Symmetry-adapted Cluster-configuration Interaction Research Articles

The excited states of azulene: A study of the vibrational energy levels for the lower ππ*-valence states by configuration interaction and density functional calculations, and theoretical studies of the Rydberg states.

A new vacuum ultraviolet absorption (VUV) spectrum of azulene vapor has been obtained by using a synchrotron radiation source. The onset of the ultraviolet spectrum, previously reported by Sidman et al., has been analyzed in detail by Franck-Condon (FC) and Herzberg-Teller (HT) methods. The photoelectron spectral profile identifies the 3px-Rydberg state 00 band to be 131cm-1 from the VUV maximum. Excited state energy levels were calculated by three independent methods: the wide scan VUV spectrum was correlated with symmetry adapted cluster configuration interaction calculations. The low energy portion of the spectrum was studied by both time dependent density functional theoretical methods (TDDFT) and multi-reference multi-root CI (MRD-CI). Equilibrium structures were determined for valence states at the TDDFT level. Rydberg states were determined by both TDDFT and MRD-CI. The FC + HT analyses were performed on the TDDFT wave-functions. The HT intensity profiles are generally low in intensity, relative to the FC ones; however, HT is dominant in the second singlet state (S2, 11A1). As a result, numerous non-symmetric modes, their overtones, and combination bands show considerable intensity in that band. Energies obtained from use of extremely diffuse s-, p-, d-, or f-character functions enabled realistic extrapolation to the IE1 for many Rydberg states (RS). The lowest RS (3b13s) based on IE2 lies at 4.804eV with a quantum defect of 0.714. Differentiation between valence and RS is readily made using the second moments of the charge distribution.

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.

Geometric, electronic and spectral properties of germanium and Eu-doped germanium clusters

This study explored the geometric, electronic, and spectral properties of germanium and Eu-doped germanium clusters and compared their simulated PES spectra. The stability of Gen and EuGen nanoclusters was examined with density functional theory using FHI-aims as a software package. Symmetry-adapted cluster-configuration interaction general-R (SAC-CI-General-R) method and the SDD basis set were adopted to determine ionization energies and calculate their intensities. Natural bond orbital (NBO) calculations were performed to determine the effective molecular orbitals in the ionization and study the electron density on the nanoclusters. The shape of molecular orbitals (MO) involved in the ionization process for each structure was displayed and interpreted. The analyses of binding energies showed that increasing the size of nanoclusters leads to more stability for nanoclusters. Ionization energy and the photoelectron spectrum of these clusters indicated that Eu as an impurity reduces ionization energy and shifts the ionization processes towards infrared wavelengths. Interestingly, pure and doped clusters followed a similar pattern in terms of an increase/decrease in the first ionization energy compared to n-1 and n + 1 clusters.

The ionized states of 6,6-dimethylfulvene; the vibrational energy levels studied by photoionization, configuration interaction and density functional calculations

A new synchrotron-based photoionization spectrum of 6,6-dimethylfulvene shows significant vibrational fine structure (VFS), in contrast to previous studies; this was successfully analysed by Franck-Condon (FC) methods. The sequence of ionic states in the range 7 to 19 eV has been determined by both symmetry adapted cluster configuration interaction and density functional methods, especially using the long-range corrected version of the Becke three-parameter hybrid functional (B3LYP) using the Coulomb-attenuating method (CAM-B3LYP). Both lead to reliable theoretical values for both the calculated vertical and adiabatic ionization energies. The FC profile for the lowest ionization energy (IE1, X2A2) shows extensive VFS which is analysed successfully. The second IE (A2B1) shows truncated structure owing to overlap with IE1.

Electronic Couplings and Electrostatic Interactions Behind the Light Absorption of Retinal Proteins.

The photo-functional chromophore retinal exhibits a wide variety of optical absorption properties depending on its intermolecular interactions with surrounding proteins and other chromophores. By utilizing these properties, microbial and animal rhodopsins express biological functions such as ion-transport and signal transduction. In this review, we present the molecular mechanisms underlying light absorption in rhodopsins, as revealed by quantum chemical calculations. Here, symmetry-adapted cluster-configuration interaction (SAC-CI), combined quantum mechanical and molecular mechanical (QM/MM), and transition-density-fragment interaction (TDFI) methods are used to describe the electronic structure of the retinal, the surrounding protein environment, and the electronic coupling between chromophores, respectively. These computational approaches provide successful reproductions of experimentally observed absorption and circular dichroism (CD) spectra, as well as insights into the mechanisms of unique optical properties in terms of chromophore-protein electrostatic interactions and chromophore-chromophore electronic couplings. On the basis of the molecular mechanisms revealed in these studies, we also discuss strategies for artificial design of the optical absorption properties of rhodopsins.

Photoionization of zwitterionic liquids

The electronic structures of three zwitterionic liquids (Zw-IL) have been studied using high-level ab initio method: equation-of-motion coupled-cluster (IP-EOM-CCSD) method. This method was found to match experimental UV photoelectron spectrum (UPS) of related N-methylimidazole better than Outer-valence Green’s Function (OVGF) method or Symmetry Adapted Cluster-Configuration Interaction (SAC-CI) method. Zw-IL gas phase photoelectron spectra have been simulated in the valence ionization regions based on the vertical ionization energies calculated. Valence ionizations have been attributed to specific molecular orbitals to specific functional groups The calculated electronic structures and valence ionization energies of Zw-ILs can be related to their polarizability which in turn influences the AC conductivity. AC conductivity is one of the principal properties of Zw-ILs which makes them very useful as electrolyte materials in lithium ion batteries or electric capacitors.

Advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase.

The purpose of this review is to demonstrate advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase. Molecular spectroscopy, particularly vibrational spectroscopy and electronic spectroscopy, has been used extensively for a wide range of areas of chemical sciences and materials science as well as nano- and biosciences because it provides valuable information about structure, functions, and reactions of molecules. In the meantime, quantum chemical approaches play crucial roles in the spectral analysis. They also yield important knowledge about molecular and electronic structures as well as electronic transitions. The combination of spectroscopic approaches and quantum chemical calculations is a powerful tool for science, in general. Thus, our article, which treats various spectroscopy and quantum chemical approaches, should have strong implications in the wider scientific community. This review covers a wide area of molecular spectroscopy from far-ultraviolet (FUV, 120-200 nm) to far-infrared (FIR, 400-10 cm-1)/terahertz and Raman spectroscopy. As quantum chemical approaches, we introduce several anharmonic approaches such as vibrational self-consistent field (VSCF) and the combination of periodic harmonic calculations with anharmonic corrections based on finite models, grid-based techniques like the Numerov approach, the Cartesian coordinate tensor transfer (CCT) method, Symmetry-Adapted Cluster Configuration-Interaction (SAC-CI), and the ZINDO (Semi-empirical calculations at Zerner's Intermediate Neglect of Differential Overlap). One can use anharmonic approaches and grid-based approaches for both infrared (IR) and near-infrared (NIR) spectroscopy, while CCT methods are employed for Raman, Raman optical activity (ROA), FIR/terahertz and low-frequency Raman spectroscopy. Therefore, this review overviews cross relations between molecular spectroscopy and quantum chemical approaches, and provides various kinds of close-reality advanced spectral simulation for condensed phases.

Structural and electronic properties of Gen and EuGen-1 nanoclusters: A full potential DFT study

In this work, the stability, structure and electronic properties of the nanoclusters of germanium (Gen) and europium atom doped germanium clusters (EuGen-1) with n=2 to 12, 15 and 20 were investigated. First, the stability of nanoclusters such as Gen and EuGen-1 was addressed using FHI-aims as a software package based on the density functional theory. Then the lowest-energy structures were selected for calculating the first vertical ionization with the symmetry adapted cluster-configuration interaction General-R (SAC-CI-General-R) method. The results of this research show that there is a good agreement between calculation and experiment ionization potential for Gen nanoclusters. Generally, the analyses of binding energies show that increasing the size of nanoclusters leads to more stability for nanoclusters. The most stable nanoclusters for EuGen can be created with exchanging the Eu atom in the most stable Gen+1 nanoclusters, but there is an exception for n=11 case. Here, the second difference in energy (∆2E) and gap energy are computed for the stable nanoclusters. The results of ionization energy and second difference in energy confirm that Ge7 and Ge10 also EuGe8 and EuGe10 have the most stability.

Ionization of adenine in the presence of Na+ in the gas phase and water

In this work, the photoelectron spectrum of adenine (AD) interacting with one and two bare and microhydrated Na+ (AD-Na+, Na+-AD-Na+, AD-[Na(H2O)n]+ and [Na(H2O)n]+-AD-[Na(H2O)n]+) have been calculated in two environments including the gas phase and water, separately. The symmetry adapted/cluster configuration interaction (SAC-CI) methodology was employed to calculate the first to fifth ionization energies (IEs) of AD. The photoelectron spectra of AD-Na+ structures showed a shift to higher binding energy, depending on the interaction site of Na+, compared to the spectrum of bare AD in gas phase while a slight shift to lower binding energy was observed for the spectrum of the most stable structure of AD-Na+ in water compared to bare AD in the same condition. In the presence of second Na+, the gas phase photoelectron spectrum of Na+-AD-Na+ shifted to higher binding energy compared to the spectrum of AD-Na+. This energy shift was nearly independent of the interaction site of second Na+ and decreased, considerably in the water environment. The microhydration of Na+ in the AD-Na+ and Na+-AD-Na+ structures in the gas phase, shifted their photoelectron spectra to lower binding energies compared to the spectra of corresponding unhydrated structures depending on the number of water molecules. A similar trend was also observed in water. The hydration of Na+ could decrease the ionization energy of AD-Na+ and Na+-AD-Na+ structures in water even lower than the ionization energies of bare AD in the same conditions. The main electronic configurations of the ionization bands of each structure were determined and their changing with the Na+ interaction site, number of Na+ and hydration of Na+ were explained. The kind and contribution of Lewis orbitals in the canonical molecular orbitals participating in ionization were determined for each structure.

Electronic and vibrational structure in the S0 and S1 states of corannulene.

Corannulene is a nonplanar aromatic hydrocarbon also known as a buckybowl. Its electronic and vibrational structure has been investigated by analyzing its fluorescence excitation spectrum and dispersed fluorescence spectrum in a supersonic jet. Its spectral features are in keeping with the expectation, confirmed by some previous results, that it has fivefold or C5v symmetry. The observed prominent vibronic bands in the S1 ← S0 transition have been assigned to e1 and e2 bands on the basis of theoretical calculations so that the S1 state was assigned to 1E2. The symmetry adapted cluster configuration interaction calculation supports this assignment of the S1 electronic state, although the time-dependent density functional theory calculation suggests that the S1 state is 1A2. It has also been shown that the normal coordinates for strong vibronic bands mainly include out-of-plane vibrational motion. The rotational envelopes are well explained by taking account of the Coriolis interaction between the degenerate vibrational and rotational levels. The mechanism of bowl-to-bowl inversion is also discussed with the results of theoretical calculations regarding the barrier to inversion and metastable conformation.

Exploring response of Li2 molecule to external electric field: A DFT and SAC-CI study

Quantum chemical calculations are performed using Gaussian16W Software to theoretically investigate the response of Li2 molecule to external electric field. Ground state properties of Li2 molecule in the presence of electric field are modelled using Density Functional Theory(CAM-B3LYP/[6-311++G(3df,3pd)]) while excited states are modelled using Symmetry Adapted Cluster Configuration Interaction theory and 6-311++G∗∗ basis set. The effect of external electric field on ground state properties including structural (bond length, total energy, harmonic vibrational frequency and rotational constant), electronic (dipole moment and polarizability) and thermodynamic (vibrational contribution to partition function, thermal energy, entropy and specific heat capacity) properties is reported. External electric field modifies the charge distribution within the molecule and hence its molecular properties. Potential energy curves and transition dipole moments of 22 states (13Singlet + 9Triplet) are also reported. The computed parameters agree well with the experimental data wherever the later is available.

The low-lying electronic states of HfO2− and HfO2: A symmetry adapted cluster–configuration interaction (SAC-CI) study

The low-lying doublet states of the HfO2− anion and low-lying triplet excited states of the HfO2 molecule were studied using the SAC-CI method. The vertical excitation spectrum of HfO2− was calculated. Four valence-bound states (X2A1, 12B1, 22A1, and 12B2(II)) and one dipole-bound state (2A1(DB)) of HfO2− were found. The vertical electron detachment energies (VDEs) from the ground electronic state of HfO2− to the triplet excited states of HfO2 were obtained. These VDEs were compared with previous photoelectron spectra of HfO2−, and possible candidate states for the unassigned bands were found. The equilibrium geometries of the valence-bound states of HfO2− and some selected triplet excited states of HfO2 were optimized. The adiabatic relative energies of these electronic states were obtained. Density functional theory (BPW91) was also applied to compute some electronic states of HfO2− and HfO2 for comparison.

The Intriguing Case of the One‐Photon and Two‐Photon Absorption of a Prototypical Symmetric Squaraine: Comparison of TDDFT and Wave‐Function Methods

AbstractWe face the challenging description of the excited states responsible for one photon (OPA) and two photon (TPA) absorption in squaraine dyes, adopting both time‐dependent density functional theory and a large variety of post Hartree Fock methods, including coupled cluster and adiabatic diagrammatic construction methods (ADC, to the second and third order), symmetry adapted cluster configuration interaction (CI), CASSCF including second‐order perturbative corrections (PT2) with standard CASPT2 recipe and according to n‐electron valence PT2 (NEVPT2) approach, and an additional multireference CI with PT2 corrections (CI‐MRPT2). We selected a small prototypical symmetric squaraine dye for which these accurate computations are feasible and experimental data are available. We show that while all methods reasonably reproduce the OPA spectrum, an acceptable description and assignment of the states responsible for TPA is only possible through ADC(3) and multireference calculations, due to the strong involvement of double‐excitations. The nature of ground state and OPA and TPA states is investigated with a rigorous reading of the CASSCF wavefunctions in terms of localized molecular orbitals on the donor and acceptor fragments, showing that these states arise from a balance of many different contributions including biradical configurations, local excitations and charge‐transfer states where both side aromatic rings and the carbonyls of the squarylium moiety act as donors.

Theoretical study of the absolute inner-shell photoionization cross sections of the formic acid and some of its hydrogen-bonded clusters.

Inner-shell absolute photoabsorption and photoionization cross sections of the formic acid, HCOOH, and its small hydrogen-bonded clusters, i.e., (HCOOH)2, HCOOH2 +, HCOHOH+, and HCOOH·H3O+, were calculated at the time-dependent density functional theory (TDDFT) level, and the results were used to analyze the effect of the formic acid clustering on the carbon and oxygen K-edge photoionization cross sections. The discrete electronic pseudospectra obtained with square-integrable (L2) basis set calculations were used in an analytic continuation procedure based on continued fraction functions to obtain the photoabsorption cross sections. Symmetry adapted cluster configuration interaction calculations on the small formic acid clusters have also been performed at the oxygen K-edge to assign the discrete transitions and ionization potentials in support to the TDDFT results.

Theoretical prediction of valence and Rydberg excited states: Minnesota exchange‐correlation functionals vs symmetry adapted cluster‐configuration interaction

AbstractDuring this contribution, we present a benchmark investigation on the applicability of several Minnesota functionals from various classes like local meta‐generalized and meta‐nonseparable gradient approximations, hybrids, and range‐separated hybrids for describing the valence and Rydberg excitation energies of some organic compounds from different categories. Furthermore, the performances of Minnesota density functionals from density functional theory are also assessed against a wave function theory based approach in the context of excite states calculations, symmetry adapted cluster‐configuration interaction (SAC‐CI) method. Pragmatically, the singles and doubles linked excitation operators are considered in the SAC‐CI wave functions. With more or less different accountabilities of the considered methods, it is shown that the M06‐2X, M05‐2X, and M11 functionals have the best performances for valence excited states. On the other hand, for Rydberg excited states although the SAC‐CI method outperforms others, the statistical analyses reveal that the efficiency of some Minnesota functionals is also respectable.

Conformational analysis and electronic structure of folic acid: A theoretical study

Folic acid decomposes when exposed to UV light so understanding of photochemical behavior of this biomolecule is very important. In this study, sixty conformers of folic acid were investigated by two methods of DFT calculations. The three most stable conformers were selected for study of their electronic structure and photoelectron spectra. The valence photoelectron spectra were simulated by two high level ab initio computational methods: general-R symmetry adapted cluster-configuration interaction (general-R-SAC-CI) method and outer-valence Green's function (OVGF). The SAC-CI computations were performed with two different basis sets: D95 (df,pd) and 6-311G(d,p). Because of the size of the folic acid molecule, OVGF calculations were only performed with 6-311G(d,p) basis set. The simulated photoelectron spectrum of folic acid was obtained by the superposition of weighted photoelectron spectra of individual conformers. natural bonding orbitals (NBO) calculations were performed for the assignment of simulated photoelectron spectra. The lowest energy ionizations occurs mostly from the orbitals localized on p-aminobenzoyl moiety in folic acid. Koopmans' approximation is only valid for describing the five lowest energy orbital ionizations (photoelectron bands). The in-situ molecular structures of two bio-active conformers were extracted from X-ray diffraction data obtained for folic acid bound to folate receptor protein. These molecular structures were then used for calculating ionization energies. We discuss the consequences for ligand-receptor binding of switching from the most stable conformation to bioactive conformation in FA.

The low-lying triplet electronic excited states of TiO2 and ZrO2: A symmetry adapted cluster–configuration interaction (SAC-CI) study

The vertical excitation energies and electron detachment energies to the low-lying triplet excited states of TiO2 and ZrO2 were calculated at the SAC-CI/aug-cc-pVTZ levels. The vertical electron detachment energies to the triplet excited states of TiO2 were compared with previous photoelectron spectra. Possible candidate states for the unassigned bands in the previous photoelectron spectrum were found. Several triplet excited states of TiO2 and ZrO2 were optimized at the SAC-CI/aug-cc-pVDZ and SAC-CI/def2-SVPD levels, respectively. The adiabatic excitation energies and electron detachment energies to the selected triplet states were obtained by single-point calculations at the SAC-CI/def2-TZVPPD level. Density functional theory (BPW91/aug-cc-pVTZ) was also applied to study some of the triplet states for comparison. The calculated electron detachment energies and optimized geometries of the triplet excited states should be useful for studying the photoelectron spectra of TiO2− and ZrO2−.

The theoretical study of the ground and excited states properties for F2BO and H2BO free radicals

The symmetry-adapted-cluster (SAC)/symmetry-adapted-cluster configuration interaction (SAC-CI) method has been applied to study the ground and low-lying excited states properties of F2BO and H2BO free radicals. The geometry of the ground states for F2BO and H2BO are optimized by SAC method with the different basis sets and the results show that the SAC/D95** and SAC/6-311G(df) levels are in excellent agreement with the available experimental and existing theoretical data. Based on SAC-CI/D95** and SAC-CI/6-311G(df) levels, the vertical excitation energies, transition dipole moments and oscillation strengths of F2BO and H2BO in the low-lying excited states are calculated. Besides, the optimized geometries, adiabatic excitation energies, and rotational constants of low-lying excited states of F2BO and H2BO are also obtained. We hope that the calculated results can provide the useful data for the studies of the excited state properties for F2BO, H2BO, and similar free radicals.

Electronic Structure of Biotin Conformers Studied with SAC-CI and OVGF Methods.

In this work, the study was performed with 37 gas-phase conformers of biotin and two biologically active conformers of biotin in the ligand-receptor complexes with astavidin and streptavidin. The ionization energies and photoelectron spectra of conformers were calculated by two methods: the general-R symmetry-adapted cluster-configuration interaction (general-R-SAC-CI) method and the outer-valence Green's function (OVGF) method. The photoelectron spectrum of each conformer was calculated using basis set D95 (df,pd) for both methods. The simulated photoelectron spectra of free molecules and bioactive conformers calculated by the two methods were compared. Natural bonding orbital (NBO) calculations were also performed for the assignment of ionization bands of each conformer. NBO calculation indicated that the first to five ionization bands correspond to ionizations from orbitals localized in the two rings. The most important point about the ionization of all conformers is that the removal of an electron from the σ-bonding orbital (C-S) takes place above 10.0 eV.

Temporary Anion States of Ethene Interacting with Single Molecules of Methane, Ethane, and Water.

When an excess electron is added into the π* orbital of ethene, the resulting anion decays by electron autodetachment; that is, it represents an electronic state referred to as a temporary anion or resonance state. Here, the influence of a cluster environment on the energy and lifetime of this state is investigated. The clusters considered are ethene···CH4, ethene···C2H6, and ethene···H2O. Most of these clusters are systematically constructed so that the solvent interacts with the π system in a specific way, and are thus by construction not minima with respect to all intermolecular degrees of freedom. However, for water, in addition, a minimal energy structure is examined. Systematic variation of the solvent and solvation geometry allows us to identify trends regarding effects due to polarizability, excluded volume, and polarity of the solvent molecules. The resonance parameters of ethene and all temporary cluster anions are computed with the symmetry-adapted cluster-configuration interaction electronic structure method in combination with a complex absorbing potential. This method is well-established for small to intermediate sized molecules. In addition to the study of the solvation effects themselves, the question of how many basis functions are needed on the closed-shell solvating unit is examined.