Secular (Latin : saeculum, lit. 'century') approximation is an approximation valid for long times t . The time evolution of the Redfield relaxation tensor is neglected as the Redfield equation describes weak coupling to the environment. Therefore, it is assumed that the relaxation tensor changes slowly in time, and it can be assumed constant for the duration of the interaction described by the interaction Hamiltonian. In general, the time evolution of the reduced density matrix can be written for the element a b as