  Time- and frequency resolved optical signals provide insights into the
properties of light harvesting molecular complexes, including excitation
energies, dipole strengths and orientations, as well as in the exciton energy
flow through the complex. The hierarchical equations of motion (HEOM) provide a
unifying theory, which allows one to study the combined effects of
system-environment dissipation and non-Markovian memory without making
restrictive assumptions about weak or strong couplings or separability of
vibrational and electronic degrees of freedom. With increasing system size the
exact solution of the open quantum system dynamics requires memory and compute
resources beyond a single compute node. To overcome this barrier, we developed
a scalable variant of HEOM. Our distributed memory HEOM, DM-HEOM, is a
universal tool for open quantum system dynamics. It is used to accurately
compute all experimentally accessible time- and frequency resolved processes in
light harvesting molecular complexes with arbitrary system-environment
couplings for a wide range of temperatures and complex sizes.
