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Register a new userDensity functional theory calculations and vibrational spectroscopy on iron spin-crossover compounds
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Iron complexes with a suitable ligand field undergo spin-crossover (SCO), which can be induced reversibly by temperature, pressure or even light. Therefore, these compounds are highly interesting candidates for optical information storage, for display devices and pressure sensors. The SCO phenomenon can be conveniently studied by spectroscopic techniques like Raman and infrared spectroscopy as well as nuclear inelastic scattering, a technique which makes use of the Mossbauer effect. This review covers new developments which have evolved during the last years like, e.g. picosecond infrared spectroscopy and thin film studies but also gives an overviewon newtechniques for the theoretical calculation of spin transition phenomena and vibrational spectroscopic data of SCO complexes.
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Density functional theory (DFT) provides a theoretical framework for efficient and fairly accurate calculations of the electronic structure of molecules and crystals. The main features of density functional theory are described and DFT methods are compared with wavefunction-based methods like the Hartree-Fock approach. Some recent applications of DFT to spin crossover complexes are reviewed, e.g., the calculation of Mossbauer parameters, of vibrational modes and of differences of entropy, vibrational energy, and total electronic energy between high-spin and low-spin isomers.
Al4SiC4 is a wide band gap semiconductor with numerous potential technological applications. We report here the first thorough experimental Raman and Infrared (IR) investigation of vibrational properties of Al4SiC4 single crystals grown by high temperature solution growth method. The experimental results are compared with the full theoretical analysis of vibrational properties based on Density Functional Theory calculations that are revisited here. We have obtained a good agreement between the experimental and calculated Raman phonon modes and this allowed the symmetry assignment of all the measured Raman modes. We have revisited the DFT calculation of the IR active phonon modes and our results for LO-TO splitting indicate a substantial decrease of the variation of omega(LO-TO) compared with the previous reported calculation. Moreover, most of the IR modes have been symmetry assigned from the comparison of the experimental IR spectra with the corresponding Raman spectra and the Al4SiC4 calculated phonon modes.
Standard flavors of density-functional theory (DFT) calculations are known to fail in describing anions, due to large self-interaction errors. The problem may be circumvented by using localized basis sets of reduced size, leaving no variational flexibility for the extra electron to delocalize. Alternatively, a recent approach exploiting DFT evaluations of total energies on electronic densities optimized at the Hartree-Fock (HF) level has been reported, showing that the self-interaction-free HF densities are able to lead to an improved description of the additional electron, returning affinities in close agreement with the experiments. Nonetheless, such an approach can fail when the HF densities are too inaccurate. Here, an alternative approach is presented, in which an embedding environment is used to stabilize the anion in a bound configuration. Similarly to the HF case, when computing total energies at the DFT level on these corrected densities, electron affinities in very good agreement with experiments can be recovered. The effect of the environment can be evaluated and removed by an extrapolation of the results to the limit of vanishing embedding. Moreover, the approach can be easily applied to DFT calculations with delocalized basis sets, e.g. plane-waves, for which alternative approaches are either not viable or more computationally demanding. The proposed extrapolation strategy can be thus applied also to extended systems, as often studied in condensed-matter physics and materials science, and we illustrate how the embedding environment can be exploited to determine the energy of an adsorbing anion - here a chloride ion on a metal surface - whose charge configuration would be incorrectly predicted by standard density functionals.
Octahedral Fe$^{2+}$ molecules are particularly interesting as they often exhibit a spin-crossover transition. In spite of the many efforts aimed at assessing the performances of density functional theory for such systems, an exchange-correlation functional able to account accurately for the energetic of the various possible spin-states has not been identified yet. Here we critically discuss the issues related to the theoretical description of this class of molecules from first principles. In particular we present a comparison between different density functionals for four ions, namely [Fe(H$_2$O)$_6$]$^{2+}$, [Fe(NH$_3$)$_6$]$^{2+}$, [Fe(NCH)$_6$]$^{2+}$ and [Fe(CO)$_6$]$^{2+}$. These are characterized by different ligand-field splittings and ground state spin multiplicities. Since no experimental data are available for the gas phase, the density functional theory results are benchmarked against those obtained with diffusion Monte Carlo, one of the most accurate methods available to compute ground state total energies of quantum systems. On the one hand, we show that most of the functionals considered provide a good description of the geometry and of the shape of the potential energy surfaces. On the other hand, the same functionals fail badly in predicting the energy differences between the various spin states. In the case of [Fe(H$_2$O)$_6$]$^{2+}$, [Fe(NH$_3$)$_6$]$^{2+}$, [Fe(NCH)$_6$]$^{2+}$, this failure is related to the drastic underestimation of the exchange energy. Therefore quite accurate results can be achieved with hybrid functionals including about 50% of Hartree-Fock exchange. In contrast, in the case of [Fe(CO)$_6$]$^{2+}$, the failure is likely to be caused by the multiconfigurational character of the ground state wave-function and no suitable exchange and correlation functional has been identified.
Forty-five years after the point de depart [1] of density functional theory, its applications in chemistry and the study of electronic structures keep steadily growing. However, the precise form of the energy functional in terms of the electron density still eludes us -- and possibly will do so forever [2]. In what follows we examine a formulation in the same spirit with phase space variables. The validity of Hohenberg-Kohn-Levy-type theorems on phase space is recalled. We study the representability problem for reduced Wigner functions, and proceed to analyze properties of the new functional. Along the way, new results on states in the phase-space formalism of quantum mechanics are established. Natural Wigner orbital theory is developed in depth, with the final aim of constructing accurate correlation-exchange functionals on phase space. A new proof of the overbinding property of the Mueller functional is given. This exact theory supplies its home at long last to that illustrious ancestor, the Thomas-Fermi model.
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