  The photoexcitation dynamics of molecular materials on the 10-100 nm length
scale depend on complex interactions between the electronic and vibrational
degrees of freedom, rendering exact calculations difficult or intractable. The
adaptive Hierarchy of Pure States (adHOPS) is a formally exact method that
leverages the locality imposed by interactions between thermal environments and
electronic excitations to achieve size-invariant scaling calculations for
single-excitation processes in systems described by a Frenkel-Holstein
Hamiltonian. Here, we extend adHOPS to account for arbitrary couplings between
the thermal environments and vertical excitation energies, enabling formally
exact, size-invariant calculations that involve multiple excitations or states
with shared thermal environments. In addition, we introduce a low-temperature
correction and effective integration of the noise to reduce the computational
expense of including ultrafast vibrational relaxation in HOPS simulations. We
present these advances in the latest version of the open-source MesoHOPS
library and use MesoHOPS to characterize charge separation at a one-dimensional
organic heterojunction when both the electron and hole are mobile.
