“…However, when dealing with Kramers systems in zero external magnetic field, additional terms of R2italicab,italiccd2−ph also survive the secular approximation in virtue of the spectrum degeneracy that makes additional terms (ω ac + ω db ) vanish. For instance, the population-coherence transfer process R2italicaa,italicbcn−ph, with E b = E c , and coherence transfer terms R2italicad,italicbcn−ph, with E b = E c and E a = E d , would not be washed out by the secular approximation and could, in principle, contribute to the spin-phonon dynamics ( 40 , 41 ).…”