Accurate and efficient spin-phonon coupling and spin dynamics calculations for molecular solids

Molecular materials are poised to play a significant role in the development future opto-electronic and quantum technologies. A crucial aspect of these areas is the role of spin-phonon coupling and how it facilitates energy-transfer processes such as intersystem crossing, quantum decoherence, and magnetic relaxation. Thus, it is of significant interest to be able to accurately calculate molecular spin-phonon coupling and spin dynamics in the condensed phase. Here we examine the various approximations inherent in spin-phonon coupling and spin dynamics calculations on molecular solids by performing a case study on a single-molecule magnet. Three key results are: i) finite crystalline slab calculations should be avoided; ii) the phonon spectrum in reciprocal space should be sampled as densely as possible; and iii) phonon linewidths, as calculated by periodic density-functional theory, are likely overestimated at low temperature, but are not essential to obtain accurate magnetic relaxation rates provided point ii is adhered to. Calculations using this approach are facilitated by the open-source packages we have developed, which enable cost-effective and accurate spin-phonon coupling calculations on molecular solids with quantitative accuracy.