In the limit of Δ Δ ω ω approaching zero, the fraction ( e i Δ Δ ω ω t − − 1 ) i Δ Δ ω ω approaches t , therefore the contribution of one element of the reduced density matrix to another element is proportional to time (and therefore dominates for long times t ). In case Δ Δ ω ω is not approaching zero, the contribution of one element of the reduced density matrix to another oscillates with an amplitude proportional to 1 Δ Δ ω ω (and therefore is negligible for long times t ). It is therefore appropriate to neglect any contribution from non-diagonal elements ( c d ) to other non-diagonal elements ( a b ) and from a non-diagonal elements ( c d ) to diagonal elements ( a a , a = b ), since the only case when frequencies of different modes are equal is the case of random degeneracy. The only elements left in the Redfield tensor to evaluate after the Secular approximation are therefore: