Correlated Ab Initio Calculations Research Articles

Theoretical Spectroscopic Study of Isopropyl Alcohol (CH3-CHOH-CH3)

Accurate spectroscopic parameters of isopropyl alcohol, a volatile organic compound present on Earth and in extraterrestrial atmospheres, are provided. The work pursues the study of the far-infrared region, describing the distribution of the low-lying vibrational energy levels that can be populated at very low temperatures, having effects on the detectability and identification. The potential energy surface shows 27 almost equivalent minima producing 27 subcomponents of the levels due to tunneling effects. Computations have been performed using highly correlated ab initio calculations, accurate enough to distinguish between the rotational parameters of the two quasi-identical conformers gauche and trans. A variational procedure in three dimensions depending on three interacting internal rotations, the internal rotation of the two methyl groups and the internal rotation of the hydroxyl group, is employed to compute the energies. The 27 subcomponents of the ground vibrational state can be grouped into three series of nine energy levels located around 0.000, 1.693, and 81.927 cm−1 whose energy difference is due to the OH torsion effect. The nine subcomponents integrated in each series derive from the torsion of the two methyl groups. The computations reproduce accurately the available experimental data. New predicted properties can help the spectroscopic analysis of the rotational-vibrational spectra in the gas phase and further detections of vibrationally excited isopropyl alcohol.

Insights into dioxygen binding on metal centers: an ab initio multireference electronic structure analysis.

Why does binding of dioxygen (O2) to metal centers, the initial step of O2 storage, transportation, and activation, almost inevitably induce metal-to-O2 single-electron transfer and generate superoxo (O2-˙) species, instead of genuine O02 adducts? To address this question, this study describes highly correlated wavefunction-based ab initio calculations using CASSCF/NEVPT2 (CASSCF = complete active space self-consistent field, and NEVPT2 = N-electron valence state second-order perturbation theory) approaches to explore the electronic-structure evolution of O2 association on Fe(II)(BDPP) (H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Co(II)(BDPP) to produce S = 3 Fe(III)(BDPP)(O2-˙) (1) and Co(III)(BDPP)(O2-˙) (2). CASSCF/NEVPT2 calculations suggest that the processes furnishing 1 and 2 feature an avoided crossing resulting from interactions of two diabatic curves, of which one is characterized as Co(II) and Fe(II) centers interacting with a triplet O2 ligand and the other as Co(III) and Fe(III) centers bound to a superoxo ligand. In both cases, the avoided crossing induces a one-electron transfer from the divalent metal center to the incoming O2 ligand and leads to formation of trivalent metal-O2-˙ complexes. To facilitate the interpretation of complicated multireference wavefunctions, we formulated two-fragment spin eigenfunctions utilizing Clebsch-Gordan coefficients (CGCs) to rationalize computed spin populations on the metal centers and the O2 ligand and compared these results with usual valence bonding (VB) analyses. It turns out that both methods give the same results and are complementary to each other. Finally, the limitation of DFT approaches in describing complex electronic structures involving metal-ligand magnetic couplings is delineated.

A Crystalline Dimeric Steroidal Diboronate with Electronically Impeded Rotation

The dimeric steroid SMR-3, featuring a 1,4-phenyldiboronic ester flanked by two pregnan-triol frameworks, was synthesized to explore the intramolecular dynamics of its central component. The structural data from single-crystal X-ray diffraction studies and the Hirshfeld analyses indicate small steric effects around the aromatic ring that should favor the intended motion. However, solid-state NMR data obtained through VT 13C{1H} CPMAS and 2H spin-echo experiments, using the deuterated analogue SMR-3D4, revealed that this component is rigid even at temperatures where other reported steroidal molecular rotors experience fast rotation (85 °C). A combination of classical molecular dynamics, molecular mechanics, and correlated ab initio calculations allowed us to distinguish the steric and electronic factors that restrict the potential motion in this compound. The experimental and computational data reveal that electronic components dominate the behavior and are responsible for the high rotational barrier in the SMR-3 crystal.

Noncanonical Isomers of Nucleoside Cation Radicals: An Ab Initio Study of the Dark Matter of DNA Ionization.

Cation radicals of DNA nucleosides, 2'-deoxyadenosine, 2'-deoxyguanosine, 2'-deoxycytidine, and 2'-deoxythymidine, can exist in standard canonical forms or as noncanonical isomers in which the charge is introduced by protonation of the nucleobase, whereas the radical predominantly resides in the deoxyribose moiety. Density functional theory as well as correlated ab initio calculations with coupled clusters (CCSD(T)) that were extrapolated to the complete basis set limit showed that noncanonical nucleoside ion isomers were thermodynamically more stable than their canonical forms in both the gas phase and as water-solvated ions. This indicated the possibility of exothermic conversion of canonical to noncanonical forms. The noncanonical isomers were calculated to have very low adiabatic ion-electron recombination energies (REad) for the lowest-energy isomers 2'-deoxy-(N-3H)adenos-1'-yl (4.74 eV), 2'-deoxy-(N-7H)guanos-1'-yl (4.66 eV), 2'-deoxy-(N-3H)cytid-1'-yl (5.12 eV), and 2'-deoxy-5-methylene-(O-2H)uridine (5.24 eV). These were substantially lower than the REad value calculated for the canonical 2'-deoxyadenosine, 2'-deoxy guanosine, 2'-deoxy cytidine, and 2'-deoxy thymidine cation radicals, which were 7.82, 7.46, 8.14, and 8.20 eV, respectively, for the lowest-energy ion conformers of each type. Charge and spin distributions in noncovalent cation-radical dA⊂dT and dG⊂dC nucleoside pairs and dAT, dCT, dTC, and dGC dinucleotides were analyzed to elucidate the electronic structure of the cation radicals. Born-Oppenheimer molecular dynamics trajectory calculations of the dinucleotides and nucleoside pairs indicated rapid exothermic proton transfer from noncanonical T+· to A in both dAT+· and dA⊂dT+·, leading to charge and radical separation. Noncanonical T+· in dCT+· and dTC+· initiated rapid proton transfer to cytosine, whereas the canonical dCT+· dinucleotide ion retained the cation radical structure without isomerization. No spontaneous proton transfer was found in dGC+· and dG⊂dC+· containing canonical neutral and noncanonical ionized deoxycytidine.

Theoretical study of the low-lying electronic states of diatomic antimony oxide including the spin-orbit coupling effect

In this paper, accurate determinations of electronic structures and spectroscopic features for the Λ−S and Ω low-lying electronic states of SbO are investigated by highly correlated ab initio calculations. Energies of twenty-four Λ−S states corresponding to the first two dissociation asymptotes, as well as the lowest twelve Ω states, are obtained and then used to construct their potential energy curves. Spectroscopic constants and vibrational states for all bound states are determined and are consistent with experimental data. The spin-orbit coupling effect has introduced a minor impact on the electronic states of SbO. Additional calculations for the low-lying electronic states of PO, AsO and BiO are also carried out to further verify the computational accuracy of SbO. Our derived results for these VA group oxides are in reasonable agreement with available experimental data. In addition, emission spectra of the excited Ω states to the ground one are simulated. The predictive results are anticipated to serve as guidelines for further research such as assisting laboratorial detections and analyzing the observed spectrum of SbO.

Correlated ab initio calculations of one-bond 31 P77 Se and 31 P125 Te spin-spin coupling constants in a series of PSe and PTe systems accounting for relativistic effects (part 2).

Synthetic chalcogen-phosphorus chemistry permanently makes new challenges to computational Nuclear Magnetic Resonance (NMR) spectroscopy, which has proven to be a powerful tool of structural analysis of chalcogen-phosphorus compounds. This paper reports on the calculations of one-bond 31 P77 Se and 31 P125 Te NMR spin-spin coupling constants (SSCCs) in the series of phosphine selenides and tellurides. The applicability of the combined computational approach to the one-bond 31 P77 Se and 31 P125 Te SSCCs, incorporating the composite nonrelativistic scheme, built of high-accuracy correlated SOPPA (CC2) and Coupled Cluster Single and Double (CCSD) methods and the Density Functional Theory (DFT) relativistic corrections (four-component level), was examined against the experiment and another scheme based on the four-component relativistic DFT method. A special J-oriented basis set (acv3z-J) for selenium and tellurium atoms, developed previously by the authors, was used throughout the NMR calculations in this work at the first time. The proposed computational methodologies (combined and 'pure') provided a reasonable accuracy for 31 P77 Se and 31 P125 Te SSCCs against experimental data, characterizing by the mean absolute percentage errors of about 4% and 1%, and 12% and 8% for selenium and tellurium species, respectively. The present study reports typical relativistic corrections to 77 Se31 P and 125 Te31 P SSCCs, calculated within the four-component DFT formalism for a broad series of tertiary phosphine selenides and tellurides with different substituents at phosphorus.

Bootstrap Embedding For Large Molecular Systems.

Recent developments in quantum embedding theories have provided attractive approaches to correlated calculations for large systems. In this work, we extend our previous work [J. Chem. Theory Comput. 2019, 15, 4497-4506; J. Phys. Chem. Lett. 2019, 10, 6368-6374] on bootstrap embedding (BE) to enable correlated ab initio calculations at the coupled cluster with singles and doubles (CCSD) level for large molecules. We introduce several new algorithmic developments that significantly reduce the computational cost of BE, while maintaining its accuracy. The resulting implementation scales as O(N3) for the integral transform and O(N) for the CCSD calculation. Numerical results on a series of conjugated molecules suggest that BE with reasonably sized fragments can recover more than 99.5% of the total correlation energy of a full CCSD calculation, while the required computational resources (time and storage) compare favorably to one popular local correlation scheme: domain localized pair natural orbital (DLPNO). The largest BE calculation in this work involves ∼2900 basis functions and can be performed on a single node with 16 CPU cores and 64 GB of memory in a few days. We anticipate that these developments represent an important step toward the application of BE to solve practical problems.

The interplay and the formation of σ-hole in the π···LiX and pseudo-π···LiX (X = F, Cl and CN) lithium bonds involving unsaturated and homocyclic hydrocarbons

A theoretical study at the level of the density functional theory and ab initio correlation calculations was carried out in order to better understand the intermolecular properties of π∙∙∙Li and pseudo-π∙∙∙Li interactions formed between the C2H2, C2H4 or C3H6 and LiX (X = F, Cl and CN). The extremely high values of the lithium bond energies followed by red-shifts and blue-shifts are the cornerstone of these systems, although the magnitude of the σ-hole in the Li–X bond agrees with the interaction strength profile. The atomic charge on the Li-atom had to be also analyzed because this parameter is mandatory to assure the lithium bond formation. Ideally, the σ-hole is predicted by variations in the occupancy of the p-orbitals of the F, Cl or C elements. Lastly, the energy decomposition analyses provided a suitable explanation for the frequency shifts to red or blue according to exchange energy calculation.

Inelastic rate coefficients for collisions of N2H+ with H2

ABSTRACT N2H+ is one of the first molecular ions observed in the interstellar medium and it is of particular interest to probe the physical conditions of cold molecular clouds. Accurate modelling of the observed lines requires the knowledge of collisional excitation rate coefficients. Thus, we have calculated rate coefficients for the excitation of N2H+ by H2, the most abundant collisional partner. The calculations are based on a new potential energy surface obtained from highly correlated ab initio calculations. This 4D-interaction surface exhibits a very deep well of ≈2530 cm−1 making fully converged scattering calculations very difficult to carry out, when one takes into account the rotational structure of H2. To overcome this difficulty, two approximate approaches, the adiabatic hindered rotor approach (AHR) and the statistical adiabatic channel model, were tested by comparing the results with those obtained from full 4D close-coupling calculations. The AHR treatment, which reduces the scattering calculations to a 2D problem was found to give the best results at all temperatures and even for transitions involving high N2H+ rotational levels. State-to-state rate coefficients between the 26 first N2H+ rotational levels were calculated for temperatures ranging from 5 K up to 500 K. Using a recoupling technique, rate coefficients are obtained among hyperfine transitions.

New Spectral Characterization of Dimethyl Ether Isotopologues CH3OCH3 and 13CH3OCH3 in the THz Region

Abstract The torsional Raman spectra of two astrophysically detected isotopologues of dimethyl ether (DME, 12CH3O12CH3 and 13CH3O12CH3) have been recorded at room temperature and cooled in a supersonic jet and interpreted with the help of highly correlated ab initio calculations. DME displays excited torsional and vibrational levels at low energy that can be populated at the temperatures of the star-forming regions, obliging to extend the analysis of the rotational spectrum over the ground state. Its spectrum in the THz region is rather complex due to the coupling of the torsional overtones 2ν 11 and 2ν 15 with the COC-bending mode and the presence of many hot bands. The torsional overtones are set here at 2ν 11 = 385.2 cm−1 and 2ν 15 = 482.0 cm−1 for 12CH3O12CH3 and 2ν 11 = 385.0 cm−1 and 2ν 15 = 481.1 cm−1 for 13CH3O12CH3. The new assignment of 2ν 11 is downshifted around ∼10 cm−1 with respect to the literature. All the other (hot) bands have been reassigned consistently. In addition, the infrared-forbidden torsional fundamental band ν 11 is observed here at 197.8 cm−1. The new spectral characterization in the THz region reported here provides improved values of the Hamiltonian parameters to be used in the analysis of the rotational spectra of DME isotopologues for further astrophysical detections.

Electronic Structures of an [Fe(NNR2)]+/0/– Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation

The intermediacy of metal-NNH2 complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N2) fixation. In this context, we have shown that triphosphine-supported Fe(N2) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH2) complexes over an array of charge and oxidation states. Upon exposure to further H+/e- equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N-H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe2) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the "NNMe2" moiety. Specifically, although two closed-shell configurations of the "NNR2" ligand have been commonly considered in the literature-isodiazene and hydrazido(2-)-we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR2]•- ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N2 fixation activity.

Theoretical Characterization of C3H and C5H and Their Anions

Abstract Highly correlated ab initio calculations are employed for the structural and spectroscopic characterization of small odd chains of type C2n+1H, considering neutral forms, cations, and giving special attention to the anions. This work confirms the stability of the linear carbon chains and carbon clusters containing three-body rings. The smallest species, C3H, displays three stable structures, whereas C5H possesses at least 8 neutral isomers and 11 and 10 isomers with a negative or a positive charge. The equilibrium geometries, which can be candidates for laboratory and astrophysical detection, are studied using the RCCSD(T)-F12 and MRCI/CASSCF levels of theory, specifying properties for various electronic states. Four different stable isomers are confirmed for the C5H− anion. They are two rings and two chains, all showing singlet ground electronic states. The viability of the triplet linear form of C5H− ( (X3Σ−)) postulated in previous works, is not confirmed because it appears to be really dependent on the electron correlation energy denoting instability. A quasi-linear singlet (Cs (X1 A′)) represents a secondary minimum. Electronic state crossing occurs close to the linear structure where spin–orbit effects are negligible. The most stable structure of C5H− is a three-carbon cycle in which rotational constants have been determined to be A 0 = 35479.86 MHz, B 0 = 3618.29 MHz, and C 0 = 3280.10 MHz. Its dipole moment is relatively large (6.4086 D).

Understanding propyl cyanide and its isomers formation: ab initio study of the spectroscopy and reaction mechanisms.

Recent detection of propyl cyanide (C3H7CN) with both linear and branched structures has stimulated many experimental and theoretical studies. In this theoretical work, we present the spectroscopic properties of the far infrared spectra of these species and we investigate their different paths of formation in the gas phase. Our spectroscopic study concerns the far infrared spectra of iso, anti and gauche propyl cyanide isomers. The equilibrium structures and the potential energy surfaces are calculated using explicitly correlated cluster ab initio methods (CCSD(T)-F12) and a variational procedure designed for non-rigid species and large amplitude motions. Accurate rotational constants, centrifugal distortion constants, potential energy barriers and surfaces are provided. The rovibrational parameters in the ground vibrational states compare very well with experimental data. The low energy vibrational levels correspond to torsional modes. Far infrared energies are calculated up to 500 cm-1 using the variational approach and the vibrational second order theory (VPT2), and a good agreement with previous experimental values is found. We have also investigated the gas phase formation of the different C3H7CN isomers. After several trials of reacting gaseous species, we considered that a possible formation route of the C3H7CN isomers can be from the bimolecular reaction of HCN with propene. At the UMP2(full)/aug-cc-pVTZ level of theory, this reaction involves two steps for each isomer; the first one corresponds to the association of the two radicals while the second one corresponds to H transfer. From highly correlated ab initio calculations by means of CCSD(T)/aug-cc-pVTZ//UMP2(full)/aug-cc-pVTZ, the geometries, energetics and minimum energy paths of the reactions are obtained. Also, the first step's transition state disappears, as the diradical minimum is much less stabilized with UCCSD(T) than by UMP2(full). We employ the zero curvature tunneling and canonical variational (CVT/ZCT) semiclassical method to predict rate constants for propyl cyanide isomers formation in the gas phase. However, due to the presence of a significant barrier, this reaction leads to very small rate constants. Very recently, we probed reaction mechanisms involving radical addition and we found that these reactions are barrierless and highly exothermic leading to expected fast reactive processes for the formation of propyl cyanide products. The kinetics of such diradical reactions is under study.

Electronic and vibrational spectroscopy of the low-lying states of potassium mono-sulphide KS, and comparison in the series of MS (M = Li, Na, K, Rb, Cs)

ABSTRACTPotential energy curves (PECs) of the lowest electronic states of the potassium mono-sulphide KS have been determined with highly correlated ab initio calculations, using internally contracted multi reference interaction configuration methods including Davidson correction (MRCI + Q) with and without considering spin-orbit effects. For the three low-lying bound states, we report a set of spectroscopic parameters including equilibrium distances, dissociation energies, vibrational and rotational constants. The effects of spin-orbit-induced changes on these parameters are also discussed. An analysis of the properties of the three bound states, X2Π, 12Σ+ and 22Π, illustrates the common characteristics of the whole series of compounds in the MS family (M = Li, Na, K, Rb, Cs). Indeed, the shapes of the PECs of these bound states are strongly affected by the interactions between the two ionic states, 2Σ+ and 2Π, correlating at large internuclear separations (RMS) to the first ionic dissociation limit [M+ + S−] and the electronic states correlating to the three/four lowest dissociation limits. The spectroscopy of these low-lying electronic states and the lifetime of their vibrational levels are thus affected by the spin-orbit interactions which are mainly related to the S atom and consequently common to all alkali-metal mono-sulphides.

Crystalline phases in Zr9Ni11 and Hf9Ni11 intermetallics; Investigations by perturbed angular correlation spectroscopy and ab initio calculations

Crystalline phases formed in stoichiometric Zr9Ni11 and Hf9Ni11 have been studied by perturbed angular correlation (PAC) spectroscopy, XRD and TEM/SAED measurements. In Zr9Ni11, the phases Zr9Ni11 (∼89%) and Zr8Ni21 (∼11%) have been found at room temperature from PAC measurements. At 773 K, Zr9Ni11 partially decomposes to Zr7Ni10 and at 973 K, it is completely decomposed to ZrNi and Zr7Ni10. In Hf9Ni11, a predominant phase (∼81%) due to HfNi is found at room temperature while the phase Hf9Ni11 is produced as a minor phase (∼19%). No compositional phase change at higher temperature is found in Hf9Ni11. Phase components found from XRD and TEM/SAED measurements are similar to those observed from PAC measurements. Electric field gradients in Zr9Ni11 and Hf9Ni11 have been calculated by density functional theory (DFT) using all electron full potential (linearized) augmented plane wave plus local orbitals [FP-(L)APW+lo] method in order to assign the phase components.

Spectral Study of A1Π–X1Σ+ Transitions of CO Relevant to Interstellar Clouds

Abstract Highly correlated ab initio calculations were performed for an accurate determination of the A 1Π–X 1Σ+ system of the CO molecule. A highly accurate multi-reference configuration interaction approach was used to investigate the potential energy curves (PECs) and the transition dipole moment curve (TDMC). The resultant PECs and TDMC found by using the aug-cc-pV5Z (aV5Z) basis set and 5330 active spaces are in good agreement with the experimental data. Moreover, the Einstein A coefficients, lifetimes, ro-vibrational intensities, absorption oscillator strengths, and integrated cross sections are calculated so that the vibrational bands include v″ = 0–39 v′ = 0–23. For applications in the atmosphere and interstellar clouds, we studied the transition lineshapes to Gaussian and Lorentzian profiles at different temperatures and pressures. The intensities were calculated at high temperature that was used to satisfy some astrophysical applications, such as in planetary atmospheres. The results are potentially useful for important SAO/NASA Astrophysics Data System and databases such as HITRAN, HITEMP, and the National Institute of Standards and Technology. Because the results from many laboratory techniques and our calculations now agree, analyses of interstellar CO based on absorption from A 1Π–X 1Σ+ are no longer hindered by present spectral parameters.

Attaching Be or Mg to BH3 results in the formation of BeBH3 and MgBH3 molecules capable of forming stable anions

Stability of BeBH3 and MgBH3 molecules and their (BeBH3)− and (MgBH3)− anions is investigated on the basis of correlated ab initio calculations. The electronic and thermodynamic stability of all species is confirmed by estimating the excess electron binding energies of the anions and by evaluating the Gibbs free energies for various fragmentation paths. The bonding effects in BeBH3 and MgBH3 have been identified as the result of alkaline earth metal ns2 lone-pair donation to the empty 2p boron orbital. Adiabatic and vertical electronic stabilities of the (BeBH3)− and (MgBH3)− anions were found to span 1.114–1.301 and 0.675–0.744 eV range, respectively.

Electronic Structure and Dipole Moment Calculations of the Electronic States of the Molecule ZnS

In this study, the low lying electronic states and spectroscopy of diatomic molecule ZnS in addition to its dipole moments have been investigated by performing highly correlated ab initio calculations, the Complete Active Space Self Consistent Field (CASSCF) method with Multi Reference Configuration Interaction (MRCI+Q) for an accurate picture for these states. The proposed study includes information about the potential energy curves of the lowest 12 singlet and 9 triplet electronic states of the molecule ZnS, in the representation 2s+1Λ(+/-). Nine of these states have been studied here for the first time. The harmonic frequency we, the internuclear distance re, the electronic energy with respect to the ground state Te, the rotational constant Be, and the permanent dipole moment μ have been calculated, compared and compiled with the available existing data to illustrate the electronic characteristics. The comparison of these values reveals a very good agreement.

Spin-Orbit Effects in the Spectroscopy of the X2Π and a4Σ- Electronic States of Carbon Iodide, CI.

Highly correlated ab initio calculations have been performed to describe the potential energy curves (PECs) and the spectroscopic properties of the X2Π state and of the first excited state of the CI radical. Multi Reference configuration interaction calculations with Davidson correction (MRCI+Q) and relativistic effective core potential for the iodine atom have been performed. It is found that the two lowest electronic states, the X2Π and the a4Σ- states, are stable against dissociation and well separated from the other electronic states. Spectroscopic constants of these two states have been evaluated using their calculated PECs. Because of the presence of the iodine atom in this molecular system, spin-orbit (SO) interactions are playing an important role in the molecular and in the dissociation regions. The excitation energy of the a4Σ- state is calculated 1.67 eV (MRCI) above the X2Π ground state and 1.70/1.62 eV (MRCI with SO correction) for the Ω = 1/2 and 3/2 transitions, respectively. The dissociation energy D0 of the X2Π ground state is evaluated 2.66 eV (MRCI calculation) without SO correction and 2.46/2.36 eV with SO correction for the Ω = 1/2 and 3/2 components, respectively. The dissociation energy D0 of the a4Σ- state is evaluated 0.99 eV (MRCI calculation) without SO correction and 0.83/0.72 eV with SO correction for the Ω = 1/2 and 3/2 components, respectively. This work should help for the identification of this radical in laboratory and in atmospheric media.

Calculation of linear and nonlinear optical properties of azobenzene derivatives with Kohn-Sham and coupled-cluster methods.

Linear polarizabilities (α) and second hyperpolarizabilities (γ) of unsubstituted azobenzenes and 'push-pull' azobenzene derivatives are investigated using Kohn-Sham theory (KST) and coupled-cluster (CC) approaches. Various standard exchange-correlation functionals as well as a non-empirically 'tuned' long-range corrected (LC) functional with range-separated exchange are used in the KST calculations. When compared to correlated ab initio calculations and measurements, the tuned functional gives accurate low-energy excitation energies, especially for charge transfer (CT) transitions, and performs well for α. Basis set and solvent effects are also studied. In contrast to expectations, but in agreement with a prior study of π-conjugated systems that do not have low-energy CT excitations, the improvements of the CT excitation energies for the push-pull π-chromophores due to tuning do not go along with clear improvements of γ toward the CC reference data, likely due to the importance of the dynamic electron correlation for this property.