Second-Order Polarization Propagator Approximation (SOPPA) method coupled to the polarizable continuum model

We present an implementation of the Polarizable Continuum Model (PCM) in combination with the Second-Order Polarization Propagator Approximation (SOPPA) electronic structure method. In analogy with the most common way of designing ground state calculations based on a Second-Order Møller–Plesset (MP2) wave function coupled to PCM, we introduce dynamical PCM solvent effects only in the Random Phase Approximation (RPA) part of the SOPPA response equations while the static solvent contribution is kept in both the RPA terms as well as in the higher order correlation matrix components of the SOPPA response equations. By dynamic terms, we refer to contributions that describe a change in environmental polarization which, in turn, reflects a change in the core molecular charge distribution upon an electronic excitation. This new combination of methods is termed PCM–SOPPA/RPA. We apply this newly defined method to the challenging cases of solvent effects on the lowest and intense electronic transitions in o-, m- and p-nitroaniline and o-, m- and p-nitrophenol and compare the performance of PCM–SOPPA/RPA with more conventional approaches. Compared to calculations based on time-dependent density functional theory employing a range-separated exchange–correlation functional, we find the PCM–SOPPA/RPA approach to be slightly superior with respect to systematicity. On the other hand, the absolute values of the predicted excitation energies are largely underestimated. This – however – is a well-know feature of the SOPPA model itself and is not connected to its combination with the PCM.

Lowest negative-parity states inBe12

A very simple model is applied to the first four negative-parity states of $^{12}\mathrm{Be}$. Energies of the corresponding four states in $^{14}\mathrm{C}$ are used to validate the model and to determine the doublet splitting parameters. Predictions for $^{12}\mathrm{Be}$ are in remarkable agreement with excitation energies of the known 1${}^{\ensuremath{-}}$ at 2.70 MeV and the suspected (3${}^{\ensuremath{-}}$) at 4.56 MeV. Predicted excitation energies of 0${}^{\ensuremath{-}}$ and 2${}^{\ensuremath{-}}$ are 3.59 and 5.12 MeV, respectively.

How accurate is the strongly orthogonal geminal theory in predicting excitation energies? Comparison of the extended random phase approximation and the linear response theory approaches

Performance of the antisymmetrized product of strongly orthogonal geminal (APSG) ansatz in describing ground states of molecules has been extensively explored in the recent years. Not much is known, however, about possibilities of obtaining excitation energies from methods that would rely on the APSG ansatz. In the paper we investigate the recently proposed extended random phase approximations, ERPA and ERPA2, that employ APSG reduced density matrices. We also propose a time-dependent linear response APSG method (TD-APSG). Its relation to the recently proposed phase including natural orbital theory is elucidated. The methods are applied to Li2, BH, H2O, and CH2O molecules at equilibrium geometries and in the dissociating limits. It is shown that ERPA2 and TD-APSG perform better in describing double excitations than ERPA due to inclusion of the so-called diagonal double elements. Analysis of the potential energy curves of Li2, BH, and H2O reveals that ERPA2 and TD-APSG describe correctly excitation energies of dissociating molecules if orbitals involved in breaking bonds are involved. For single excitations of molecules at equilibrium geometries the accuracy of the APSG-based methods approaches that of the time-dependent Hartree-Fock method with the increase of the system size. A possibility of improving the accuracy of the TD-APSG method for single excitations by splitting the electron-electron interaction operator into the long- and short-range terms and employing density functionals to treat the latter is presented.

New Studies on the Aspects of Nuclear Shapes

Using the pairing-deformation-frequency self-consistent total-Routhiansurface and configuration-constrained potential-energy-surface calculations, we have studied nuclear deformation and its effect on the structure of nuclei. It was found that the high-order multipolarity-six (β6) deformation plays a significant role in superheavy nuclei. Possible non-collective highspin isomeric states which locate in the second well of actinide nuclei have been investigated with the predictions of excitation energies and configurations. High-spin isomers can extend shape coexistence in A ∼ 190 neutrondeficient nuclei. Triaxiality with γ 30 is found in the ground and excited rotational states of the A ∼ 70 germanium isotopes. Octupole correlations have also been discussed in different mass regions. In recent experiments, the textbook nucleus 158Er has been reached at ultrahigh spins around 65∼. We have studied 158Er ultrahigh-spin states by means of the self-consistent tilted-axis-cranking method based on the Nilsson shell correction and the Skyrme-Hartree-Fock model. The calculation with a ≈ 12 ° triaxialstrongly-deformed (TSD) excited configuration can well reproduce the observed large transitional quadrupole moment. It is demonstrated that the TSD minimum at negative γdeformation which appears in the principalaxis-cranking approach is a saddle point if allowing the rotational axis to change direction.

A Density Functional Theory Investigation on the Properties of Supramolecular Catalysts for Photoinitiated Electron Collection

In this work we investigated by density functional theory (DFT)/time-dependent DFT (TDDFT) supramolecular complexes for photoinitiated electron collection, in particular [{(bpy)2Ru(dpp)}2RhCl2]5+ and related catalysts derived by variation of the ligand/metal. The electron collection in this class of catalysts enables hydrogen production or DNA cleavage, among other applications. Changes in excitation energies upon variation of the ligand/metal were mostly consistent with experiment, and within the expected TDDFT accuracy, thus enabling their use as the basis for further analysis. Indeed, the consistency observed between the predicted excitation energy and the bridging ligand's reduction potential can assist in catalyst design for electron collection. Calculated fragment orbital energies could explain, in part, changes in the propensity towards photocleavage of DNA.

Ultra-violet Treatment as a Strategy for Destruction of Degradation Products from Amine Based Post Combustion CO2 Capture

The amine-based post-combustion capture (PCC) of CO2 is an option being considered for the reduction of anthropogenic CO2 emissions. PCC plants will be attached or retro-fitted to fossil-fuel fired power generating infrastructure (i.e. large stationary point sources) during the transition to renewable energy, which is expected to take place over several decades. Despite being an established technology widely used to remove CO2 from small scale-commercial process such as natural gas sweetening, there are many challenges that must be overcome before large scale implementation of the process can be realised. Some of these challenges include:•Reducing plant capital costs•Reducing the energy required for solvent regeneration/release of captured CO•Reducing solvent degradation in the presence of O2 and other flue gas contaminants, such as NOx and SOx,and•Reducing and understanding the environmental impact i.e. the potential of solvents and solvent degradation products to be released into the environment through off-gases, leaks or spills, and any associated impact this might have.Several recent research articles1-3 have highlighted the propensity of amine-based solvents to undergo certain chemical transformations, both in the condensed and aqueous phases, which produce harmful materials such as nitrosamines and nitramines. The formation and emission of nitrosated amines from the PCC process has not been fully investigated, although it is now the focus of some research agencies in Europe and Australasia. Nitrosamine formation mechanisms are presented in relation to the post- combustion capture of CO2 with aqueous amine solvents. The relative merits of nitrosamine mitigation strategies are also discussed. In particular the susceptibility of several nitrosamines to UV destruction is explored; the results are compared with TD-DFT results which predict excitation energies with good accuracy. The rate of destruction versus UV wavelength is also discussed, with a view to understanding which electronic transition facilitates decomposition

Electronic excitation in bulk and nanocrystalline alkali halides

The lowest energy excitations in bulk alkali halides are investigated by considering five different excited state descriptions. It is concluded that excitation transfers one outermost halide electron in the fully ionic ground state to the lowest energy vacant s orbital of one closest cation neighbour to produce the excited state termed dipolar. The excitation energies of seven salts were computed using shell model description of the lattice polarization produced by the effective dipole moment of the excited state neutral halogen-neutral metal pair. Ab initio uncorrelated short-range inter-ionic interactions computed from anion wavefunctions adapted to the in-crystal environment were augmented by short-range electron correlation contributions derived from uniform electron-gas density functional theory. Dispersive attractions including wavefunction overlap damping were introduced using reliable semi-empirical dispersion coefficients. The good agreement between the predicted excitation energies and experiment provides strong evidence that the excited state is dipolar. In alkali halide nanocrystals in which each ionic plane contains only four ions, the Madelung energies are significantly reduced compared with the bulk. This predicts that the corresponding intra-crystal excitation energies in the nanocrystals, where there are two excited states depending on whether the halide electron is transferred to a cation in the same or in the neighbouring plane, will be reduced by almost 2 eV. For such an encapsulated KI crystal, it has been shown that the greater polarization in the excited state of the bulk crystal causes these reductions to be lowered to a 1.1 eV-1.5 eV range for the case of charge transfer to a neighbouring plane. For intra-plane charge transfer the magnitude of the polarization energy is further reduced thus causing the excitation in these encapsulated materials to be only 0.2 eV less than in the bulk crystal.

Excitation energies from range-separated time-dependent density and density matrix functional theory

Time-dependent density functional theory (TD-DFT) in the adiabatic formulation exhibits known failures when applied to predicting excitation energies. One of them is the lack of the doubly excited configurations. On the other hand, the time-dependent theory based on a one-electron reduced density matrix functional (time-dependent density matrix functional theory, TD-DMFT) has proven accurate in determining single and double excitations of H(2) molecule if the exact functional is employed in the adiabatic approximation. We propose a new approach for computing excited state energies that relies on functionals of electron density and one-electron reduced density matrix, where the latter is applied in the long-range region of electron-electron interactions. A similar approach has been recently successfully employed in predicting ground state potential energy curves of diatomic molecules even in the dissociation limit, where static correlation effects are dominating. In the paper, a time-dependent functional theory based on the range-separation of electronic interaction operator is rigorously formulated. To turn the approach into a practical scheme the adiabatic approximation is proposed for the short- and long-range components of the coupling matrix present in the linear response equations. In the end, the problem of finding excitation energies is turned into an eigenproblem for a symmetric matrix. Assignment of obtained excitations is discussed and it is shown how to identify double excitations from the analysis of approximate transition density matrix elements. The proposed method used with the short-range local density approximation (srLDA) and the long-range Buijse-Baerends density matrix functional (lrBB) is applied to H(2) molecule (at equilibrium geometry and in the dissociation limit) and to Be atom. The method accounts for double excitations in the investigated systems but, unfortunately, the accuracy of some of them is poor. The quality of the other excitations is in general much better than that offered by TD-DFT-LDA or TD-DMFT-BB approximations if the range-separation parameter is properly chosen. The latter remains an open problem.

Verdict: Time-Dependent Density Functional Theory “Not Guilty” of Large Errors for Cyanines

We assess the accuracy of eight Minnesota density functionals (M05 through M08-SO) and two others (PBE and PBE0) for the prediction of electronic excitation energies of a family of four cyanine dyes. We find that time-dependent density functional theory (TDDFT) with the five most recent of these functionals (from M06-HF through M08-SO) is able to predict excitation energies for cyanine dyes within 0.10-0.36 eV accuracy with respect to the most accurate available Quantum Monte Carlo calculations, providing a comparable accuracy to the latest generation of CASPT2 calculations, which have errors of 0.16-0.34 eV. Therefore previous conclusions that TDDFT cannot treat cyanine dyes reasonably accurately must be revised.

Quantum mechanical modeling of electronic excitations in metal oxides: Magnesia as a prototype

We compare embedded correlated wavefunction (ECW) approaches for predicting excited states within MgO as a prototypical metal oxide. The crystal is partitioned into a cluster treated with CW methods and a background described by various electrostatic or orbital-free-density-functional-theory (DFT)-based embedding potentials. The excited singlet and triplet states are found to be nearly degenerate and of charge-transfer type, consistent with experiment. Although the prediction of excitation energies by ECW theory with an electrostatic description of the background falls slightly short of more expensive Green’s function methods, it is significantly improved over standard DFT or non-embedded CW methods.

Theoretical Study on the Chiroptical Optical Properties of Chiral Fullerene C60Derivative

Time-dependent density functional theory (TDDFT) calculations have been used to investigate UV/CD spectra and nonlinear optical (NLO) property of the C(60)-fullerene bisadduct (R,R,(f,s)A)-[CD(+)280] for the first time. The electron transition natures of the four main measured bands are analyzed, and their results are used to designate the excited states involved in an electron-transfer process of the studied compound. On a comparative scale, the predicted excitation energies and oscillator strengths are in reasonable agreement with the observed values, demonstrating the efficiency of TDDFT in predicting the localized and charge transfer transitions. The good agreement between the experimental and the simulated CD spectra shows that TDDFT calculations can be used to assign the absolute configurations (ACs) of chiral fullerene C(60) derivatives with high confidence. The observed large dissymmetry ratio g (g = Δε/ε) at about 700 nm results from the orbital characters of the local fullerene excited state, which leads to large transition magnetic dipole moment and small transition electronic dipole moment. The different functionals and solvent effects on UV/CD spectra were also considered. The studied compound has a possibility to be an excellent second-order NLO material from the standpoint of transparency and large second-order polarizability value.

Theoretical studies on structures and electronic spectra of linear HC nN + ( n = 2–14)

With unrestricted B3LYP and CAM-B3LYP calculations, we have investigated the linear HC n N + ( n = 2–14) clusters focusing on the ground-state geometries, vibrational frequencies, rotational constants, dipole moments, energy differences (Δ E n ), and incremental binding energies (Δ E I). The results indicate that the odd-numbered HC n N + clusters have polyacetylene-like structures with significant single–triple bond length alternation, while the even-numbered analogues exhibit some sort of cumulenic character, with the former being more stable than the latter. The CASPT2/cc-pVTZ approach has been employed to estimate the vertical excitation energies for the dipole-allowed (2, 3) 2Π ← X 2Π and dipole-forbidden 1 2Φ ← X 2Π transitions in HC n N + ( n = 5–14) clusters. The predicted excitation energies of 2 2Π ← X 2Π transitions for HC n N + ( n = 5–13) clusters are 2.27, 2.33, 2.04, 2.02, 1.84, 1.76, 1.62, 1.58, and 1.49 eV, respectively, in good agreement with the available observed values (2.12, 2.17, 1.84, 1.89, 1.61, 1.66, 1.42, 1.49, and 1.27 eV). In addition, the higher electronic transitions of HC n N + ( n = 5–14) are also calculated. We expect that calculations for 3 2Π ← X 2Π transitions in odd- n clusters are able to contribute to further experimental and theoretical researches because of their relatively higher oscillator strengths. Finally, in the paper are discussed the possible dissociation channels and corresponding fragmentation energies of HC n N + clusters.

Singlet Oxygen Generation by Novel NIR BODIPY Dyes

Five novel near-infrared BODIPY dyes were prepared for improved singlet oxygen generation using thiophene and bromine. Theoretical, optical, photostable, and singlet oxygen generation characteristics of these dyes were assessed. Predicted excitation energies by TDDFT calculations were in good agreement (ΔE ≈ 0.06 eV) with experimental data. All five dyes showed both excitation and emission in the NIR range. In particular, two dyes having sulfur and bromine atoms showed efficient singlet oxygen generation with high photostability.

Predicting the UV–vis spectra of oxazine dyes

In the current work we have investigated the ability of time-dependent density functional theory (TD-DFT) to predict the absorption spectra of a series of oxazine dyes and the effect of solvent on the accuracy of these predictions. Based on the results of this study, it is clear that for the series of oxazine dyes an accurate prediction of the excitation energy requires the inclusion of solvent. Implicit solvent included via a polarizable continuum approach was found to be sufficient in reproducing the excitation energies accurately in the majority of cases. Moreover, we found that the SMD solvent model, which is dependent on the full electron density of the solute without partitioning into partial charges, gave more reliable results for our systems relative to the conductor-like polarizable continuum model (CPCM), as implemented in Gaussian 09. In all cases the inclusion of solvent reduces the error in the predicted excitation energy to <0.3 eV and in the majority of cases to <0.1 eV.