Vibronic Coupling Parameters Research Articles

On the ground state T g ⊗ εg (T g = 2T 2g, 3T 1g) Jahn–Teller-coupling in hexacyano complexes of 3d-transition metals

The linear and quadratic T g ⊗ ε g Jahn–Teller effect in the T g (T g = 2T 2g, 3T 1g) ground states of low-spin octahedral cyano complexes of 3d-transition metals (M = Ti III, V III, Mn III, Fe III, Cr II, Mn II) has been studied. Vibronic coupling parameters have been derived using density functional theory to calculate energies of Slater determinants, which result from various electron distributions within the t 2 g n configuration due to the metal based t 2g(3d) molecular orbitals ( n = 1, 2, 3, 4). Tetragonal elongations are found in the case of Ti III, Mn III and Cr II, and compressions for V III, Fe III and Mn II, and these establish the metal–ligand π-back donation as the dominant effect and driving force for the geometry distortions and energy stabilizations towards non-degenerate 2 B 2 g ( t 2 g 1 , t 2 g 5 ) and 3 A 2 g ( t 2 g 2 , t 2 g 4 ) ground states. The strength of the Jahn–Teller coupling is very weak and found not to follow a monotonic trend across the transition metal series but is shown to be weakest for Ti III, V III and Fe III, slightly larger for Mn III, increases further to Mn II and is found to be the strongest but is still weak for Cr II.

The bending vibrational levels of the acetylene cation: A case study of the Renner-Teller effect in a molecule with two degenerate bending vibrations

Forty three vibronic levels of C2H2+, X 2Pi u, with upsilon4 = 0-6, upsilon5 = 0-3, and K = 0-4, lying at energies of 0-3520 cm(-1) above the zero-point level, have been recorded at rotational resolution. These levels were observed by double resonance, using 1+1' two-color pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. The intermediate states were single rovibrational levels chosen from the A1Au, 4nu3 (K = 1-2), 5nu3 (K = 1), nu2+4nu3 (K = 0), and 47,206 cm(-1) (K = 1) levels of C2H2. Seven of the trans-bending levels of C2H2+ (upsilon4 = 0-3, K = 0-2) had been reported previously by Pratt et al. [J. Chem. Phys. 99, 6233 (1993)]; our results for these levels agree well with theirs. A full analysis has been carried out, including the Renner-Teller effect and the vibrational anharmonicity for both the trans- and cis-bending vibrations. The rotational structure of the lowest 16 vibronic levels (consisting of the complete set of levels with upsilon4 + upsilon5 < or = 2, except for the unobserved upper (2Pi u component of the 2nu4 overtone) could be fitted by least squares using 16 parameters to give an rms deviation of 0.21 cm(-1). The vibronic coupling parameter epsilon5 (about whose magnitude there has been controversy) was determined to be -0.0273(7). For the higher vibronic levels, an additional parameter, r45, was needed to allow for the Darling-Dennison resonance between the two bending manifolds. Almost all the observed levels of the upsilon4 + upsilon5 = 3 and 4 polyads (about half of the predicted number) could then be assigned. In a final fit to 39 vibronic levels with upsilon4 + upsilon5 < or = 5, an rms deviation of 0.34 cm(-1) was obtained using 20 parameters. An interesting finding is that Hund's spin-coupling cases (a) and (b) both occur in the Sigmau components of the nu4 + 2nu5 combination level. The ionization potential of C2H2 (from the lowest rotational level of the ground state to the lowest rotational level of the cation) is found to be 91,953.77 +/- 0.09 cm(-1) (3sigma).

Theoretical treatment of pseudorotation in the Jahn-TellerC60+ion

A theoretical investigation of the pseudorotational dynamics in the ${\mathrm{C}}_{60}^{+}$ cation is made using a single-mode $H\ensuremath{\bigotimes}h$ model. Analytical expressions for the rate of pseudorotation are formulated as a function of the vibronic coupling parameters and used to probe the dynamics of the Jahn-Teller effect. For particular values of the coupling constants, it is known that the ground state of the system may change from the expected $H$ symmetry to one of $A$ symmetry. Therefore, we examine the dynamics in the coupling regime where this ground-state $H$-$A$ crossover occurs. Using coupling constants taken from the literature, a pseudorotational period of $\ensuremath{\sim}210\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ is estimated for this ion. Thus there is the potential that pseudorotation could be observed in ultrafast optical experiments.

Theoretical calculation of pseudorotation rates applicable to theC60−ion

The dynamical nature of the Jahn-Teller effect in the $T\ensuremath{\bigotimes}h$ Jahn-Teller system is explored theoretically using the time-evolution operator to obtain analytical expressions for the rate of pseudorotation between equivalent potential wells in the adiabatic potential energy surface as a function of the vibronic coupling parameters. We outline experiments that should be capable of measuring the pseudorotation rate in the ${\mathrm{C}}_{60}^{\ensuremath{-}}$ ion, and other systems derived from ${\mathrm{C}}_{60}$. The theory presented here could then be combined with such data to probe the nature of the vibronic coupling in these fullerene systems.

Functionally Relevant Electric-Field Induced Perturbations of the Prosthetic Group of Yeast Ferrocytochrome c Mutants Obtained from a Vibronic Analysis of Low-Temperature Absorption Spectra

We have measured the low temperature (T = 20 K) absorption spectra of the N52A, N52V, N52I, Y67F, and N52AY67F mutants of ferrous Saccharomyces cerevisiae (baker's yeast) cytochrome c. All the bands in the Q0- and Q(v)-band region are split, and the intensity distributions among the split bands are highly asymmetric. The spectra were analyzed by a decomposition into Voigtian profiles. The spectral parameters thus obtained were further analyzed in terms of the vibronic coupling model of Schweitzer-Stenner and Bigman (Schweitzer-Stenner, R.; Bigman, D. J. Phys. Chem. B 2001, 7064-7073) to identify parameters related to electronic and vibronic perturbations of the heme macrocycle. We report that the electronic perturbation is of B(1g) symmetry and reflects the heterogeneity of the electric field at the heme, that is, the difference between the gradients along the perpendicular N-Fe-N axis of the heme core. We found that all the investigated mutations substantially increase this electronic perturbation, so that the spectral properties become similar to those of horse heart cytochrome c. Moreover, the electronic perturbation was found to correlate nonlinearly with the enthalpy changes associated with the reduction of the heme iron. Group theoretical arguments are invoked to propose a simple model which explains how a perturbation of the obtained symmetry can stabilize the reduced state of the heme iron. Finally, vibronic coupling parameters obtained from the analysis of the Q(v)-band region suggest that the investigated mutations decrease the nonplanar deformations of the heme group. This finding was reproduced by a normal mode structural decomposition (NSD) analysis of the N52V and N52VY67F heme conformations obtained from a 1 ns molecular dynamics simulation. We argue that the reduced nonplanarity contributes to the stabilization of the reduced state.

Vibronic coupling in the icosahedralC602+Jahn-Teller cation: Repercussions of the nonsimple reducibility of theH⊗Hproduct

Coulomb coupling between the two holes in the fullerene cation ${\mathrm{C}}_{60}^{2+}$ results in high- and low-spin terms. Vibronic coupling of the high-spin terms to a single mode of vibration of ${h}_{g}$ symmetry can distort the lowest adiabatic potential-energy surface into wells of ${D}_{3d}$, ${D}_{5d}$, ${D}_{2h}$, or ${C}_{2h}$ symmetry depending upon the precise nature of the coupling produced by different admixtures of the two sets of coupling coefficients arising from the two $H$-type irreducible representations appearing in the symmetric part of the $H\ensuremath{\bigotimes}H$ Kronecker product. The symmetry of dynamic distortions of ${\mathrm{C}}_{60}^{2+}$ ions when coupling to all eight of the possible ${h}_{g}$ modes is taken into account depends very closely on both the vibronic coupling parameters and the splittings between different Coulomb terms. Using estimates for these parameters from the literature leads us to conclude that the distortional symmetry is most likely to be ${D}_{2h}$, but there is also the possibility that it could be ${D}_{3d}$.

Theoretical Study of the Electronic Nonadiabatic Transitions in the Photoelectron Spectroscopy of F2O

The photoelectron spectrum of F2O pertaining to ionizations to the ground (X2B1) and low-lying excited electronic states (A2B2, B2A1, and C2A2) of F2O+ is investigated theoretically. The near equilibrium potential energy surfaces of the ground electronic state (X2B1) of F2O and the mentioned ground and excited electronic states of F2O+ reported by Wang et al. ( J. Chem. Phys. 2001, 114, 10682) for the C2v configuration are extended for the Cs geometry assuming a harmonic vibration along the asymmetric stretching mode. The vibronic interactions between the A2B2 and B2A1 electronic states of F2O+ are treated within a linear coupling approach, and the strength of the vibronic coupling parameter is calculated by an ab initio method. The nuclear dynamics is simulated by both time-independent quantum mechanical and time-dependent wave packet approaches. Although the first photoelectron band exhibits resolved vibrational progression along the symmetric stretching mode, the second one is highly overlapping. The latter is attributed to the nonadiabatic interactions among the energetically close A2B2, B2A1, and C2A2 electronic states of F2O+. The theoretical findings are in good accord with the available experimental results.

Nonplanar Heme Deformations and Excited State Displacements in Nickel Porphyrins Detected by Raman Spectroscopy at Soret Excitation

We have correlated the Raman intensities of out-of-plane modes of nickel porphyrins with the nonplanar deformations of specific symmetries, i.e., static normal coordinate deformations (SNCDs) expressed in terms of irreducible representations of the unperturbed D(4h) point group. The model porphyrins Ni(II) octaethyltetraphenylporphyrin (NiOETPP), Ni(II) tetra(isopropyl)porphyrin (NiT((i)Pr)P), Ni(II) tetra(tert-butyl)porphyrin (NiT((t)Bu)P), and Ni(II) meso-tetraphenylporphyrin (NiTPP) were chosen because they exhibit significant out-of-plane deformations of different symmetries. At B-band excitation, the Raman scattering of out-of-plane modes becomes activated mostly via the Franck-Condon mechanism. Some characteristic bands from out-of-plane modes in the spectra were identified as reliable predictors of the type and magnitude of out-of-plane deformation. The gamma(10)-gamma(13) bands are indicators of ruffling (B(1u)) deformations for porphyrins, as confirmed by data for NiTPP, NiT((i)Pr)P, and NiT((t)Bu)P, where the Raman intensity increases with the magnitude of the ruffling deformation. The gamma(15)-gamma(17) bands are indicators of saddling (B(2u)) deformations, as shown by data for NiOETPP, which is highly saddled. By comparing the relative intensities of these prominent Raman bands we estimated the vibronic coupling parameters using a self-consistent analysis, and showed that they reproduce the respective B-band absorption profiles. We also identified the deformations along the lowest wavenumber normal coordinates as the predominant reason for the Raman activity of out-of-plane modes. Our results suggest that some of the normal coordinates (gamma(10) and gamma(13)) may be used as tools to quantitatively probe the nonplanar deformations of metalloporphyrins with alkyl substituents at the meso-positions. Out-of-plane deformations also increase the vibronic coupling strength of some low frequency in-plane Raman modes, namely, nu(7) and nu(8). Generally, the Raman data suggest that the excited B-state is substantially more nonplanar than the ground state. The overall larger vibronic coupling of ruffled porphyrins yields substantially larger dipole strengths for the vibronic sidebands associated with the B-state transition, so that the relative absorptivity of the B(v) band can be used as a convenient tool to probe the nonplanarity of the porphyrin macrocycle.

Jahn-Teller vibronic coupling in II–VI compounds with Cr 2+ ion

A theoretical study of the vibronic Jahn-Teller interaction in a series of II–VI crystals doped with Cr 2+ ion is reported. The vibronic coupling parameters for the tetrahedrally coordinated Cr 2+ ion (high-spin d 4 configuration) are evaluated in the framework of the exchange charge model of the crystal field accounting for the exchange and covalence effects. The calculations demonstrate the leading role of the trigonal vibrations for the 5T 2 term although the contribution of tetragonal modes is also important; the full symmetric and tetragonal vibrations for the 5E term significantly contribute to the phonon broadening of the absorption and emission bands related to the 5E ↔ 5T 2 transition in Cr 2+ ion.

Non‐planar heme deformations and excited state displacements in horseradish peroxidase detected by Raman spectroscopy at Soret excitation

AbstractWe studied the out‐of‐plane modes of horseradish peroxidase (HRP) in different spin, oxidation and ligation states by measuring the respective resonance Raman spectra with (near) Soret excitation. The non‐planar modes of the heme are Raman inactive for planar macrocycles, but become resonance Raman active in the presence of out‐of‐plane deformations. The thus induced Raman scattering results mostly from Franck–Condon type coupling. We observed bands from a variety of out‐of‐plane modes such as γ5, γ6, γ7(A2u) modes, γ21 and γ22(Eg), γ15(B2u) and γ11(B1u). The appearance of these bands is clearly indicative of non‐planar deformations of the same symmetry. We determined the relative intensities of all sufficiently intense Raman modes of ferric pentacoordinated, quantum mixed and hexacoordinated low‐spin as well as of ferrous pentacoordinated high‐spin HRP C. The apparent vibronic coupling parameters were obtained from a self‐consistent analysis of the Raman intensities and the respective optical absorption spectrum. They revealed significantly reduced displacements of the excited B‐state for the ferrous, deoxy‐like state compared with the resting ferric state. An analysis of the obtained coupling strengths of out‐of‐plane modes revealed that they are predominantly induced by static non‐planar deformations along the normal coordinates of the lowest wavenumber modes of a given symmetry type. We used the deformations of the heme in resting HRP C to determine the vibronic coupling strength of the experimentally detectable out‐of‐plane modes and subsequently employed this information to obtain the non‐planar deformations for the investigated ferric hexacoordinated low‐spin state. This yielded a substantial reduction of the ruffling distortion, whereas the doming deformation remained mostly unaffected by the change of the iron's spin and ligation state. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Ligand K-edge X-ray Absorption Spectroscopy and DFT Calculations on [Fe3S4]0,+ Clusters: Delocalization, Redox, and Effect of the Protein Environment

Ligand K-edge XAS of an [Fe3S4]0 model complex is reported. The pre-edge can be resolved into contributions from the mu(2)S(sulfide), mu(3)S(sulfide), and S(thiolate) ligands. The average ligand-metal bond covalencies obtained from these pre-edges are further distributed between Fe(3+) and Fe(2.5+) components using DFT calculations. The bridging ligand covalency in the [Fe2S2]+ subsite of the [Fe3S4]0 cluster is found to be significantly lower than its value in a reduced [Fe2S2] cluster (38% vs 61%, respectively). This lowered bridging ligand covalency reduces the superexchange coupling parameter J relative to its value in a reduced [Fe2S2]+ site (-146 cm(-1) vs -360 cm(-1), respectively). This decrease in J, along with estimates of the double exchange parameter B and vibronic coupling parameter lambda2/k(-), leads to an S = 2 delocalized ground state in the [Fe3S4]0 cluster. The S K-edge XAS of the protein ferredoxin II (Fd II) from the D. gigas active site shows a decrease in covalency compared to the model complex, in the same oxidation state, which correlates with the number of H-bonding interactions to specific sulfur ligands present in the active site. The changes in ligand-metal bond covalencies upon redox compared with DFT calculations indicate that the redox reaction involves a two-electron change (one-electron ionization plus a spin change of a second electron) with significant electronic relaxation. The presence of the redox inactive Fe(3+) center is found to decrease the barrier of the redox process in the [Fe3S4] cluster due to its strong antiferromagnetic coupling with the redox active Fe2S2 subsite.

DFT-based pseudo-Jahn-Teller coupling studies on the steric and energetic lone pair effect of four- and five-coordinate halide molecules and complexes with central ions from the fifth, sixth, and seventh main groups.

The energetical and stereochemical effect of the s(2) lone pair in the title molecules and complexes is investigated using a pseudo-Jahn-Teller coupling model with parameters adjusted to energies and wave functions from DFT calculations. Vibronic coupling parameters were calculated and compared with those of the coordination number (CN) 3. Inspecting the correlation between the chemical hardness and the vibronic coupling energy (hardness rule), it is found that the tendency to distort decreases with increasing CN. While all considered molecules AX(3) (A(III) = P to Bi; X(-) = F to I) undergo lone pair deformations (D(3h)---> C(3v)), only part of the AX(4)(-) and BX(4) species (B(IV) = S to Po) do so (T(d)---> C(2v)-and even less the ones with CN = 5 (D(3h)---> C(2v) (congruent with C(4v)), AX(5)(2-), BX(5)(-), and CF(5) (C(V); Cl to I). The distorted polyhedra of minimum energy possess usually the butterfly C(2v) shape (CN = 4, tau(2)(zeta) displacement path) and a C(2v) = C(4v)geometry (CN = 5, epsilon' (epsilon) distortion path). A further symmetry lowering to C(s) occurs, if the central ion becomes too small with respect to the ligands (ionic size influence, PCl(Br)(4)(-), PCl(5)(2-)), with the tendency to reduce the CN toward 3 + 1 and 4 + 1, respectively. For CN = 4 the various stationary points of, for example, compressed and elongated C(3v), C(4v), etc. in the multidimensional ground-state potential surface have been characterized. Though of higher energy than the absolute C(2v) minimum, they are shown to govern the dynamics and reactivity of the CN = 4 species to a large extent. To simulate the chemical environment (positively charged counterions, polar solvents), the DFT calculations were performed using the polarizable continuum model COSMO (conductor-like screening model). Though the electronic energy gain upon distortion is not significantly affected by the solvent, the total stabilization energy is distinctly enhanced, frequently leading to lone pair deformations of otherwise electronically stable species. All results obtained by the combined vibronic/DFT approach are well in accord with available experimental data.

Multistate vibronic interactions and nonadiabatic wave packet dynamics in the second photoelectron band of chlorine dioxide

Abstract We report theoretical investigations on the second photoelectron band of chlorine dioxide molecule by ab initio quantum dynamical methods. This band exhibits a highly complex structure and represents a composite portrait of five excited energetically close-lying electronic states of ClO2+. Much of this complexity is likely to be arising due to strong vibronic interactions among these electronic states – which we address and examine herein. The near equilibrium MRCI potential energy surfaces (PESs) of these five cationic states reported by Peterson and Werner [J. Chem. Phys. 99 (1993) 302] for the C2v configuration, are extended for the Cs geometry assuming a harmonic vibration along the asymmetric stretching mode. The strength of the vibronic coupling parameters of the Hamiltonian are calculated by ab initio CASSCF-MRCI method and conical intersections of the PESs are established. The diabatic Hamiltonian matrix is constructed within a linear vibronic coupling scheme and the resulting PESs are employed in the nuclear dynamical simulations, carried out with the aid of a time-dependent wave packet approach. Companion calculations are performed for transitions to the uncoupled electronic states in order to reveal explicitly the impact of the nonadiabatic coupling on the photoelectron dynamics. The theoretical findings are in good accord with the experimental observations. The femtosecond nonradiative decay dynamics of ClO2+ excited electronic states mediated by conical intersections is also examined and discussed.

Transform method in resonance Raman scattering: effect of mode mixing

AbstractThe effect of mode mixing on resonance Raman scattering is examined on the basis of exact transform relations for the resonance Raman excitation profiles and absorption lineshape in the time domain. General transform relations are presented which hold for an arbitrary number of mixed modes. A detailed description of the excitation profiles of two mixed Raman modes is given, taking into account an arbitrary number of other modes. The dependence of the mixing parameters, change of vibrational frequencies and equilibrium positions in the excited state on the vibronic coupling parameters is found. It is shown that in the case of a substantial change in chemical bonds at the electronic transition, the mode mixing drastically changes the Raman cross‐sections/excitation profiles. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Mode mixing via resonance Raman excitation profiles

In the case of linear+quadratic vibronic coupling with an arbitrary number of configurational coordinates, the exact equation for the resonance Raman amplitude has been derived via the absorption Fourier transform. There are some actual cases when this equation has simple solutions: change of vibrational frequency at the electronic transition without mode mixing, mixing of two modes, mixing of modes with close frequency. The corresponding transform relations between Raman amplitudes (→excitation profiles) and the absorption are given, which allow to determine the mixing parameters, vibrational frequencies as well as the linear vibronic coupling parameters.

Conformational Distortions of Metalloporphyrins with Electron-Withdrawing NO2 Substituents at Different Meso Positions. A Structural Analysis by Polarized Resonance Raman Dispersion Spectroscopy and Molecular Mechanics Calculations

The meso substituted Ni(II)(5,15-diNO2−octaethylporphyrin) coexists in at least three different conformers in CS2. To explore the structural properties of these conformers, we measured the resonance excitation profiles and depolarization ratio dispersions of various prominent Raman lines of Ni(5,15-diNO2−octaethylporphyrin) in CS2. The data were analyzed by a theoretical approach, which formulates the Raman tensor in terms of vibronic coupling parameters that depend on static deformations along the normal coordinates. The coupling parameters were determined by simultaneously fitting the depolarization ratio dispersion data and the corresponding resonance excitation profiles. We have also performed molecular mechanics calculations to identify all possible stable conformers of the molecule. To quantify the out-of-plane distortions of the calculated structures, we subjected them to normal coordinate deformation analysis (Jentzen, W.; Song, X.-Z.; Shelnutt, J. A. J. Phys. Chem. B 1997, 101, 1684). The results...

Configuration vibronic mixing for a neutral vacancy in silicon and diamond

Configuration vibronic mixing is considered for a fully symmetric Jahn-Teller electronic term with orientation-degenerate terms (due to the distortion direction) including a correlation correction in a single-open-shell approximation. The approach is nonempirical and involves only linear vibronic coupling. The adiabatic potential is a multiwell one, because the different configurations involved in the exact Jahn-Teller term have different vibronic coupling with a lattice distortion. The stabilization energy, the frequencies of local lattice vibrations, the vibronic coupling parameter, and the energy barriers to migration and to distortion-axis reorientation are estimated for a neutral vacancy in silicon and diamond with allowance made for configuration vibronic coupling. The estimates agree with the results obtained by different experimental and theoretical methods for a wide range of properties associated with the Jahn-Teller effect.

Multiple lines of conical intersections and nondegenerate ground state in T⊗t2 Jahn–Teller systems

We have investigated the T⊗t2 Jahn–Teller problem with linear and quadratic vibronic coupling including a fourth-order term. First, numerical calculations of the lowest vibronic states were carried out by direct diagonalization of the Hamiltonian. The results show that the energy level of the ground vibronic state, which is triply degenerate T for small quadratic coupling g values, intersects the next A level with increasing g, thus realizing the nondegenerate ground state at sufficiently large g values. This result reverses the long-standing belief that the ground vibronic state for the T⊗t2 system has the same degeneracy and symmetry T as the initial electronic state. To explain these results in terms of Berry phase requirements and conical intersections, the adiabatic potential energy surface of the system is analyzed, and the relationships among the type and number of minima, conical intersections, and relevant tunneling paths are revealed. Depending on the vibronic coupling parameter values, there are four trigonal minima and six orthorhombic saddle points, which become minima at large g values, plus ten lines of conical intersections on the lowest potential energy surface. The barriers between the minima are significantly increased near the lines of conical intersections where the Born–Huang terms diverge. For small enough quadratic coupling, only four lines of conical intersections that originate from the highest symmetry point and proceed along the four trigonal directions are significant in determining the Berry phase, and the triply degenerate ground T state is obtained. By increasing the quadratic coupling parameter, the remaining six lines of conical intersections approach the point of highest symmetry, thus allowing for alternative tunneling paths and Berry phases which lead to the nondegenerate A ground state. This explanation of the origin of the nondegenerate ground state for some range of values of the vibronic coupling parameters is strengthened by model calculations of tunneling splitting.

Anharmonic Protein Motions and Heme Deformations in Myoglobin Cyanide Probed by Absorption and Resonance Raman Spectroscopy

The Soret absorption of myoglobin cyanide in a 65% glycerol/water mixture was measured as a function of temperature between 20 and 300 K. The data were analyzed by using an earlier model relating each transition into the vibronic manifold of the electronic B-state to a Voigtian band profile (Cupane et al. Eur. Biophys. J. 1995, 23, 385, 1995). Its Gaussian part contains a temperature-dependent component due to the coupling of low-frequency modes to the Soret transition. The analysis of the vibronic substructure was facilitated by comparison with vibronic coupling parameters derived from the line intensities in the polarized Raman spectra taken with Soret excitation. From the depolarization ratios of several Raman lines, the existence of asymmetric heme macrocycle distortions was inferred, which lift the degeneracy of the excited B state. Raman intensities and depolarization ratios were then analyzed by a theory that formulates the polarizability tensor in terms of a time-independent perturbation theory. T...

Cooperative jahn-teller coupling in the manganites

The cooperative Jahn-Teller coupling between the Mn e(g) electrons and the oxygen octahedral distortions in LaMnO3 is studied using ab initio density-functional calculations and tight-binding models. The linear and quadratic vibronic coupling parameters are calculated using density-functional methods. It is shown that the cooperative Jahn-Teller coupling, primarily due to the interoctahedral electron hopping (band structure term), leads to the ordering of the octahedral distortion and simultaneously to orbital ordering. The coupling results in a two-minima adiabatic potential surface in the solid, instead of the three-minima "Mexican-hat" surface for the isolated octahedron.