Electron correlation effects in cobalt fluorides CoFn

The molecular cobalt fluorides CoF₂, CoF₃ and CoF₄ are studied and compared by employing different basis sets as well as Quantum Information Theory (QIT) to investigate their correlation effects. These prototypical monomers may be systematically extended in size yielding a novel quasi 1-dimensional, strongly correlated model system consisting of cobalt atoms bridged by oxygen atoms and fluorine termination on both ends. Accurate correlation energies are obtained using Full Configuration Interaction (FCI) and Full Configuration Interaction Quantum Monte Carlo (FCIQMC) calculations and the results are compared to Coupled Cluster and Density Matrix Renormalization Group (DMRG) energies. The analysis indicates the cobalt atom requires a larger number of one-electron basis functions than fluorine and the use of localized molecular orbitals may facilitate calculations for the extended systems.     

Electron correlation effects in the Fe dimer

The potential energy surface of the Fe dimer is investigated on the basis of density functional theory in the generalized gradient approximation (GGA). Electron correlation effects are taken into account explicitly within the GGA+U approach. We find a value of 2.20 eV for the Coulomb repulsion parameter U to describe the Fe dimer best, yielding a 9 Sigma(g)- ground state with an interatomic separation of 2.143 A. Agreement of the associated vibrational frequency, binding energy, ionization potential, and electron affinity with experimental data as well as corresponding results calculated within a high-level ab initio approach is improved significantly compared to conventional GGA. The effect of U on calculated geometric and magnetic properties of larger Fe clusters is discussed.     

Electron correlation effects in ionic hydrogen clusters

We employ density functional, post‐Hartree–Fock, and quantum Monte Carlo methods to study the electronic structure, geometries, and behavior of positively charged H_m⁺ clusters with m=3,5,…,17. Their structure consists of a tightly bound H₃⁺ core ion surrounded by successive solvation shells of H₂ molecules. For the largest clusters, we propose new geometries. We find that correlated methods yield the stepwise decrease of enthalpies for dissociation of H₂ from the clusters observed in experiments. Our best results are obtained by the diffusion Monte Carlo method, and by including finite temperature entropic effects, we are able to reproduce the experimental dissociation enthalpies with an unprecedented accuracy of less than 0.5 kcal/mol. These benchmark results contrast with erroneous predictions discovered in the density functional approaches. Finally, our analysis of the cluster energy surfaces indicates that under quantum and thermal fluctuations, the outer solvation shells will exhibit pronounced fluctional behavior. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 86–95, 2001     

Electron density and localizability in BH

Three partitioning methods are compared for the molecule BH. They are by minimum fluctuations, intersections of isodensity contours for localized molecular orbitals, and the virial partitioning method. The first two methods were found to partition BH very similarly, such that the centroids of charge can be calculated for these loges defined by minimum fluctuation and using the localized molecular orbitals.     

Electron correlation and relativistic effects in xenon tetrafluoride

Ab initio all-electron fully relativistic Dirac–Fock self-consistent field and Dirac–Fock–Breit calculations are reported for the XeF₄ molecule at various internuclear distances assuming the experimental D_4h geometry with our recently developed relativistic universal Gaussian basis set. The nonrelativistic limit Hartree–Fock calculations were also performed for XeF₄ at various internuclear distances. The calculated relativistic correction to the total energy of molecule at the Dirac–Fock level is ～ ?5856 eV, whereas the magnetic part of the Breit correction to the electron-electron interaction is calculated as ～ 177 eV. The electron correlation effects were included in the nonrelativistic Hartree–Fock calculations using the second-order Møller-Plesset (MP 2) theory, and the calculated correlation energy for XeF₄ is ?71 eV. The basis-set superposition error (BSSE ) was estimated by using the counterpoise method for Xe and F. The inclusion of both the relativistic and electron correlation effects in the calculated total energies of F, Xe, and XeF₄ predicts the Xe—F bond length and dissociation energy of XeF₄ as 1.952 Å and 5.59 eV, respectively, which are in excellent agreement with the experimental values of 1.953 Å and 5.69 eV, respectively, for XeF₄. The contribution of the electron correlation and relativistic effects to the dissociation energy of XeF₄ is 8.11 and 0.05 eV, respectively. The Breit interaction, however, contributes only 0.02 eV to the dissociation energy of XeF₄. Electron correlation is most significant for the prediction of an accurate value of dissociation energy, whereas relativistic effects are very important for the prediction of spin-orbital splitting as well as the energies of the orbitals, especially the inner orbitals of XeF₄. © 1995 John Wiley & Sons, Inc.     

Vibrational and correlation effects on properties of water

Large basis set, ab initio potential energy and property surfaces of water have been used with quantum Monte Carlo vibrational analysis in the evaluation of the molecule's rotational constants, zero-point energy, and dipole moment. While there are clearly differences in vibrational state parameters due to including correlation effects, the vibrational averaging effect on rotational constants is very nearly additive with the correlation effect. This has implications for evaluation and estimation of properties of molecules in specific vibrational states. Received: 9 February 1999 / Accepted: 11 February 1999 / Published online: 5 May 1999     

Electron transport and magnetic properties of some mixed-valent alkalithiocuprates

Electrical transport properties of some two-dimensional isostructural compounds KCu₄S₃, RbCu₄S₃, and CsCu₄S₃ and a one dimensional conductor Na₃Cu₄S₄ have been investigated. Conductivities have been measured on pelletized materials in the temperature range 300 to 150 K. All of them behave as metallic conductors. The room temperature conductivities of these compounds are KCu₄S₃, 1600 Ω^?1 cm^?1; RbCu₄S₃, 1400 Ω^?1 cm^?1; CsCu₄S₃, 1250 Ω^?1 cm^?1; Na₃Cu₄S₄, 700 Ω^?1 cm^?1. The observed trend in conductivities among the isostructural compounds may be rationalized either on the basis of shortest CuCu distances or on the carrier charge concentrations per unit volume. All of these compounds behave as Pauli paramagnetic materials.     

Electron impact polarization and correlation properties of the inert gases

For the heavier rare-gas targets, Ne, Ar, Kr, there is now a reasonable amount of experimental electron impact coherence parameter data available for excitation of the lowestJ=1 states. Theoretical results for those rare-gas targets, have been restricted to distorted-wave approximation (DWA) type theories. We present a systemization of the experimental data and compare them with available theoretical results. In the case of the heavy rare gases, we compare the experimental and theoretical data available for the three species, Ne, Ar, Kr, in order to identify trends. We compare the experimental data with results from available theories (mainly DWA type) and discuss the importance of spin-orbit coupling effects and “shell” effects. We present our point-of-view as to the physical picture that is emerging from all collisional data, and conclude by recommending future experimental and theoretical activities that will, from our perspective, provide new insight into the physics of these processes.     

Electron correlation effects in reduced or oxidized mixed-valency molecules

The vibronic model ordinarily written for mixed-valence molecules predicts an intervalence transfer (IVT) frequency which, in the limit of delocalized states, is unchanged upon reduction by one further electron. This is in sharp disagreement with experiment, but can be remedied, qualitatively, by adding one-site Coulomb repulsion which shifts the analog of the IVT into the near ultraviolet, and an exchange term which produces the correct ordering of the energy levels, thus making a transition analogous to IVT spin-forbidden.     

Electron correlation effects in the 4f14 shell

This paper serves a twofold purpose. First, Löwdin's inner projection in both nonperturbative and perturbative forms is applied to the quartic anharmonic oscillator. Inner projection with perturbation theory yields rational approximations to Brillouin–Wigner-type perturbation expansions. These lower bounds are compared with [N ? 1, N] Padé approximants to the Rayleigh–Schrödinger perturbation series for this problem. These Padés are also expressible as the even convergents, w_2N, of a Stieltjes-type continued fraction. The latter representation has certain advantages with respect to its Padé counterpart. Inner projection without perturbation theory provides significantly better results than the perturbative version. The application of inner projection techniques to a perturbed hydrogen atom is not straightforward. The usual problems associated with the continuum spectrum of hydrogen are present. By means of a nonunitary “tilting” transformation associated with the Lie group SO(4, 2), these problems may be bypassed. In the SO(4, 2)-reformulated eigenvalue problem, a reinterpretation of the basic variables, as developed by Silverstone and Moats, yields a new Hamiltonian that permits direct use of the inner projection method. This method has been applied to the ground state of the hydrogen atom in a magnetic field, using both four- and eight-dimensional basis manifolds. This represents the first application of inner projection to this problem.     

Electron correlation effects on the shape of the Kohn–Sham molecular orbital

Even systems in which strong electron correlation effects are present, such as the large near-degeneracy correlation in a dissociating electron pair bond exemplified by stretched H₂, are represented in the Kohn–Sham (KS) model of non-interacting electrons by a determinantal wavefunction built from the KS molecular orbitals. As a contribution to the discussion on the status and meaning of the KS orbitals we investigate, for the prototype system of H₂ at large bond distance, and also for a one-dimensional molecular model, how the electron correlation effects show up in the shape of the KS σ _g orbital. KS orbitals φ^HL and φ^FCI obtained from the correlated Heitler-London and full configuration interaction wavefunctions are compared to the orbital φ^LCAO, the traditional linear combination of atomic orbitals (LCAO) form of the (approximate) Hartree-Fock orbital. Electron correlation manifests itself in an essentially non-LCAO structure of the KS orbitals φ^HL and φ^FCI around the bond midpoint, which shows up particularly clearly in the Laplacian of the KS orbital. There are corresponding features in the kinetic energy density t _s of the KS system (a well around the bond midpoint) and in the one-electron KS potential v _s (a peak). The KS features are lacking in the Hartree-Fock orbital, in a minimal LCAO approximation as well as in the exact one. Received: 11 December 1996 / Accepted: 10 January 1997     

Magnetic properties of the BH molecule

Summary The magnetic susceptibility of the BH molecule, as well as the magnetic shielding at the boron nucleus, have been calculated using CASSCF wavefunctions. A variety of active spaces have been employed, thus including more and more dynamical correlation. With very large active spaces, nearly all the valence correlation can be retrieved. The effect of core correlation is discussed in a second series of calculations and found to be small. Final results are +12.5 ppm cgs for the susceptibility and –175 ppm for the magnetic shielding at the boron nucleus.Paper dedicated to Prof. Werner Kutzelnigg on the occasion of his 60th anniversary     

Electron correlation effects in the valence shell photoelectron spectra of haloethenes

Calculations have show that ionization from the inner valence shells of haloethenes cannot be treated on a one-electron basis and that photoelectron spectra should show additional bands corresponding to final ionized states of mixed character. The spectra of a range of haloethenes have been recorded using synchrotron radiation with photon energies of up to 75 eV and the inner valence shell spectra have been shown to be in good agreement with predictions. The measurements have also been used to extend existing photoelectron assignments providing complete valence shell assignments for the molecules studied.     

Susceptibility and magnetic shieldings of the BH molecule

Susceptibility and magnetic shieldings of both nuclei of BH are calculated by use of the finite perturbation method and gauge invariant gaussian basis sets. The results provide further evidence for the temperature independent paramagnetism of BH.     

Electron density distribution of an oxamato bridged Mn(II)-Cu(II) bimetallic chain and correlation to magnetic properties

The electron density distribution of the ferrimagnetic MnCu(pba)(H2O)3.2H2O chain compound, where pba stands for 1,3-propylenebis(oxamato), has been derived from high resolution X-ray diffraction measurements at 114 K using a multipolar model. The analysis of the chemical bonding has been carried out through the "Atoms in Molecules" formalism and thoroughly interpreted with regards to the strong intrachain and weak interchain magnetic couplings. The topological properties of the electron density on the oxamato bridge indicate large electron delocalization and conjugation effects, in addition to high charge transfer from both metals to the bridge. The resulting positive charges on Mn (+1.45 e) and Cu (+1.56 e) induce charge polarization of the bridge, leading to a shift of electron density from the central C atoms to the metal coordinating O and N atoms. The Mn-bridge interactions are mainly closed-shell interactions with low electron density at the corresponding bond critical points, whereas the Cu-bridge interactions exhibit significant covalent character. The Cu-N bonds are moreover stronger than the Cu-O bonds. The 3d Cu and Mn orbital populations are consistent with pyramidal and regular octahedral environments, respectively, in agreement with the loss of degeneracy due to ligand field effects. Interchain interaction pathways are evidenced by the existence of four bond critical points in hydrogen bond regions. Finally, these intrachain and interchain bonding features are correlated to the results of experimental and theoretical spin density distributions, as well as magnetic measurements.     

Electron correlation and relativistic effects in atomic structure calculations of the thorium atom

Relativistic two-component ab initio calculations have been performed for the Th atom. The spin free low lying states have been calculated at state-averaged complete active space self-consistent field (SA-CASSCF) and multi-state complete active space second-order perturbation (MS-CASPT2) level of theories using different sets of active orbitals. The spin-orbit states have been computed using Douglas-Kroll type of atomic mean-field integral approach. The effects of dynamic electron correlation have been studied at the MS-CASPT2 level. The energy levels of spin-orbit states below 30,000 cm(-1) obtained by the inclusion of dynamic electron correlation are in very good agreement with the experimental values. The radiative properties such as weighted transition probabilities (gA) and oscillator strengths (gf) among several spin-orbit states have been calculated at the SA-CASSCF and MS-CASPT2 levels and are expected to be very helpful for future experiments.     

Size and surface effects on the magnetic properties of NiO nanoparticles

NiO nanoparticles (NPs) were prepared by a sol-gel process using the citrate route. The sol-gel parameters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (K(eff)) with decreasing particle size. The magnetic moment per particle was calculated for all samples using the susceptibility value at T = 300 K. The number of uncompensated spins per NP was found to be proportional to n (n(S)≡ total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of K(eff) with particle size. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (～6.5 kOe in the field axis and ～0.18 emu g(-1) vertically), coercive field enhancement (H(C)≈ 4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.