The ground state of a spin-crossover molecule calculated by diffusion Monte Carlo

TL;DR

This study uses diffusion Monte Carlo to accurately determine the ground state of a spin-crossover molecule, revealing phase-dependent spin states and evaluating the performance of density functional theory in such complex systems.

Contribution

It provides the first diffusion Monte Carlo benchmark for a spin crossover molecule and assesses the limitations of density functional theory for these systems.

Findings

Ground state varies between gas and solid phases.

Density functional theory over-stabilizes the low spin state.

Hybrid functionals with ~50% exact exchange improve accuracy.

Abstract

Spin crossover molecules have recently emerged as a family of compounds potentially useful for implementing molecular spintronics devices. The calculations of the electronic properties of such molecules is a formidable theoretical challenge as one has to describe the spin ground state of a transition metal as the legand field changes. The problem is dominated by the interplay between strong electron correlation at the transition metal site and charge delocalization over the ligands, and thus it fits into a class of problems where density functional theory may be inadequate. Furthermore, the crossover activity is extremely sensitive to environmental conditions, which are difficult to fully characterize. Here we discuss the phase transition of a prototypical spin crossover molecule as obtained with diffusion Monte Carlo simulations. We demonstrate that the ground state changes depending on whether the molecule is in the gas or in the solid phase. As our calculation provides a solid benchmark for the theory we then assess the performances of density functional theory. We find that the low spin state is always over-stabilized, not only by the (semi-)local functionals, but even by the most commonly used hybrids (such as B3LYP and PBE0). We then propose that reliable results can be obtained by using hybrid functionals containing about 50% of exact-exchange.