This project aims to design efficient molecular magnets for quantum technologies by introducing a new quantum chemical approach that combines the accuracy of equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) with the efficiency of density functional theory (DFT). Our focus is on two types of systems: linear single-molecule magnets with a twofold-coordinated cobalt(II) magnetic center and cobalt atoms adsorbed on MgO(001) and (111) metallic surfaces.
These systems exhibit high spin-reversal energy barriers (i.e. slow magnetic relaxation), which is key to designing efficient molecular magnets. However, a comprehensive understanding of the origins of such magnetic behavior is still lacking.
One standard strategy to tackle molecular magnets is to use a multireference method and extract magnetic properties from phenomenological spin Hamiltonians. This approach often yields accurate results; however, outcomes are sensitive to the choice of active-orbital space, and achieving high accuracy requires including dynamic correlation through additional and costly computational steps. For surface-bound metal atoms, DFT+U (i.e. Hubbard correction to DFT) methods are commonly used. However, DFT+U is not parameter-free, and results depend on the functional choice. In contrast, EOM-CCSD offers significant advantages: it does not introduce empirical parameters nor requires active-space selection and accounts for both dynamic and non-dynamic correlation. However, the high computational cost of EOM-CCSD restricts its applications to small molecules. To address this, we present a new embedding approach that applies EOM-CCSD to the magnetic center while using DFT for the remainder, namely EOM-CCSD-in-DFT. Furthermore, embedded EOM-CCSD can be combined with post-processing tools available in the ezMagnet software to predict the spin-reversal barrier, magnetic anisotropy, magnetization, and susceptibility. Equipped with these new tools, our objectives are: (i) to establish new design rules for maximizing spin-reversal barriers in Co(II) complexes, (ii) to identify the preferred adsorption structures of Co atoms on various substrates, and (iii) to assess the influence of the substrate on the Co magnetic behavior. These findings will impact a range of scientific fields, including molecular magnetism, solid-state physics, and quantum chemistry.
Furthermore, this project provides a predictive tool for the design of efficient molecular magnets, which will be beneficial to major industry players in developing next-generation molecular quantum devices. The outcomes of this project will also be used to develop additional methods, such as periodic embedding theories for strongly correlated materials, and to describe other complex chemical systems with easily localized active sites, with implications for catalysis and electronics.
