  Non-Markovian Quantum State Diffusion (NMQSD) has turned out to be an
efficient method to calculate excitonic properties of aggregates composed of
organic chromophores, taking into account the coupling of electronic
transitions to vibrational modes of the chromophores. NMQSD is an open quantum
system approach that incorporates environmental degrees of freedom (the
vibrations in our case) in a stochastic way. We show in this paper that for
linear optical spectra (absorption, circular dichroism) no stochastics is
needed, even for finite temperatures. Thus, the spectra can be obtained by
propagating a single trajectory. To this end we map a finite temperature
environment to the zero temperature case using the so-called thermofield
method. The resulting equations can then be solved efficiently by standard
integrators.
