Exponential unitary integrators for nonseparable quantum Hamiltonians Maximilian Ćirić, Denys I. Bondar and Ole Steuernagel Eur. Phys. J. Plus, 138 3 (2023) 238 Published online: 13 March 2023 DOI: 10.1140/epjp/s13360-023-03819-3