  In many important cases, the rate of excitation of a system embedded in an
environment is significantly smaller than the internal system relaxation rates.
An important example is that of light-induced processes under natural
conditions, in which the system is excited by weak, incoherent (e.g., solar)
radiation. Simulating the dynamics on the time scale of the excitation source
can thus be computationally intractable. Here we describe a method for
obtaining the dynamics of quantum systems without directly solving the master
equation. We present an algorithm for the numerical implementation of this
method, and, as an example, use it to reconstruct the internal conversion
dynamics of pyrazine excited by sunlight. Significantly, this approach also
allows us to assess the role of quantum coherence on biological time scales,
which is a topic of ongoing interest.
