Symmetry-adapted Cluster-configuration Interaction Calculations Research Articles

X-ray photoelectron spectroscopy of Thymine and 5-Bromouracil studied by Symmetry-Adapted-Cluster Configuration-Interaction (SAC-CI) theory

The X-ray Photoelectron Spectra (XPS) of thymine (T) and 5-Bromouracil (BrU) were studied and assigned using Symmetry-Adapted-Cluster Configuration-Interaction (SAC-CI) theory. The differences between theory and experiment were within 1.0 eV when compared with the spectra measured in vacuum conditions. It was clarified that inner-core ionization processes involve many electronic processes like ionizations, accompanying excitations, reorganizations, and electron correlations. Therefore, accurate calculations require both good theory and good basis set. The present SAC-CI calculations with extended basis sets including even Rydberg basis functions may satisfy these requirements.

On the performance of time-dependent double-hybrid density functionals for description of absorption and emission spectra of heteroaromatic compounds

Theoretical chemistry provides a panel of powerful tools to investigate the geometries and electronic properties of excited states. However, it is necessary to benchmark the accuracy of existing models to determine the range of their applicability and accountability. In the present work, the performance of two related methodologies in this context, symmetry-adapted cluster-configuration interaction (SAC-CI) and time-dependent double-hybrid density functional theory (TD-DHDFT), is compared in detail for the calculation of absorption and fluorescence energies of small organic molecules and π-conjugated heteroaromatic compounds. Pragmatically, for the SAC-CI calculations the singles and doubles linked excitation operators are considered in the wave functions. On the other hand, for the TD-DHDFT calculations the linear-response Tamm–Dancoff formalism has been imposed. The considered DH density functionals include the approximations from both families of adjusted and non-adjusted models. Our numerical data reveal that of the tested adjusted DHs, B2-PLYP performs the best, while among the category of non-adjusted approximations the PBE0-DH functional outperforms others. Furthermore, the DH density functionals show deviations less than those obtained from SAC-CI method. Putting all the results together, the B2-PLYP and B2GP-PLYP functionals are found to be superior for overall performance. Altogether, the present contribution and recent efforts in this arena point out that besides the ground-state properties the DH approximations have also unquestionably entered into the field of excited-state calculations, where the DHDFT again comes into play and further evidence on the quality of the corresponding models is pronounced.

Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers.

Naphthalene, anthracene, and phenanthrene are shown to have very long-lived triplet lifetimes when the isolated molecules are excited with nanosecond pulsed lasers resonant with the lowest singlet state. For naphthalene, triplet state populations are created only during the laser pulse, excluding the possibility of normal intersystem crossing at the one photon level, and all molecules have triplet lifetimes greater than hundreds of microseconds, similar to the behavior previously reported for phenylacetylene. Although containing 7-12 thousand cm(-1) of vibrational energy, the triplet molecules have ionization thresholds appropriate to vibrationless T1 states. The laser power dependences (slopes of log-log power plots) of the excited singlet and triplet populations are about 0.7 for naphthalene and about 0.5 for anthracene. Kinetic modeling of the power dependences successfully reproduces the experimental results and suggests that the triplet formation mechanism involves an enhanced spin orbit coupling caused by sigma character in states at the 2-photon level. Symmetry adapted cluster-configuration interaction calculations produced excited state absorption spectra to provide guidance for estimating kinetic rates and the sigma character present in higher electronic states. It is concluded that higher excited state populations are significant when larger molecules are excited with pulsed lasers and need to be taken into account whenever discussing the molecular photodynamics.

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of pyridine: determination of accurate ionization energy and cationic structure.

Ionization energies and cationic structures of pyridine were intensively investigated utilizing one-photon mass-analyzed threshold ionization (MATI) spectroscopy with vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The present one-photon high-resolution MATI spectrum of pyridine demonstrated a much finer and richer vibrational structure than that of the previously reported two-photon MATI spectrum. From the MATI spectrum and photoionization efficiency curve, the accurate ionization energy of the ionic ground state of pyridine was confidently determined to be 73,570 ± 6 cm(-1) (9.1215 ± 0.0007 eV). The observed spectrum was almost completely assigned by utilizing Franck-Condon factors and vibrational frequencies calculated through adjustments of the geometrical parameters of cationic pyridine at the B3LYP/cc-pVTZ level. A unique feature unveiled through rigorous analysis was the prominent progression of the 10 vibrational mode, which corresponds to in-plane ring bending, and the combination of other totally symmetric fundamentals with the ring bending overtones, which contribute to the geometrical change upon ionization. Notably, the remaining peaks originate from the upper electronic state ((2)A2), as predicted by high-resolution photoelectron spectroscopy studies and symmetry-adapted cluster configuration interaction calculations. Based on the quantitatively good agreement between the experimental and calculated results, it was concluded that upon ionization the pyridine cation in the ground electronic state should have a planar structure of C(2v) symmetry through the C-N axis.

Efficiency of perturbation-selection and its orbital dependence in the SAC-CI calculations for valence excitations of medium-size molecules.

The efficiency and accuracy of the perturbation-selection used in the symmetry-adapted cluster-configuration interaction (SAC-CI) calculations are investigated for several low-lying valence excited states of 21 medium-size molecules, including typical chromophores with heterocyclic macrocycles (free-base porphine, coumarin, indole, and BODIPY), nucleobases, amino acids (tyrosine and tryptophan), polycyclic aromatic hydrocarbons, and organometallics (ferrocene and Re(bpy)(CO)4+1). Benchmark SAC-CI calculations with up to 110 million operators are performed. The efficiency of the perturbation-selection depends on the molecular orbitals (MOs); therefore, the canonical MO and localized MO (LMO) obtained by Pipek-Mezey's method are examined. Except for the highly symmetric molecules, using LMOs improves the efficiency and accuracy of the perturbation-selection. With using LMOs and perturbation-selection, sufficiently reliable results can be obtained in less than 10% of the computational costs required for the full-dimensional calculations. The perturbation-selection with LMOs is suggested to be a promising method for excited states in larger molecular systems.

Symmetry adapted cluster–configuration interaction calculation of the photoelectron spectra of famous biological active steroids

The photoelectron spectra of some famous steroids, important in biology, were calculated in the gas phase. The selected steroids were 5α-androstane-3,11,17-trione, 4-androstane-3,11,17-trione, cortisol, cortisone, corticosterone, dexamethasone, estradiol and cholesterol. The calculations were performed employing symmetry-adapted cluster/configuration interaction (SAC–CI) method using the 6-311++G(2df,pd) basis set. The population ratios of conformers of each steroid were calculated and used for simulating the photoelectron spectrum of steroid. It was found that more than one conformer contribute to the photoelectron spectra of some steroids. To confirm the calculated photoelectron spectra, they compared with their corresponding experimental spectra. There were no experimental gas phase HeI photoelectron spectra for some of the steroids of this work in the literature and their calculated spectra can show a part of intrinsic characteristics of this molecules in the gas phase. The canonical molecular orbitals involved in the ionization of each steroid were calculated at the HF/6-311++g(d,p) level of theory. The spectral bands of each steroid were assigned by natural bonding orbital (NBO) calculations. Knowing the electronic structures of steroids helps us to understand their biological activities and find which sites of steroid become active when a modification is performing under a biological pathway.

Electronic excitation spectra of molecules in solution calculated using the symmetry-adapted cluster-configuration interaction method in the polarizable continuum model with perturbative approach

A perturbative approximation of the state specific polarizable continuum model (PCM) symmetry-adapted cluster-configuration interaction (SAC-CI) method is proposed for efficient calculations of the electronic excitations and absorption spectra of molecules in solutions. This first-order PCM SAC-CI method considers the solvent effects on the energies of excited states up to the first-order with using the zeroth-order wavefunctions. This method can avoid the costly iterative procedure of the self-consistent reaction field calculations. The first-order PCM SAC-CI calculations well reproduce the results obtained by the iterative method for various types of excitations of molecules in polar and nonpolar solvents. The first-order contribution is significant for the excitation energies. The results obtained by the zeroth-order PCM SAC-CI, which considers the fixed ground-state reaction field for the excited-state calculations, are deviated from the results by the iterative method about 0.1 eV, and the zeroth-order PCM SAC-CI cannot predict even the direction of solvent shifts in n-hexane for many cases. The first-order PCM SAC-CI is applied to studying the solvatochromisms of (2,2(')-bipyridine)tetracarbonyltungsten [W(CO)4(bpy), bpy = 2,2(')-bipyridine] and bis(pentacarbonyltungsten)pyrazine [(OC)5W(pyz)W(CO)5, pyz = pyrazine]. The SAC-CI calculations reveal the detailed character of the excited states and the mechanisms of solvent shifts. The energies of metal to ligand charge transfer states are significantly sensitive to solvents. The first-order PCM SAC-CI well reproduces the observed absorption spectra of the tungsten carbonyl complexes in several solvents.

Chemically intuitive indices for charge-transfer excitation based on SAC-CI and TD-DFT calculations

A recently proposed charge-transfer (CT) index and some related quantities aimed to the description of CT excitations in push-pull donor-acceptor model systems were computed in vacuum and in ethanol by the direct symmetry-adapted cluster-configuration interaction (SAC-CI) method including solvent effects by means of the nonequilibrium state-specific approach. The effects of both solvation and electron correlations on these quantities were found to be significant. The present results are also in line with previous time-dependent (TD) density functional theory calculations and they demonstrate that SAC-CI provides a description of the excitation character close to that of TD-PBE0. Indeed, CT indices evaluated by the SAC-CI and TD-PBE0 would be useful in the field of materials chemistry, for the design and development of novel molecular materials.

Excited-State Geometries of Heteroaromatic Compounds: A Comparative TD-DFT and SAC-CI Study.

The structures of low-lying singlet excited states of nine π-conjugated heteroaromatic compounds have been investigated by the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the time-dependent density functional theory (TDDFT) using the PBE0 functional (TD-PBE0).In particular, the geometry relaxation in some ππ* and nπ* excited states of furan, pyrrole, pyridine, p-benzoquinone, uracil, adenine, 9,10-anthraquinone, coumarin, and 1,8-naphthalimide as well as the corresponding vertical transitions, including Rydberg excited states, have been analyzed in detail. The basis set and functional dependence of the results was also examined. The SAC-CI and TD-PBE0 calculations showed reasonable agreement in both transition energies and excited-state equilibrium structures for these heteroaromatic compounds.

Theoretical Study on the Excited Electronic States of Coronene and Its π-Extended Molecules Using the Symmetry-Adapted Cluster-Configuration Interaction Method

Abstract The excited electronic states and optical absorption spectra of coronene (C24H12), hexa-peri-hexabenzocoronene (HBC) (C42H18), and circumcoronene (C54H18) were studied using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. For coronene and HBC, the SAC-CI calculations reproduced the experimental spectra well and predicted optically forbidden excited states. For HBC, the symmetry lowering enhanced the intensity of the S2 state that corresponds to the p-band, and the SAC-CI calculation predicted the existence of the second and third optically allowed states around the β-band region near 4.0 eV. For circumcoronene, the SAC-CI calculation predicted a strong absorption of the β-band in the visible light region. The mechanisms of energy splitting for the HOMO–LUMO transition were investigated. Electron correlation was the most important factor for the energy splitting between the lowest and the next-lowest states. Configuration interaction with single excitations (CIS) calculations could not correctly predict the relative energies of these states in coronene and circumcoronene. For HBC, on the other hand, the CIS calculation provided the same energy order as the SAC-CI calculation.

Electronic excited states and electronic spectra of biphenyl: a study using many-body wavefunction methods and density functional theories

The low-lying electronic excited states of biphenyl were studied using the symmetry-adapted cluster-configuration interaction (SAC-CI), complete active space self-consistent field (CASSCF), complete active space perturbation theory of the second-order (CASPT2), and the time-dependent density functional theory (TDDFT). The molecular geometries in the ground and excited states were optimized using the SAC-CI and TDDFT for singlet and triplet states. The energies of vertical excitations, emissions, and adiabatic transitions were calculated. The TDDFT calculations significantly underestimated the excitation energy of the 1(1)B1 state, while the SAC-CI and CASPT2 provided essentially similar results. The present SAC-CI and CASPT2 calculations concluded that the lowest singlet state of isolated biphenyl is the 1(1)B3 state that takes a planar geometry and the second lowest state is the 1(1)B2 state with a twisted geometry. The present results were consistent with the previous experimental findings. The 1(1)B1 state that has a charge-separated biracial character in the vertical excitation relaxed into a planar quinoid structure in which bond alternations were emphasized. The other states took a benzenoid structure. The ultraviolet (UV) absorption and circular dichroism (CD) spectra below 7 eV were calculated with the SAC-CI method. The valence-Rydberg mixings were found to be significant in the second and higher series of excited states.

Elucidating Electronic Transitions from σ Orbitals of Liquid n- and Branched Alkanes by Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations

Attenuated total reflection far-ultraviolet (ATR-FUV) spectra containing Rydberg states of n-alkanes (C(m)H(2m+2); m varies in the range 5-9) and branched alkanes observed in the liquid phase were investigated by quantum chemical calculations with the aim of elucidating electronic transitions from σ orbitals of liquid n- and branched alkanes. New assignments are proposed based on the time-dependent density functional theory (TD-DFT) and symmetry-adapted cluster configuration interaction (SAC-CI) calculations, and the differences in these spectra are analyzed in detail. The FUV spectra of n-alkanes show a broad asymmetric feature near 8.3 eV. The strong band at ∼8.3 eV shows a red shift with a significant increase in intensity as the carbon chain length increases, which is attributed to the overlapping transitions from the third (or fourth) highest occupied molecular orbitals HOMO-2 (or HOMO-3) and HOMO-1 to Rydberg 3p(y) by the TD-DFT and SAC-CI calculations. This band was previously assigned to the overlap of two peaks arising from the transition from the HOMO to 3p and from the HOMO-1 to 3s based on their term values. Although the most intense transition, T1, is from HOMO-2 for m = 5 and 6 and HOMO-3 for m varying in the range of 7-9, the shape of Kohn-Sham molecular orbital for T1 is similar among the all-alkanes investigated. The theoretical result also has demonstrated that the red shift originates in both stabilization of the Rydberg 3p(y) and destabilization of the occupied orbitals. The intensity of the shoulder at 7.7 eV drastically increases in the spectra of the branched alkanes, especially for those with quaternary carbon atoms such as 2,2-dimethyl butane. This increase in intensity is caused by a reduction in symmetry in the branched alkanes, which leads the forbidden transitions to Rydberg 3s to allowed transitions. In this way, the present study has provided new insight into the existence of their Rydberg transitions and the shape of the relevant MOs of the transitions.

Optical absorption and fluorescence of PRODAN in solution: Quantum chemical study based on the symmetry-adapted cluster-configuration interaction method

The optical absorption and fluorescence properties of 6-propionyl-2-(N,N-dimethyl)aminonaphthalene (PRODAN), a biological probe, were investigated using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. The solvent effects were included using the polarizable continuum model (PCM). The PCM SAC/SAC-CI calculations well reproduced the significant solvatochromic shifts, without assuming twisted charge-transfer conformations. The geometry relaxation in naphthalene ring was significant for the lowest excited state. The nonequilibrium solvation was important in explaining the large solvatochromic shifts of the fluorescence. The present SAC-CI calculation showed that the absorption maximum corresponds to the second π–π∗ state, not the lowest excited state.

Excited states and electronic spectra of annulated dinuclear free-base phthalocyanines: A theoretical study on near-infrared-absorbing dyes

The electronic excited states and electronic absorption spectra of annulated dinuclear free-base phthalocyanine (C(58)H(30)N(16)) are studied through quantum chemical calculations using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. Three tautomers are possible with respect to the position of the pyrrole protons; therefore, the SAC-CI calculations for these tautomers were performed. The structures of the Q-band states are discussed based on the character of their molecular orbitals. The lower energy shift of the Q-bands because of dimerization is explained by the decrease in the HOMO-LUMO gaps resulting from the bonding and antibonding interactions between the monomer units. The electronic dipole moments of the nonsymmetric tautomer were calculated, and the possibility of charge-separated excited states is discussed. The relative energies of these tautomers are examined using density functional theory (DFT) calculations for several peripheral substituents. The relative energies of these tautomers significantly depend on the substituents, and therefore, the abundance ratios of the three tautomers were affected by the substituents. The absorption spectra were simulated from the SAC-CI results weighted by the Boltzmann factors obtained from the DFT calculations. The SAC-CI spectra reproduce the experimental findings well. The thermal-averaged SAC-CI spectra could explain the observed substituent effect on the structure of the Q-bands in terms of the relative stabilities and the abundance ratios of the tautomers. The SAC-CI and time-dependent density functional theory calculations are also compared. The CAM-B3LYP results agreed with the trends of the SAC-CI results; however, the CAM-B3LYP calculation overestimated the excitation energies in comparison with the SAC-CI and experimental results.

Polymethine dyes for all-optical switching applications: a quantum-chemical characterization of counter-ion and aggregation effects on the third-order nonlinear optical response

Polymethine dyes have recently demonstrated promise for all-optical switching applications at telecommunications wavelengths as they can combine large refractive optical nonlinearities with low single-photon and two-photon optical losses. Here, we use density functional theory and symmetry-adapted cluster configuration interaction calculations to characterize model streptocyanine molecules. We first consider the isolated, closed-shell cationic molecules and then complexes formed by the molecules with chloride counter-ions and a series of aggregates. Our goal is to examine the influence of: (i) the presence of counter-ions and (ii) aggregation on the electronic structure and nonlinear optical properties. We find that the counter-ions increase the degree of bond-length alternation along the cyanine backbone, while aggregation significantly reduces the energy window between the lowest one-photon and two-photon excited states. Our results provide insight toward the design of new polymethine derivatives that could maintain large figures-of-merit for all-optical switching applications in the solid state.

Dyson orbitals of N2O: Electron momentum spectroscopy and symmetry adapted cluster-configuration interaction calculations

Electron momentum spectroscopy and symmetry adapted cluster-configuration interaction (SAC-CI) theory were combined to study electron correlation effects in nitrous oxide molecule (N(2)O). The SAC-CI General-R method accurately reproduced the experimental ionization spectrum. This bench-marked method was also introduced for calculating the momentum distributions of N(2)O Dyson orbitals. Several calculated momentum distributions with different theoretical methods were compared with the high resolution experimental results. In the outer-valence region, Hartree-Fock (HF), density functional theory (DFT), and SAC-CI theory can well describe the experimental momentum distributions. SAC-CI presented a best performance among them. In the inner-valence region, HF and DFT cannot work well due to the severe breaking of the molecular orbital picture, while SAC-CI still produced an excellent description of experimental momentum profiles because it can accurately take into account electron correlations. Moreover, the thermally averaged calculation showed that the geometrical changes induced by the vibration at room temperature have no noticeable effects on momentum distribution of valence orbitals of N(2)O.

Photophysical properties and vibrational structure of ladder-type penta p-phenylene and carbazole derivatives based on SAC-CI calculations

The π-conjugated ladder-type molecules constitute an attractive field of organic photoactive materials. In this work, the photophysical properties of ladder-type penta-p-phenylene (LPP) and carbazole derivatives (bisindenocarbazole and diindolocarbazole) have been investigated theoretically using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. The equilibrium geometries in the ground (S 0) and first excited (S 1) states were calculated to be planar, and the excitation is delocalized over the molecules. SAC-CI/DZP calculations have been applied to the absorption and emission spectra of these molecules. The absorption spectra were well reproduced in both peak positions and the shape of the absorption bands. The strong absorption is attributed to the highest occupied molecular orbital to the lowest unoccupied molecular orbital (H–L) transition; however, in carbazoles, the H–1→L transition is located below the H–L transition. The vibrational structure in the S 0–S 1 absorption band of LPP was analyzed by calculating the Franck–Condon (FC) factors based on the potential energy surfaces (PESs) along the normal coordinates that are relevant to the geometry change. The vibrational structure was well reproduced by the theoretical simulation. The C–C stretching mode dominantly contributes to the vibrational structure, while the breathing motion of the molecular frame does not influence the structure. The emission energies calculated by the SAC-CI method also agree well with the experimental values. The vibrational structure in the fluorescence band was also examined by the FC analysis; the theoretical spectrum is satisfactory for the two carbazoles, while the 0–0 transition is overestimated in LPP. In diindolocarbazole, the S 2 state has a large oscillator strength, while the S 1 state has a small oscillator strength.

Electron-impact ionization of the water molecule at large momentum transfer above the double-ionization threshold

The single and double ionization of the water molecule at large momentum transfer has been studied using a combination of ($e$,$2e$) and ($e$,3--$1e$) spectroscopy, with the binding energy spectrum being measured from 0 to 100 eV. The experiment has been performed in the symmetric noncoplanar geometry at an incident electron energy of 2055 eV. In this way we have achieved a large momentum transfer of 9 a.u. In particular, we present an observation of a relatively intense band at around 58 eV. Symmetry-adapted cluster configuration interaction (SAC-CI) general-R calculations for single ionization indicate that the observed band is at least partly generated by a cluster of satellites with small intensities, which predominantly belong to states possessing ${}^{2}{A}_{1}$ symmetry originating from the mixing of the $(2{a}_{1}){}^{\ensuremath{-}1}$ state with two electron processes. Nevertheless, it has been found that the entire spectrum above the lowest double-ionization threshold cannot be understood, even qualitatively, with the SAC-CI calculations. This result suggests that the ($e$,3--$1e$) double-ionization processes have a significant contribution to the observations.

Electronic excited states of macrocyclic compounds: direct SAC-CI study

Abstract We developed the direct SAC-CI (symmetry-adapted cluster-configuration interaction) method for accurate calculations of electronic excited states of large molecules. The direct SAC-CI method uses an efficient MO (molecular orbital) direct scheme that is compatible with the perturbation-selection technique with using sparse linear algebra techniques. The direct SAC-CI can calculate large molecules with moderate computational cost. Using the direct SAC-CI method, excited states of free-base tetraazaporphyrin, phthalocyanine, naphthalocyanine, and anthracocyanine were studied. An interesting feature of the observed spectra is that the splitting of the Q-bands decreases with increasing molecular size. The SAC-CI calculation well reproduced the observed trend of excitation energies and showed the importance of electron correlation effects in the excited states.

Excited states and electronic spectra of extended tetraazaporphyrins

Electronic excited states, electronic absorption, and magnetic circular dichroism (MCD) spectra of free-base tetraazaporphyrin (TAP), phthalocyanine (Pc), naphthalocyanine (Nc), and anthracocyanine (Ac) were studied by quantum chemical calculations using the symmetry-adapted cluster-configuration interaction (SAC-CI) method. Not only optically allowed states including the Q- and B-bands but also optically forbidden states were calculated for transitions whose excitation energies were lower than 4.5 eV. The present SAC-CI calculations consistently assigned the absorption and MCD peaks as optically allowed π→π(∗) excitations, although these calculations using double-zeta basis limit quantitative agreement and discussion. For Nc and Ac, excited states beyond the four-orbital model appeared in the low-energy region. The low-energy shifts of the Q-bands with the extension of molecular size were explained by the orbital energies. The splitting of the Q-bands decreases with extension of the molecular size. This feature was reproduced by the SAC-CI calculations but the configuration interaction with single excitations and time-dependent density functional theory calculations failed to reproduce this trend. Electron correlation in the excited states is important in reproducing this splitting of the Q-bands and in describing the energy difference between the B(2u) and B(3u) states of free-base porphyrins.