Ab initio simulations of spin-phonon relaxation time in molecular qubits*

Theoretical and computational methods are essential tools for understanding the interplay between electronic and nuclear degrees of freedom in single-molecule magnets (SMMs). Electron-phonon interactions are responsible for the thermally induced spin relaxation in SMMs, limiting their application as building blocks for quantum technologies and memory devices.

Previous efforts to compute spin relaxation time have successfully combined the theory of open-quantum systems with the spin Hamiltonian formalism [1]. However, the use of an effective Hamiltonian restricts the size of the Hilbert space considered in simulations, thus preventing the investigation of relaxation paths involving high-energy electronic states.

In this study, we introduce a generalization of the aforementioned approach [2]. This modification enables us to bypass the use of an effective Hamiltonian and directly utilize ab initio wavefunctions obtained at the Multi-State Complete Active Space Self-Consistent Field (MS-CASSCF) level of theory. We apply this methodology to investigate the thermalization process of spin-1/2 systems and to elucidate the role of electronic excited states in the relaxation process.

[1] A. Lunghi, Sci. Adv.,8, eabn7880 (2022).

[2] A.L. Mariano et al., preprint, arXiv:2310.04278 (2023).