Accurate And General Formalism For Spin-Mixing Parameter Calculations

The spin-admixture parameter y is key to understanding spin dynamics in molecules. It measures the influence of spin-orbit coupling on spin dynamics in virtually all models for molecular materials. However, the predictive quality of its current first-principles electronic structure theory formulation [Z. G. Yu, Phys. Rev. B 85, 115201 (2012)] is limited by multiple approximations. Consequently, current literature is lacking further first-principles predictions of gamma beyond those of the original paper, and the application of such in computational studies of the role of spin mixing in organic and molecular spintronics. Here, we generalize the previous formulation of gamma to remove all approximations limiting its predictive accuracy. The result is dramatic qualitative improvements in several model systems for organic spintronics. The significantly increased transferability of our method also allows its application to a range of heavier, metal-center "molecular qubit" molecules without modification. For these molecules, we identify molecular spin-orbit coupling as the key to their spin relaxation, with unprecedented accuracy. The accuracy and computational robustness of our reformulated method makes it ideal for the kind of large-scale, high-throughput computational exploration of chemical spaces used in several other areas of molecular nanotechnology.