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 fun
ctional 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.
The electronic origin of a large resistance change in nanoscale junctions incorporating spin crossover molecules is demonstrated theoretically by using a combination of density functional theory and the non-equilibrium Greens functions method for qua
ntum transport. At the spin crossover phase transition there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two terminal device, the current increases by up to four orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3,000 %. Since the typical phase transition critical temperature for spin crossover compounds can be extended to well above room temperature, spin crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level.
The magnetic properties of the transition metal monoxides MnO and NiO are investigated at equilibrium and under pressure via several advanced first-principles methods coupled with Heisenberg Hamiltonian MonteCarlo. The comparative first-principles an
alysis involves two promising beyond-local density functionals approaches, namely the hybrid density functional theory and the recently developed variational pseudo-self-interaction correction method, implemented with both plane-wave and atomic-orbital basis sets. The advanced functionals deliver a very satisfying rendition, curing the main drawbacks of the local functionals and improving over many other previous theoretical predictions. Furthermore, and most importantly, they convincingly demonstrate a degree of internal consistency, despite differences emerging due to methodological details (e.g. plane waves vs. atomic orbitals)
The Bethe-Ansatz local density approximation (LDA) to lattice density functional theory (LDFT) for the one-dimensional repulsive Hubbard model is extended to current-LDFT (CLDFT). The transport properties of mesoscopic Hubbard rings threaded by a mag
netic flux are then systematically investigated by this scheme. In particular we present calculations of ground state energies, persistent currents and Drude weights for both a repulsive homogeneous and a single impurity Hubbard model. Our results for the ground state energies in the metallic phase compares favorably well with those obtained with numerically accurate many-body techniques. Also the dependence of the persistent currents on the Coulomb and the impurity interaction strength, and on the ring size are all well captured by LDA-CLDFT. Our study demonstrates that CLDFT is a powerful tool for studying one-dimensional correlated electron systems with high accuracy and low computational costs.
We present a Monte Carlo study of the finite temperature properties of an extended Hubbard-Peierls model describing one dimensional $pi$-conjugated polymers. The model incorporates electron-phonon and hyperfine interaction and it is solved at the mea
n field level for half filling. In particular we explore the model as a function of the strength of electron-electron and electron-phonon interactions. At low temperature the system presents a diamagnetic to antiferromagnetic transition as the electron-electron interaction strength increases. At the same time by increasing the electron-phonon coupling there is a transition from a homogeneous to a Peierls dimerized geometry. As expected such a Peierls dimerized phase disappears at finite temperature as a result of thermal vibrations. More intriguing is the interplay between the electron-phonon and the electron-electron interactions at finite temperature. In particular we demonstrate that for a certain region of the parameter space there is a spin-crossover, where the system transits from a low-spin to a high-spin state as the temperature increases. In close analogy to standard spin-crossover in divalent magnetic molecules such a transition is entropy driven. Finally we discuss the role played by the hyperfine interaction over the phase diagram.
The electronic structure of the neutral and singly charged Mg vacancy in MgO is investigated using density functional theory. For both defects, semilocal exchange correlation functionals such as the local spin density approximation incorrectly predic
t a delocalized degenerate ground state. In contrast functionals that take strong correlation effects into account predict a localized solution, in agreement with spin resonance experiments. Our results, obtained with the HSE hybrid, atomic self-interaction corrected and LDA+U functionals, provide a number of constraints to the possibility of ferromagnetism in hole doped MgO.
Density functional theory calculations demonstrate that rocksalt MgN is a magnetic material at the verge of half-metallicity, with an electronic structure robust against strong correlations and spin-orbit interaction. Furthermore the calculated heat
of formation describes the compound as metastable and suggests that it can be fabricated by tuning the relative Mg and N abundance during growth. Intriguingly the equilibrium lattice constant is close to that of MgO, so that MgN is likely to form as an inclusion during the fabrication of N-doped MgO. We then speculate that the MgO/MgN system may represent a unique materials platform for magnetic tunnel junctions not incorporating any transition metals.
We demonstrate that the different magnetic states of a Mn12 molecule can be distinguished in a two-probe transport experiment from a complete knowledge of the current-voltage curve. Our results, obtained with state-of-the-art non-equilibrium transpor
t methods combined with density functional theory, demonstrate that spin configuration-specific negative differential resistances (NDRs) appear in the I-V curves. These originate from the interplay between electron localization and the re-hybridization of the molecular levels in an external electric field and allow the detection of the molecules spin-state.
Predicting magnetism originating from 2$p$ orbitals is a delicate problem, which depends on the subtle interplay between covalency and Hunds coupling. Calculations based on density functional theory and the local spin density approximation fail in tw
o remarkably different ways. On the one hand the excessive delocalization of spin-polarized holes leads to half-metallic ground states and the expectation of room temperature ferromagnetism. On the other hand, in some cases a magnetic ground state may not be predicted at all. We demonstrate that a simple self-interaction correction scheme modifies both these situations via an enhanced localization of the holes responsible for the magnetism and possibly Jahn-Teller distortion. In both cases the ground state becomes insulating and the magnetic coupling between the impurities weak.
