Spin signature of nonlocal-correlation binding in metal organic frameworks

TL;DR

This paper introduces a nonempirical spin-density formalism for the vdW-DF method, revealing significant spin effects in nonlocal correlation-driven adsorption in metal-organic frameworks, with surprising attraction increases in certain cases.

Contribution

The paper develops the svdW-DF formalism, integrating spin effects into the van der Waals density functional, enabling better understanding of magnetic interactions in adsorption processes.

Findings

Spin significantly influences adsorption in MOFs despite weak magnetic responses.

Including spin effects increases attraction in CO₂ adsorption in Ni-MOF74.

Counter-intuitive hybridization explains increased attraction due to spin effects.

Abstract

We develop a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the nonlocal-correlation driven adsorption of H$_2$ and CO$_2$ in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO$_2$ adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO$_2$ would suggest. We explain this counter-intuitive result, tracking the behavior to a coincidental hybridization of the O $p$ states with the Ni $d$ states in the down-spin channel. More generally, by providing insight on nonlocal correlation in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions.