Ab Initio Multiconfiguration Research Articles

ChemInform Abstract: Group VI Trimers (Se3, Te3, and Po3). Electronic States and Potential Energy Surfaces

The potential energy surfaces of several electronic states of Se3, Te3, and Po3 are computed. We employed the ab initio complete active space multiconfiguration self‐consistent field followed by multireference configuration interaction techniques which included up to 1.7 million configurations. Our computations reveal two nearly degenerate electronic states with D3h(1A’1) and C2v(1A1) symmetries as candidates for the ground states, although the C2v minimum was found to be favored at the highest level of theory. Our computations facilitate the assignment of the observed spectra of Se3 and Te3 in the visible region by Andrews and co‐workers to the 1B2– X1A1 systems. Our computed Te values were found to be in excellent agreement with experiment. We computed the dipole moments, dissociation and atomization energies for all three trimers and the vertical ionization energies for the two minima of Se3. The analysis of the periodic trends revealed significant relativistic effects for Po3.

Ab initio calculations of 14N and 15N hyperfine structures

Hyperfine structure parameters are calculated for the 2p2(3P)3s 4PJ, 2p2(3P)3p 4PoJ and 2p2(3P)3p 4DoJ levels, using the ab initio multiconfiguration Hartree–Fock method. The theoretical hyperfine coupling constants are in complete disagreement with the experimental values of Jennerich et al deduced from the analysis of the near-infrared Doppler-free saturated absorption spectra.

A novel biomimetic photochemical switch at work: design of a photomodulable peptide

We report the outcomes of our recent computational and experimental work for the development of a novel biomimetic molecular switch. Furthermore, we present the new results on the design and computational characterization of a "functional" cyclic peptidomimetic formed by the switch conjugated to a biologically active peptide: the RGD sequence involved in the control of cell adhesion. Structural properties of the construct are investigated in aqueous solution using molecular dynamics (MD) simulations. Analysis of MD trajectories reveals that, for each diastereoisomer of the switch (E or Z), different conformations are stabilized. Electrostatic and spectroscopic properties of such conformers are evaluated by means of ab initio multiconfiguration quantum chemical method implemented in a quantum-mechanical/molecular-mechanic (CASPT2//CASSCF/6-31G*/AMBER) scheme.

Density Functional and ab Initio Investigation of CF2ICF2I and CF2CF2I Radicals in Gas and Solution Phases

Quantum chemical calculations of CF(2)ICF(2)I and (*)CF(2)CF(2)I, model systems in reaction dynamics, in the gas phase and methanol solvent are performed using the density functional theory (DFT) and multiconfigurational ab initio methods. Molecular geometries, vibrational frequencies, and vertical excitation energies (T(v)) are computed and compared with available experimental results. We also evaluate the performance of four hybrid and one hybrid meta DFT functionals. The T(v) values calculated using time-dependent DFT vary depending on the exchange-correlation functionals, with the degree of variation approaching approximately 0.7 eV. The M05-2X functional well predicts molecular geometries and T(v) values, while it overestimates the vibrational frequencies. The T(v) values calculated using the M05-2X are similar to those calculated by the CASPT2. All low-lying excited states in CF(2)ICF(2)I are characterized by the excitation from the nonbonding to antibonding orbital of C-I. The excited states of (*)CF(2)CF(2)I are different in their character from those of CF(2)ICF(2)I and have considerable double excitation characters. The spin-orbit coupling of (*)CF(2)CF(2)I is larger than that of CF(2)ICF(2)I.

Theoretical Description of the Structure and Magnetic Properties of Nitroxide−Cu(II)−Nitroxide Spin Triads by Means of Multiconfigurational Ab Initio Calculations

The structural, electronic and magnetic properties of two different models of the heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides Cu(hfac)2L(R) have been studied by means of multiconfigurational perturbation theory based on a CASSCF (complete active space self-consistent field) wave function, i.e. the CASPT2 method. Our calculations reveal the presence of two minima in the electronic energy curve along the Cu-O(L) bond, separated by only 6 kcal/mol, and corresponding to the X-ray structures of the CuO6 centers in Cu(hfac)2L(Pr) at 115 and 293 K, respectively. The two energetic minima are characterized by a different electronic structure, thus giving rise to a different three-spin exchange coupling and explaining the thermally induced spin transitions in this family of compounds. The concomitant variations in the magnetic properties, i.e. g factors and magnetic moments mu(eff)(T) were calculated and compared with the experimental data of Cu(hfac)2L(Pr). Even if the correspondence is only qualitative, our calculations provide a convincing explanation of the observed magnetic peculiarities. In particular, at low temperatures, the predicted ground-state is 2A(u), well separated from the 2A(g), 4A(u) states and therefore exclusively populated. Its calculated g factors, g(parallel) = 1.848, g(perpendicular) = 1.965, 1.974, qualitatively correspond to the observed g < 2 signals in the low-temperature EPR spectra. The previously assumed formal spin assignment >N-O*-Cu-*O-N < for these linear spin triads is challenged by our calculations, pointing instead to a more important role of the end-standing NO in the exchange interactions with Cu(II).

A mystery solved? Photoelectron spectroscopic and quantum chemical studies of the ion states of CeCp3+

The electronic states of CeCp(3)(+) have been studied experimentally by variable photon energy photoelectron spectroscopy, and computationally using multi-configurational ab initio methods. Relative partial photoionisation cross section and branching ratio data are presented to confirm our previous conclusion that bands A and D in the valence photoelectron spectrum, despite their 3.2 eV separation, are produced by ionization of the single 4f electron of CeCp(3) [M. Coreno, M. de Simone, J. C. Green, N. Kaltsoyannis, N. Narband and A. Sella, Chem. Phys. Lett., 432, 2006, 17]. The origin of this effect is probed using the CASSCF/CASPT2 approach. While configurations based on the canonical CASSCF orbitals are found to be an unreliable description of the ground and excited states of CeCp(3)(+), the state-specific natural orbitals and their occupations yield greater insight, allowing us to characterize ion states in terms of the presence or otherwise of a Ce 4f-localised electron. Neither the CeCp(3)(+) ground state (assigned to band A), and two excited states ((1)A' and (1)A'', associated with band D), possess such a metal-based electron, as expected of f ionization. The (1)A' and (1)A'' states differ from the ground state in having a significant Ce 5d population, arising from Cp --> Ce charge transfer, which accompanies f ionization, and which is responsible for the energetic separation of bands A and D in the valence photoelectron spectrum.

Photostability via Sloped Conical Intersections: A Computational Study of the Pyrene Radical Cation

The photophysics of the pyrene radical cation, a polycyclic aromatic hydrocarbon (PAH) and a possible source of diffuse interstellar bands (DIBs), is investigated by means of hybrid molecular mechanics-valence bond (MMVB) force field and multiconfigurational CASSCF and CASPT2 ab initio methods. Potential energy surfaces of the first three electronic states D 0, D 1, and D 2 are calculated. MMVB geometry optimizations are carried out for the first time on a cationic system; the results show good agreement with CASSCF optimized structures, for minima and conical intersections, and errors in the energy gaps are no larger than those found in our previous studies of neutral systems. The presence of two easily accessible sloped D 1/D 2 and D 0/D 1 conical intersections suggests the pyrene radical cation is highly photostable, with ultrafast nonradiative decay back to the initial ground state geometry predicted via a mechanism similar to the one found in the naphthalene radical cation.

Mapping the d−d Excited-State Manifolds of Transition Metal β-Diiminato−Imido Complexes. Comparison of Density Functional Theory and CASPT2 Energetics

Trigonal-planar, middle transition metal diiminato-imido complexes do not exhibit high-spin states, as might be naively expected on the basis of their low coordination numbers. Instead, the known Fe(III), Co(III), and Ni(III) complexes exhibit S = 3/2, S = 0, and S = 1/2 ground states, respectively. Kohn-Sham DFT calculations have provided a basic molecular orbital picture of these compounds as well as a qualitative rationale for the observed spin states. Reported herein are ab initio multiconfiguration second-order perturbation theory (CASPT2) calculations, which provide a relatively detailed picture of the d-d excited-state manifolds of these complexes. Thus, for a C(2v) Fe(III)(diiminato)(NPh) model complex, two near-degenerate states ((4)B(2) and (4)B(1)) compete as contenders for the ground state. Moreover, the high-spin sextet, two additional quartets and even a low-spin doublet all occur at <0.5 eV, relative to the ground state. For the Co(III) system, although CASPT2 reproduces an S = 0 ground state, as observed experimentally for a related complex, the calculations also predict two exceedingly low-energy triplet states; there are, however, no other particularly low-energy d-d excited states. In contrast to the Fe(III) and Co(III) cases, the Ni(III) complex has a clearly nondegenerate (2)B(2) ground state. The CASPT2 energetics provide benchmarks against which we can evaluate the performance of several common DFT methods. Although none of the functionals examined perform entirely satisfactorily, the B3LYP hybrid functional provides the best overall spin-state energetics.

Chromophore Protonation State Controls Photoswitching of the Fluoroprotein asFP595

Fluorescent proteins have been widely used as genetically encodable fusion tags for biological imaging. Recently, a new class of fluorescent proteins was discovered that can be reversibly light-switched between a fluorescent and a non-fluorescent state. Such proteins can not only provide nanoscale resolution in far-field fluorescence optical microscopy much below the diffraction limit, but also hold promise for other nanotechnological applications, such as optical data storage. To systematically exploit the potential of such photoswitchable proteins and to enable rational improvements to their properties requires a detailed understanding of the molecular switching mechanism, which is currently unknown. Here, we have studied the photoswitching mechanism of the reversibly switchable fluoroprotein asFP595 at the atomic level by multiconfigurational ab initio (CASSCF) calculations and QM/MM excited state molecular dynamics simulations with explicit surface hopping. Our simulations explain measured quantum yields and excited state lifetimes, and also predict the structures of the hitherto unknown intermediates and of the irreversibly fluorescent state. Further, we find that the proton distribution in the active site of the asFP595 controls the photochemical conversion pathways of the chromophore in the protein matrix. Accordingly, changes in the protonation state of the chromophore and some proximal amino acids lead to different photochemical states, which all turn out to be essential for the photoswitching mechanism. These photochemical states are (i) a neutral chromophore, which can trans-cis photoisomerize, (ii) an anionic chromophore, which rapidly undergoes radiationless decay after excitation, and (iii) a putative fluorescent zwitterionic chromophore. The overall stability of the different protonation states is controlled by the isomeric state of the chromophore. We finally propose that radiation-induced decarboxylation of the glutamic acid Glu215 blocks the proton transfer pathways that enable the deactivation of the zwitterionic chromophore and thus leads to irreversible fluorescence. We have identified the tight coupling of trans-cis isomerization and proton transfers in photoswitchable proteins to be essential for their function and propose a detailed underlying mechanism, which provides a comprehensive picture that explains the available experimental data. The structural similarity between asFP595 and other fluoroproteins of interest for imaging suggests that this coupling is a quite general mechanism for photoswitchable proteins. These insights can guide the rational design and optimization of photoswitchable proteins.

First-principles calculations of magnetic circular dichroism spectra

An elaborate approach for the prediction of magnetic circular dichroism (MCD) spectra in the framework of highly correlated multiconfigurational ab initio methods is presented. The MCD transitions are computed by the explicit treatment of spin-orbit coupled (SOC) and spin-spin coupled (SSC) N-electron states. These states are obtained from the diagonalization of the SOC and SSC operators along with the spin and orbital Zeeman operators in the basis of a preselected number of roots of the spin-free Hamiltonian. Therefore, zero-field splittings due to the SOC and SSC interactions along with the magnetic field splittings are explicitly accounted for in the ground as well as the excited states. This makes it possible to calculate simultaneously all MCD A, B, and C terms even beyond the linear response limit. The SOC is computed using a multicenter mean-field approximation to the Breit-Pauli Hamiltonian. Two-electron SSC terms are included in the treatment without further approximations. The MCD transition intensities are subjected to numerical orientational averaging in order to treat the most commonly encountered case of randomly oriented molecules. The simulated MCD spectra for the OH, NH, and CH radicals as well as for [Fe(CN)(6)](3-) are in good agreement with the experimental spectra. In the former case, the significant effects of the inert gas matrices in which the experimental spectra were obtained were modeled in a phenomenological way.

Quasiparticle Bands in Cuprates by Quantum-Chemical Methods: Towards anAb InitioDescription of Strong Electron Correlations

Realistic electronic-structure calculations for correlated Mott insulators are notoriously difficult. Here we present an ab initio multiconfiguration scheme that adequately describes strong correlation effects involving Cu 3d and O 2p electrons in layered cuprates. In particular, the O 2p states giving rise to the Zhang-Rice band are explicitly considered. Renormalization effects due to nonlocal spin interactions are also treated consistently. We show that the dispersion of the lowest band observed in photoemission is reproduced with quantitative accuracy. Additionally, the evolution of the Fermi surface with doping follows directly from our ab initio data. Our results thus open a new avenue for the first-principles investigation of the electronic structure of correlated Mott insulators.

Nonlocal interactions in doped cuprates: Correlated motion of Zhang-Rice polarons

Intercarrier correlations in hole doped cuprates are investigated by ab initio multiconfiguration calculations. The dressed carriers display features that are reminiscent of both Zhang-Rice (ZR) $\mathrm{Cu}{\mathrm{O}}_{4}$ states and Jahn-Teller polarons. The interaction between these quasiparticles is repulsive. At dopings that are high enough, the interplay between long-range unscreened Coulomb interactions and long-range phase coherence among the O-ion half-breathing vibrations on the ZR plaquettes may lead to a strong reduction of the effective adiabatic energy barrier associated to each polaronic state. Tunneling effects cannot be neglected for a relatively flat, multiwell energy landscape. We suggest that the coherent, superconducting quantum state is the result of such coherent quantum lattice fluctuations involving in-plane O ions. Our findings appear to support phenomenological models where the superconductivity is related to a lowering of the in-plane kinetic energy.

Ab initio study of the optical transitions on low-coordinated sites of an intermediate F center: The F s+(OH) − center on MgO(100) surface

The optical transition energies of a point defect in MgO(100), consisting of an oxygen vacancy with one trapped electron and an OH − group adsorbed nearby the surface oxygen vacancy (the F s +(OH −) center), have been studied by means of ab initio multiconfiguration second order perturbation theory (CASPT2) and Density Functional (DFT) calculations on embedded cluster models. We find that a method that takes explicitly into account electronic correlation effects is required to properly predict the first transition energies. The lowest transition energy of the F s +(OH −) center on a terrace site is very similar to the one reported in previous studies of F + centers on a regular MgO(100) surface (F s + center). Experiments have difficulty distinguishing between these two kinds of point defects based solely on the optical spectra. We have also considered the situation where the vacancy is located at low-coordinated sites like steps and corners. The results show that the loss of coordination decreases the transition energies in agreement with what has been reported in previous studies for “classical” F s + centers.

Electron correlations and bond-length fluctuations in copper oxide superconductors: Electron versus hole doping

We investigate the nature of the electronic ground state and electron-lattice couplings for doped chains of $\mathrm{Cu}{\mathrm{O}}_{4}$ plaquettes or $\mathrm{Cu}{\mathrm{O}}_{6}$ octahedra. The undoped configuration implies here Cu $3{d}^{9}$ and O $2{p}^{6}$ formal valence states. The results of multiconfiguration calculations on 4-plaquette (or 4-octahedra) linear clusters indicate strong electron-lattice interactions and polaronic behavior of the doped particles, for both electron and hole doping. For certain phases of the oxygen-ion half-breathing distortions a multiwell energy landscape is predicted. Since each well is associated to carriers localized at different sites, the half-breathing displacements induce charge transfer along the chain. In the case of hole doping, the trends found by ab initio multiconfiguration calculations on 4-octahedra clusters are confirmed by density-matrix renormalization-group calculations for a $p\text{\ensuremath{-}}d$, extended Hubbard model with chains of few tens of $\mathrm{Cu}{\mathrm{O}}_{4}$ plaquettes. Under the assumption of charge separation and the formation of $1∕3$-doped stripes, our results seem to support the traveling charge-density wave scenario proposed in some recent contributions for superconductivity in copper oxides.

One-Dimensional Zigzag Chains of Cs-: The Structures and Properties of Li+(Cryptand[2.1.1])Cs- and Cs+(Cryptand[2.2.2])Cs-

The crystal structure and properties of lithium (cryptand[2.1.1]) ceside, Li+ (C211)Cs-, are reported. Li+ (C211)Cs- is the second ceside and third alkalide with a one-dimensional (1D) zigzag chain of alkali metal anions. The distance between adjacent Cs- anions, 6 A, is shorter than the sum of the van der Waals radii, 7 A. Optical, magic angle spinning NMR, two-probe alternating and direct current conductivity, and electron paramagnetic resonance measurements reveal unique physical properties that result from the overlap of adjacent Cs- wave functions in the chain structure. The properties of cesium (cryptand[2.2.2]) ceside, Cs+ (C222)Cs-, were also studied to compare the effects of the subtle geometric changes between the two 1D zigzag chain structures. Li+ (C211)Cs- and Cs+ (C222)Cs- are both low-band-gap semiconductors with anisotropic reflectivities and large paramagnetic 133Cs NMR chemical shifts relative to Cs- (g). An electronic structure model consistent with the experimental data has sp2-hybridized Cs- within the chain and sp-hybridized chain ends. Ab initio multiconfiguration self-consistent field calculations on the ceside trimer, Cs3(3-), support this model and indicate a net bonding interaction between nearest neighbors. The buildup of electron density between adjacent Cs- anions is visualized through an electron density difference map constructed by subtracting the density of three cesium atoms from the short Cs3(3-) fragment.

Direct observation of the ^{2}D_{3∕2}-^{2}D_{5∕2} ground-state splitting in Xe^{9+}

We have used an electron beam ion trap to observe a visible line at 598.30(13) nm that corresponds to the 4d9 2D3/2–4d9 2D5/2 magnetic dipole transition within the ground state configuration of Xe9+. We have found no evidence to support the claim by others that a line near this position originates from Xe31+. The comparison of the measured wavelength with previous indirect experimental values, and the dependence of the line intensity on the electron beam energy, confirms the validity of the present identification. Our measured wavelength is in agreement with semiempirical Hartree-Fock calculations but shows a discrepancy with ab initio multiconfiguration Dirac-Fock calculations. We propose that the line may be useful as a diagnostic for extreme ultraviolet lithography light sources.

Metal–metal bonding in molecular actinide compounds: electronic structure of [M2X8]2−(M = U, Np, Pu; X = Cl, Br, I) complexes and comparison with d-block analogues

Density functional and multiconfigurational (ab initio) calculations have been performed on [M(2)X(8)](2-) (X = Cl, Br, I) complexes of 4d (Mo, Tc, Ru), 5d (W, Re, Os), and 5f (U, Np, Pu) metals in order to investigate general trends, similarities and differences in the electronic structure and metal-metal bonding between f-block and d-block elements. Multiple metal-metal bonds consisting of a combination of sigma and pi interactions have been found in all species investigated, with delta-like interactions also occurring in the complexes of Tc, Re, Np, Ru, Os, and Pu. The molecular orbital analysis indicates that these metal-metal interactions possess predominantly d(z2) (sigma), d(xz) and d(yz) (pi), or d(xy) and d(x2-y2) (delta) character in the d-block species, and f(z3) (sigma), f(z2x) and f(z2y) (pi), or f(xyz) and f(z) (delta) character in the actinide systems. In the latter, all three (sigma, pi, delta) types of interaction exhibit bonding character, irrespective of whether the molecular symmetry is D(4h) or D(4d). By contrast, although the nature and properties of the sigma and pi bonds are largely similar for the D(4h) and D(4d) forms of the d-block complexes, the two most relevant metal-metal delta-like orbitals occur as a bonding and antibonding combination in D(4h) symmetry but as a nonbonding level in D(4d) symmetry. Multiconfigurational calculations have been performed on a subset of the actinide complexes, and show that a single electronic configuration plays a dominant role and corresponds to the lowest-energy configuration obtained using density functional theory.

Electronic structure of 1,3-dinitrobenzene radical anion: A multiconfigurational quantum chemical study

The structure of 1,3-dinitrobenzene radical anion in the doublet ground and lowest excited states was studied by ab initio multiconfiguration CASSCF methods. The results of calculations suggest the existence of one symmetrical and two asymmetrical structures of the radical anion. The energies of these structures were estimated.

Oscillator strength calculations in neutral technetium

Ab initio multiconfiguration calculations are performed for the oscillator strengths of the A = 4238, 4262 and 4297 A Tc i resonance lines of astrophysical interest. Electron correlation is treated through multiconfiguration expansions built from elaborate correlation models, while relativistic effects are introduced in the perturbation Breit-Pauli approximation or in the multiconfiguration Dirac-Fock-Breit variational approach. The calculated gf-values are sensitively lower (30 per cent) than the values obtained with the pseudo-relativistic Hartree-Fock wavefunctions calculated from a parametric analysis of Tc I by Palmeri & Wyart. The inclusion of a core-polarization potential in the latter approach confirms the present ab initio results when different ionic cores are used for the different transition arrays. The strong lines of Tc I are revisited adopting this model, giving rise to a systematic reduction in the oscillator strength scale due to core polarization. The astrophysical implications are discussed.

Complementary investigations using the MCDF and RQDO methods

AbstractTheoretical oscillator strengths for fine‐structure lines of electric dipole (E1) allowed transitions for a number of ions belonging to the Be, As, and Ge sequences, are reported. Both ab initio (multiconfiguration Dirac–Fock) and model potential (relativistic quantum defect orbital) techniques have been used. A comparative analysis of the two sets of f‐values, as well as with other data found in the literature, has been carried out. For most of the studied ions and transitions, the results achieved with the two methodologies are, overall, in rather good agreement. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005