Efficient calculation of open quantum system dynamics and time‐resolved spectroscopy with distributed memory HEOM (DM‐HEOM)

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. © 2018 Wiley Periodicals, Inc. The hierarchical equations of motions (HEOM) allow the accurate simulation of open quantum systems across a wide range of temperatures and system‐bath couplings. An application of HEOM is the computation of time‐resolved spectra of light‐harvesting complexes and the exciton dynamics, but is complicated by the compute memory demands of the method. To overcome this barrier, we introduce the computationally efficient, distributed memory implementation DM‐HEOM for the simulation of time‐resolved spectra. DM‐HEOM scales memory and compute resources across compute nodes and takes advantage of massively parallel GPU/CPU devices.