Single Configuration Interaction Calculations Research Articles

Charge transfer excitons in π-stacked thiophene oligomers and P3[Alkyl]T crystals: CIS calculations and electroabsorption spectroscopy.

Poly(3-alkylthiophenes) (P3[Alkyl]T) exhibit high mobility and efficiency of formation of polaronic charge carriers generated by light absorption, thus finding applications in field effect devices. Excited states of π-stacked dimers of tetra-thiophene oligomers (T4), infinite isolated polythiophene (PT) chains, and P3[Alkyl]T crystals are modeled using configuration interaction singles (CIS) calculations. Excited states in cofacial T4 dimers are mostly localized Frenkel states except for two low energy charge transfer (CT) exciton states, which become the ionization potential and electron affinity levels of T4 molecules at large dimer separation. The lowest excited states in infinite, isolated PT chains and P3[Alkyl]T crystals are intra-chain excitons where the electron and hole are localized on the same chain. The next lowest excited states are interchain, CT excitons in which the electron and hole reside on neighboring chains. The former capture almost all optical oscillator strength and the latter may be a route to efficient formation of polaronic charge carriers in P3[Alkyl]T systems. Changes in optical absorption energies of T4 dimers as a function of molecular separation are explained using CIS calculations with four frontier orbitals in the active space. Shifts in optical absorption energy observed on going from isolated chains to P3[Alkyl]T lamellar structures are already present in single-particle transition energies induced by direct π-π interactions at short range. The electroabsorption spectrum of T4 dimers is calculated as a function of dimer separation and states that are responsible for parallel and perpendicular components of the spectrum are identified.

Channel-specific core-valence projectors for determining partial Auger decay widths

ABSTRACT Auger decay is a relaxation process of core-vacant states in atoms and molecules, in which one valence electron fills the core vacancy while a second one is emitted. These states pose a challenge to electronic-structure theory, because they are embedded in the ionisation continuum. Recently, we showed that molecular Auger decay can be described using complex-variable coupled-cluster (CC) methods and that partial widths and branching ratios can be computed based on a decomposition of the CC energy. Here, we introduce channel-specific core-valence projectors, dubbed Auger channel projectors, as a more general technique to evaluate partial widths from complex-variable treatments. We apply this new method to core-ionised states of neon, water, ammonia and methane using CC singles and doubles (CCSD), equation-of-motion ionisation potential CCSD and configuration interaction singles (CIS) wave functions. Even though a single CIS calculation can never describe all Auger decay channels at once, we show that a combination of CIS calculations based on different reference states is able to recover partial and total decay widths from CC calculations to an excellent degree.

Reduced-Scaling Approach for Configuration Interaction Singles and Time-Dependent Density Functional Theory Calculations Using Hybrid Functionals.

An approximation is presented which can efficiently decrease the computational expenses of configuration interaction singles (CIS) and time-dependent density functional theory (TDDFT) methods employing hybrid functionals. The approach is the adaptation of the local density fitting scheme developed for Hartree-Fock (HF) calculations for excited states and reduces the quartic scaling of the methods to cubic. It can also be applied to related methods, such as the time-dependent HF and Tamm-Dancoff approximation TDDFT approaches. Our benchmark calculations show that, for molecules of 50-100 atoms, average speedups of 2-4 can be achieved for CIS and TDDFT calculations at the expense of negligible errors in the calculated excitation energies and oscillator strengths, but for bigger systems or molecules of localized electronic structure significantly larger speedups can be gained. We also demonstrate that the approximation enables excited-state calculations on a single processor even for molecules of 1000 atoms using basis sets augmented with diffuse functions including more than 17 000 atomic orbitals.

HHG probing of atomic dipoles by electronic wave-packet caustics

We exploit high-order harmonic generation spectroscopy at the caustics of the recombining electron wave-packet as a method for directly comparing experimental spectra with ab-initio theories. Experimental results in xenon and comparison with ab-initio time-dependent configuration-interaction singles calculations allowed to assess the role of the wave-packet enhancement during the giant resonance. Results in argon show that this technique can also be applied to other targets.

Configuration interaction singles natural orbitals: an orbital basis for an efficient and size intensive multireference description of electronic excited states.

Multireference quantum chemical methods, such as the complete active space self-consistent field (CASSCF) method, have long been the state of the art for computing regions of potential energy surfaces (PESs) where complex, multiconfigurational wavefunctions are required, such as near conical intersections. Herein, we present a computationally efficient alternative to the widely used CASSCF method based on a complete active space configuration interaction (CASCI) expansion built from the state-averaged natural orbitals of configuration interaction singles calculations (CISNOs). This CISNO-CASCI approach is shown to predict vertical excitation energies of molecules with closed-shell ground states similar to those predicted by state averaged (SA)-CASSCF in many cases and to provide an excellent reference for a perturbative treatment of dynamic electron correlation. Absolute energies computed at the CISNO-CASCI level are found to be variationally superior, on average, to other CASCI methods. Unlike SA-CASSCF, CISNO-CASCI provides vertical excitation energies which are both size intensive and size consistent, thus suggesting that CISNO-CASCI would be preferable to SA-CASSCF for the study of systems with multiple excitable centers. The fact that SA-CASSCF and some other CASCI methods do not provide a size intensive/consistent description of excited states is attributed to changes in the orbitals that occur upon introduction of non-interacting subsystems. Finally, CISNO-CASCI is found to provide a suitable description of the PES surrounding a biradicaloid conical intersection in ethylene.

TDDFT and CIS Studies of Optical Properties of Dimers of Silver Tetrahedra

The absorption spectra for dimers of Ag(4)(+2) and Ag(8) clusters at various interparticle distances are examined using time-dependent density functional theory (TDDFT) and configuration interaction singles (CIS) calculations. With TDDFT calculations employing the SAOP functional, minor peaks for Ag(4)(+2) and Ag(8) dimers appear as the interparticle distance decreases; these peaks are suggested to be charge transfer artifacts on the basis of CIS and TDDFT (CAM-B3LYP) calculations. The relationship of the absorption peak locations to the distance and orientation between T(d) Ag(20) dimers is also investigated. TDDFT calculations using the SAOP functional are used to determine excitation absorption spectra for eight different orientations of Ag(20) dimers. Although the Ag(20)T(d) monomer has a sharp peak, each dimer absorption spectrum is split due to lower symmetry. This splitting increases as the center of mass distance decreases. As the interparticle distance between the monomers decreases, the initially strong peaks decrease in intensity and red or blue shift depending on symmetry, while the minor peaks increase in intensity and red shift.

Primary Reactions of the LOV2 Domain of Phototropin Studied with Ultrafast Mid-Infrared Spectroscopy and Quantum Chemistry

Phototropins, major blue-light receptors in plants, are sensitive to blue light through a pair of flavin mononucleotide (FMN)-binding light oxygen and voltage (LOV) domains, LOV1 and LOV2. LOV2 undergoes a photocycle involving light-driven covalent adduct formation between a conserved cysteine and the FMN C(4a) atom. Here, the primary reactions of Avena sativa phototropin 1 LOV2 (AsLOV2) were studied using ultrafast mid-infrared spectroscopy and quantum chemistry. The singlet excited state (S1) evolves into the triplet state (T1) with a lifetime of 1.5 ns at a yield of ∼50%. The infrared signature of S1 is characterized by absorption bands at 1657 cm−1, 1495–1415 cm−1, and 1375 cm−1. The T1 state shows infrared bands at 1657 cm−1, 1645 cm−1, 1491–1438 cm−1, and 1390 cm−1. For both electronic states, these bands are assigned principally to C=O, C=N, C-C, and C-N stretch modes. The overall downshifting of C=O and C=N bond stretch modes is consistent with an overall bond-order decrease of the conjugated isoalloxazine system upon a π-π∗ transition. The configuration interaction singles (CIS) method was used to calculate the vibrational spectra of the S1 and T1 excited ππ∗ states, as well as respective electronic energies, structural parameters, electronic dipole moments, and intrinsic force constants. The harmonic frequencies of S1 and T1, as calculated by the CIS method, are in satisfactory agreement with the evident band positions and intensities. On the other hand, CIS calculations of a T1 cation that was protonated at the N(5) site did not reproduce the experimental FMN T1 spectrum. We conclude that the FMN T1 state remains nonprotonated on a nanosecond timescale, which rules out an ionic mechanism for covalent adduct formation involving cysteine-N(5) proton transfer on this timescale. Finally, we observed a heterogeneous population of singly and doubly H-bonded FMN C(4)=O conformers in the dark state, with stretch frequencies at 1714 cm−1 and 1694 cm−1, respectively.

Density-matrix calculation of surface-enhanced Raman scattering for p-mercaptoaniline on silver nanoshells

A theoretical analysis of recent experiments measuring the 782 nm surface-enhanced Raman scattering of p-mercaptoaniline bound to silver nanoshells of different core and shell radii [J. B. Jackson and N. J. Halas, Proc. Natl. Acad. Sci. U.S.A. 101, 17930 (2004)] is performed. Electronic structure Hartree-Fock and density-functional theory calculations for Ag salts of p-mercaptoaniline are used to characterize observed vibrational modes and configuration-interaction singles calculations are carried out to examine excited states. Multimode vibronic density-matrix calculations are then made including one excited electronic state, using a classical description of the strong local fields and a phenomenological treatment of relaxations. The spectral behavior as a function of both nanoshell surface-plasmon resonance position and molecular electronic spacing is examined.

Rydberg character of the higher excited states of free-base porphin

The Rydberg character of the excited states of free-base porphin (FBP) has been investigated by the ab initio configuration interaction singles (CIS) method and the state-averaged complete-active-space self-consistent-field method. Double-zeta basis sets augmented with s, p, and d Rydberg functions and d polarization functions have been employed. Two types of molecular orbitals sets, the restricted Hartree–Fock molecular orbitals obtained for the ground state (1Ag) and for the cation state (2Au), have been used in the CIS calculations. All the calculations show that Rydberg-type excitations play important roles especially in the N bands. In this article we propose applying the model of a perturbed Rydberg series to interpret the excited states of FBP. By using this model, we have succeeded in analyzing the characteristics of the excited states as well as the experimental oscillator strengths, which have considerable magnitude even in the higher excited states.

Single configuration interaction study on conjugated betainic chromophores based on DFT optimized geometries

The structure of a series of heterocyclic betaines was calculated by methods of density functional theory (DFT). The charge distribution and bond characteristics of these compounds were analyzed by Weinhold’s natural bond orbital analysis (NBO) and by natural resonance theory (NRT). In order to probe the aromatic character of the ring fragments, Schleyer’s nucleus-independent chemical shifts (NICSs) were calculated by GIAO-RHF. Ab initio single configuration interaction calculations (SCI) correctly predict intense π→π* transitions at low energies, but the transition energies of the color bands are overestimated. Torsion around the interfragmental bond increases the charge separation between the molecular fragments and the dipole moment. The molecular fragments become increasingly aromatic. The absorption wavelengths increase on torsion while the oscillator strengths decrease.

A CIS study of the solvent effects on the electronic absorption spectra of push-pull ethylenes

The lowest energy electronic transitions of two push-pull compounds, 1,1-bisdimethylamino-2-cyano-2-para-fluorophenylethene (PP-F) and 1,1-bisdimethylamino-2-cyano-2-para-nitrophenylethene (PP-NO 2) were investigated as a function of solvent polarity using experimental and quantum mechanical techniques. Ab initio calculations at the unrestricted Hartree-Fock (UHF) level were employed for structure optimization with the solvent effects simulated via a self-consistent reaction field (SCRF) using the Onsager model. The configuration interaction singles (CIS) method was used to calculate the transition energies using various solute-solvent interaction models. Experimentally, while the principal absorption for both compounds exhibited a red shift with increasing solvent polarity, PP-NO 2 demonstrated a more pronounced shift. The predicted shifts from the CIS calculations were in good agreement with the experimental data. The different behavior for the two systems with solvent was attributed to qualitative differences in the nature of their electronic transitions. While the transition of PP-F was attributed to a π (CC) to π ∗(CC) transition, that of PP-NO 2 was identified as an intramolecular charge transfer transition from π(CC) to π ∗(-para-Ph-NO 2).

The electronic spectrum of phthalazine. Theory and experiment

Abstract Ab initio configuration interaction-singles (CIS) calculations of the electronic spectra of phthalazine and pyridazine predict two neighbouring singlet ( π ∗ , n) states: 1A2 followed by 1B1 for phthalazine, and the reverse for pyridazine. A comparison of the 3–21G and 6–31G∗ basis sets for pyridazine shows that the 3–21G basis set gives satisfactory results, and may be used in CIS calculations for the larger molecule, phthalazine. The lowest-energy π ∗ ← n transition for phthalazine is predicted to be forbidden, in agreement with experiment, but in conflict with most semiempirical calculations. Ab initio calculations are used to predict the vibrational frequencies of phthalazine and phthalazine-d4 at the HF/6–31G∗ and CIS/3–21G levels for the ground and excited electronic states, respectively. The ground-state calculations for phthalazine are in good agreement with experiment, including the results of several SERS studies. The vapour absorption spectrum of phthalazine is reported for the first time and analysed using ab initio, semiempirical and rotational-contour calculations. The first 6000 cm−1 of the spectrum is assigned to a forbidden π ∗ ← n transition, confirming the findings of crystal-state studies and the CIS/3–21G predictions. The higher-energy regions of this band system show no clear evidence of a second π ∗ ← n transition, despite theoretical predictions and circular-dichroism observations to the contrary.

Ground state properties and optical response of LixNa4−x, x=0–4: An a b i n i t i o study

The ground state properties of LixNa4−x (x=0–4) were investigated using extended basis Hartree–Fock calculations. The existence of substitution isomers was found for the mixed alkali clusters and planar singlet ground states were found for all clusters, including the isomers. The optical response of the clusters was modeled via configuration interaction—singles (CIS) in the hope of helping to interpret the experimental data. CIS results were found comparable to experiment at lower excitation energies, but inadequate at higher energies due to the neglect of doubles and higher excitations. Some exploration of the excited state surfaces of Li4 and of one isomer of Li2Na2 is also reported. Comparisons are made between CIS calculations, experiment, and multireference doubly excited configuration interaction calculations.