  The simulation of open quantum systems coupled to a reservoir through
multiple channels remains a substantial challenge. This kind of open quantum
system arises when considering the radiationless decay of excited states that
are coupled to molecular vibrations, for example. We use the chain mapping
strategy in the interaction picture to study systems linearly coupled to a
harmonic bath through multiple interaction channels. In the interaction
picture, the bare bath Hamiltonian is removed by a unitary transformation (the
system-bath interactions remain), and a chain mapping transforms the bath modes
to a new basis. The transformed Hamiltonian contains time-dependent local
system-bath couplings. The open quantum system is coupled to a limited number
of (transformed) bath modes in the new basis. As such, the entanglement
generated by the system-bath interactions is local, making efficient dynamical
simulations possible with matrix product states. We use this approach to
simulate singlet fission, using a generalized spin-boson Hamiltonian. The
electronic states are coupled to a vibrational bath both diagonally and
off-diagonally. This approach generalizes the chain mapping scheme to the case
of multi-channel system-bath couplings, enabling the efficient simulation of
this class of open quantum systems using matrix product states.
