Showing papers by "Carmen Herrmann published in 2018"

Performance of range-separated hybrid exchange-correlation functionals for the calculation of magnetic exchange coupling constants of organic diradicals.

Abstract: The prediction of magnetic behavior is important for the design of new magnetic materials. Kohn-Sham density functional theory is popular for this purpose, although one should be careful about choosing the right exchange-correlation functional. Here, we perform a statistical analysis to test different range-separated hybrid density functionals for the calculation of magnetic exchange coupling constants J of fourteen organic diradicals. Our analysis suggests that in absolute terms the MN12SX functional performs best among the series of twelve functionals studied here (including the popular B3LYP), followed by N12SX functionals along with Scuseria's HSE series of functionals. LC- ωPBE was found to be the least accurate, which is in contrast with its good performance for calculating J for transition metal complexes. The HSE family of functionals and B3LYP are the only functionals to reproduce the qualitative trends of the coupling constants correctly for the ferromagnetically coupled diradicals under study. © 2017 Wiley Periodicals, Inc.

Interplay between strong correlation and adsorption distances: Co on Cu(001)

Abstract: Adsorbed transition metal atoms can have partially filled $d$- or $f$-shells due to strong on-site Coulomb interaction. Capturing all effects originating from electron correlation in such strongly correlated systems is a challenge for electronic structure methods. It requires a sufficiently accurate description of the atomistic structure (in particular bond distances and angles), which is usually obtained from Kohn-Sham density functional theory (DFT), which due to the approximate nature of the exchange-correlation functional may provide an unreliable description of strongly correlated systems. To elucidate the consequences of this popular procedure, we apply a combination of DFT with the Anderson impurity model (AIM), as well as DFT+U for a calculation of the potential energy surface along the Co/Cu(001) adsorption coordinate, and compare the results with those obtained from DFT. The adsorption minimum is shifted towards larger distances by applying DFT+AIM, or the much cheaper DFT+U method, compared to the corresponding spin-polarized DFT results, by a magnitude comparable to variations between different approximate exchange-correlation functionals (0.08 to 0.12 Angstrom). This shift originates from an increasing correlation energy at larger adsorption distances, which can be traced back to the Co 3$d_{xy}$ and 3$d_{z2}$ orbitals being more correlated as the adsorption distance is increased.

Toward an automated analysis of exchange pathways in spin-coupled systems

Abstract: Understanding (super-)exchange coupling between local spins is an important task in theoretical chemistry and solid-state physics. We show that a Green's-function approach introduced earlier (Liechtenstein et al., J. Phys. F 1984, 14, L125; Steenbock et al., J. Chem. Theory Comput. 2015, 11, 5651) can be used for analyzing exchange coupling pathways in an automated fashion rather than by visual inspection of molecular orbitals. We demonstrate the capabilities of this approach by comparing it to previously published pathway analyses for hydroxy-bridged dinuclear copper complexes and an oxo-bridged dinuclear manganese complex, and employ it for discriminating between through-space and through-bond pathways in a naphthalene-bridged bisnickelocene complex. © 2017 Wiley Periodicals, Inc.

Co(CO)n/Cu(001): Towards understanding chemical control of the Kondo effect

Abstract: The Kondo effect is a many-body phenomenon allowing insight into the electronic and atomistic structure of spin-polarized adsorbates on metal surfaces. Its chemical control is intriguing because it deepens such insight, but the underlying mechanisms are only partly understood. We study the effect of increasing the number of CO ligands attached to a cobalt adatom on copper(001), which correlates with an increase in the Kondo temperature $T_K$ experimentally (P. Wahl et al, Phy. Rev. Lett. 95, 166601 (2005)), by solving an Anderson impurity model parametrized by density functional theory (DFT++). Our results suggest that the orbital responsible for the Kondo effect is $d_{x^2-y^2}$ for the tetracarbonyl, and its combination with $d_{z^2}$ for the dicarbonyl. The molecular structures depend considerably on the approximate exchange--correlation functional, which may be related to the known difficulty of describing CO binding to metal surfaces. These structural variations strongly affect the Kondo properties, which is not only a concern for predictive studies, but also of interest for detecting mechanical deformations and for understanding the effect of tip--adsorbate interactions in the scanning tunneling microscope. Still, by constraining the tetracarbonyl to $C_{4v}$ symmetry, as suggested by experimental data, we find structures compatible with the experimental trend for $T_K$ (employing BLYP-D3+U). This is not possible for the tricarbonyl despite the range of computational parameters scanned. For the tetra- and dicarbonyl, the increased $T_K$ correlates with a larger hybridization function at the Fermi level, which we trace back to an increased interaction of the Co $3d$ orbitals with the ligands.

Designing and Understanding Building Blocks for Molecular Spintronics

Abstract: Designing and understanding spin coupling within and between molecules is important for, e.g., nanoscale spintronics, magnetic materials, catalysis, and biochemistry. We review a recently developed approach to analyzing spin coupling in terms of local pathways, which allows to evaluate how much each part of a structure contributes to coupling, and present examples of how first-principles electronic structure theory can help to understand spin coupling in molecular systems which show the potential for photo- or redoxswitching, or where the ground state is stabilized with respect to spin flips by adding unpaired spins on a bridge connecting two spin centers. Finally, we make a connection between spin coupling and conductance through molecular bridges.

Structural diradical character

Abstract: A reliable first-principles description of singlet diradical character is essential for predicting nonlinear optical and magnetic properties of molecules. Since diradical and closed-shell electronic structures differ in their distribution of single, double, triple and aromatic bonds, modeling electronic diradical character requires accurate bond-length patterns, in addition to accurate absolute bond lengths. We therefore introduce structural diradical character, which we suggest as an additional measure for comparing first-principles calculations with experimental data. We employ this measure to identify suitable exchange-correlation functionals for predicting the bond length patterns and electronic diradical character of a biscobaltocene with the potential for photoswitchable nonlinear optical activity. Out of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic validation systems and B3LYP for the organic ones. Still, none of the functionals is suitable for correctly describing relative bond lengths across the range of molecules studied, so that none can be recommended for predictive studies of (potential) diradicals without reservation.