Quantum Dynamics of Coupled Excitons and Phonons in Chain-Like Systems:
Tensor Train Approaches and Higher-Order Propagators 

Strang splitting for tensor train operators using the SLIM format: In each stage of the splitting scheme, only pairs of cores or single cores are altered.

Patrick Gelß, Sebastian Matera, Rupert Klein, and Burkhard Schmidt 

We investigate the use of tensor-train approaches to the solution of the time-dependent Schrödinger equation for chain-like quantum systems with on-site and nearest-neighbor interactions only. Using efficient low-rank tensor train representations, we aim at reducing the memory consumption as well as the computation costs. As an example, coupled excitons and phonons modeled in terms of Fröhlich-Holstein type Hamiltonians are studied here. By comparing our tensor-train based results with semi-analytical results, we demonstrate the key role of the ranks of the quantum state vectors. Typically, an excellent quality of the solutions is found only when the maximum number of ranks exceed a certain value. One class of propagation schemes builds on splitting the Hamiltonian into two groups of interleaved nearest-neighbor interactions which commutate within each of the groups. In particular, the 4-th order Yoshida-Neri and the 8-th order Kahan-Li symplectic compositions are demonstrated to yield very accurate results, close to machine precision. However, due to the computational costs, currently their use is restricted to rather short chains. That also applies to propagations based on the time-dependent variational principle, typically used in the context of matrix product states. Yet another class of propagators involves explicit, time-symmetrized Euler integrators. Especially the 4-th order variant is recommended for quantum simulations of longer chains, even though the high precision of the splitting schemes cannot be reached. Moreover, the scaling of the computational effort with the dimensions of the local Hilbert spaces is much more favorable for the differencing than for the splitting or variational schemes.