I. de P. R. Moreira

Comment on "about the calculation of exchange coupling constants using density-functional theory: The role of the self-interaction error" [J. Chem. Phys. 123, 164110 (2005)]

The use of density functional theory to obtain energy differences related to magnetic coupling constants is debated with special emphasis to the claims by Ruiz et al. [J. Chem. Phys.123, 164110 (2005)] that good agreement with experiment using the B3LYP potential is obtained by ignoring spin symmetry in case self-interaction error cannot be removed.

The structural relaxation of the α-Al2O3(0001): an investigation of potential errors

Abstract All electron, first principles, periodic calculations have been carried out to investigate the relaxation of the Al-terminated corundum surface. Different theoretical formalisms, Hartree–Fock (HF) and density functional theory using either the LDA or the hybrid B3LYP exchange-correlation functional, have been employed. In the past, many ab initio studies of surface structure have been performed using the experimental, rather than the theoretically optimized bulk structures. In the current study, we show that this leads to significant differences in the predicted geometry. In addition, we demonstrate that optimizing a limited subset of surface parameters also has a significant effect on the accuracy of the final geometry. The extent of the effect associated with each of these sources is shown to be strongly dependent on the choice of exchange-correlation potential.

Ab initio cluster model approach to the chemisorption of NH3 on Pt(111)

Abstract The chemisorption of NH 3 on Pt(111) has been studied using several Pt 10 clusters that model different adsorption sites of the Pt(111) surface. Ab initio methods have been used to obtain a theoretical estimate of several spectroscopic features that can be directly compared to experiment. The comparison includes the variation of the difference between the 3a 1 and 1e levels, the vibrational frequency shifts and the order of stability on different surface sites. Chemisorption at the on-top site is predicted to be favoured, the calculated interaction energy appears to be quite close to the experimental estimate, and it is suggested that NH 3 chemisorbs molecularly, in an N-down orientation, with an equilibrium geometry representing a small distortion from the gas-phase molecular geometry and no azimuthal preference, in good agreement with ESDIAD experiments. Constrained space orbital variation (CSOV) analysis of the interaction has also been performed using a Hartree–Fock wave function. This analysis shows that the leading bonding mechanisms are substrate polarisation and charge transfer from ammonia to the surface.

Detailed ab-initio analysis of the magnetic coupling in CuF2

Abstract In this study, previously reported results on the magnetic coupling constants of CuF 2 are completed. The most prominent coupling constants are determined qualitatively by UHF calculations on the 3D periodic system and more quantitatively through accurate quantum chemical cluster calculations that explicitly include electron correlation effects. The magnetic ground state of CuF 2 thus predicted is in agreement with experimental findings. For the dominant coupling constant, J 2 , the value −130 K is obtained. This value is about five times larger than the CASCI or the UHF values themselves, showing the importance of dynamical correlation effects.

Exchange Coupling Inversion in a High-Spin Organic Triradical Molecule

The magnetic properties of a nanoscale system are inextricably linked to its local environment. In adatoms on surfaces and inorganic layered structures, the exchange interactions result from the relative lattice positions, layer thicknesses, and other environmental parameters. Here, we report on a sample-dependent sign inversion of the magnetic exchange coupling between the three unpaired spins of an organic triradical molecule embedded in a three-terminal device. This ferro-to-antiferromagnetic transition is due to structural distortions and results in a high-to-low spin ground-state change in a molecule traditionally considered to be a robust high-spin quartet. Moreover, the flexibility of the molecule yields an in situ electric tunability of the exchange coupling via the gate electrode. These findings open a route to the controlled reversal of the magnetic states in organic molecule-based nanodevices by mechanical means, electrical gating, or chemical tailoring.

Theoretical Study of NH3 Chemisorption on Pt(111)

Abstract The chemisorption of NH 3 on Pt(111) has been studied utilizing a Pt 10 cluster model in conjunction with the ab initio Hartree–Fock and the hybrid B3LYP Density Functional Theory based methods. For the on–top chemisorption both techniques predict that NH 3 chemisorbs molecularly with an equilibrium geometry representing a small distortion of the gas phase molecular geometry and no azimuthal preference. The interaction energy calculated by means of the two methods appears to be quite close to the experimental estimate, specially for the B3LYP calculated value. The similarity between HF and B3LYP spreads to other electronic structure properties of the chemisorbed molecule such as the Δ(1e–3a 1 ) shift, the net charge transfer from NH 3 to the surface and the bonding mechanism.

A relationship between electronic structure effective parameters and Tc in monolayered cuprate superconductors

Abstract For a broad class of high-Tc superconductor cuprates with nearly independent Cu–O planes, the values of the parameters defining the t–J model Hamiltonian are obtained from accurate ab initio electronic structure calculations on embedded cluster models. From the calculated t and J values a clear-cut correlation between Tc and these values emerges. Based on the ideas of resonating-valence-bond (RVB) theory, a possible interpretation of the origin of this correlation is discussed. The present work stresses the adequacy of the t–J model and strongly supports the idea of a pairing mechanism in cuprates mediated by magnetic interactions.

Spin Hamiltonian effective parameters from periodic electronic structure calculations

This paper presents and discusses a general procedure to extract spin Hamiltonian effective parameters from periodic calculations. The methodology is illustrated through representative examples of increasing complexity covering systems with three dimensional magnetic order or with a two dimensional magnetic structure. Some more complex systems are discussed where physical intuition based on the crystal structure of the system does not provide a reliable guide but where the present approach can be applied in a straightforward way.