Pablo Rivero

Reliability of range-separated hybrid functionals for describing magnetic coupling in molecular systems

The performance of the Heyd-Scuseria-Ernzerhorf (HSE) and single parameter long-range corrected Perdew-Burke-Ernzerhorf (LC-omegaPBE) range-separated hybrids for predicting magnetic coupling constants has been investigated for a broad set of magnetic molecular systems for which accurate experimental data exist. The set includes the H-He-H model system, two organic diradicals with different magnetic behaviors, and a series of Cu dinuclear complexes with a broad range of magnetic coupling values. Both HSE and LC-omegaPBE provide a significant improvement to standard hybrids such as the well-known hybrid Becke-3-parameters exchange with Lee-Yang-Parr correlation (B3LYP) functional. Nevertheless, the performance of these two range-separated hybrid functionals is different: HSE overestimates antiferromagnetic and ferromagnetic interactions in Cu dinuclear complexes (although significantly less than B3LYP), whereas LC-omegaPBE treats ferro- and antiferromagnetic couplings on a much more balanced way. The increased accuracy of LC-omegaPBE suggests that the inclusion of 100% Hartree-Fock exchange considered in the definition of this long-range corrected hybrid functional has important consequences for an accurate description of exchange and correlation effects on the electronic structure of open shell systems. On the other hand, HSE, which was developed with periodic systems in mind, also performs quite well (and better than B3LYP) thus opening the possibility of magnetic coupling studies in metal oxides and other challenging solids.

Entanglement and Polyradical Character of Polycyclic Aromatic Hydrocarbons Predicted by Projected Hartree–Fock Theory

We study strong correlation effects in a series of fused benzene rings (acenes) of varying length and width using our recently developed projected Hartree-Fock (PHF) method. These molecules, commonly known as polycyclic aromatic hydrocarbons or nanographenes, are very challenging for electronic structure theory because of their strong multireference character. This challenge is here met by PHF at moderate computational cost optimizing a spin eigenfunction obtained by projection of an unrestricted Hartree-Fock (UHF) trial determinant. The resulting method, known as SUHF, predicts that polyradical behavior and orbital entanglement are enhanced with molecular size, especially in systems whose structural motifs are dominated by zigzag edges, like oligoacenes.

Performance of plane-wave-based LDA+U and GGA+U approaches to describe magnetic coupling in molecular systems.

This work explores the performance of periodic plane wave density functional theory calculations with an on‐site Coulomb correction to the standard LDA and GGA exchange‐correlation potential—commonly used to describe strongly correlated solids—in describing the magnetic coupling constant of a series of molecular compounds representative of dinuclear Cu complexes and of organic diradicals. The resulting LDA+U or GGA+U formalisms, lead to results comparable to experiment and to those obtained by means of standard hybrid functionals provided that the value of the U parameter is adequately chosen. Hence, these methods offer an alternative efficient computational scheme to correct LDA and GGA approaches to adequately describe the electronic structure and magnetic coupling in large molecular magnetic systems, although at the expenses of introducing an empirical (U) parameter. For all investigated copper dinuclear systems, the LDA+U and GGA+U approaches lead to an improvement in the description of magnetic properties over the original LDA and GGA schemes with an accuracy similar to that arising from the hybrid B3LYP functional, by increasing the on‐site Coulomb repulsion with a moderate U value. Nevertheless, the introduction of an arbitrary U value in the 0–10 eV range most often provides the correct ground‐state spin distribution and the correct sign of the magnetic coupling constant.

Predicting Singlet–Triplet Energy Splittings with Projected Hartree–Fock Methods

Hartree-Fock (HF) and density functional theory (DFT) methods are known for having problems in predicting singlet-triplet energy splittings when the system displays significant diradical character. Multireference methods are traditionally advocated to deal with the spin-contamination problem inherent in broken-symmetry mean-field methods. In the present work, spin-contamination is rigorously eliminated by means of a symmetry projection approach, carried out in a variation-after-projection fashion, recently implemented in our research group. We here explore the performance of a variety of projected Hartree-Fock (PHF) approaches (SUHF, KSUHF, SGHF, and KSGHF) in predicting singlet-triplet energy gaps in a broad set of diradical systems: small diatomic molecules, carbenes and silenes, and a few larger molecules (trimethylenemethane and benzyne isomers). For most of these systems, accurate experimental data is available in the literature. Additionally, we assess the quality of the geometrical parameters obtained in SUHF-based optimizations for some of the systems considered. Our results indicate that PHF methods yield high-quality multireference wave functions, providing a good description of the ground state potential surface as well as an accurate singlet-triplet splitting gap, all within a modest mean-field computational cost.

Half-metallic ferromagnetism in Sr 3 Ru 2 O 7

The bilayered member of the Ruddesden-Popper family of ruthenates, Sr

Electronic structure of HgBa2Can−1CunO2n+2 (n = 1, 2, 3) superconductor parent compounds from periodic hybrid density functional theory

_3

Spin Hamiltonian effective parameters from periodic electronic structure calculations

Ru

Electronic, structural, and magnetic properties ofLaMnO3phase transition at high temperature

_2

Electronic, structural, and magnetic properties of LaMnO

O

_3

_7