Wavepacket approaches to system-bath quantum dynamics

So-called system-bath dynamical problems are ubiquitous in chemical physics. They represent one of the most challenging issues in current rate theories, especially when the need of a quantum description arises, as in the case of inherently quantum systems (e.g. H atom transfer in biologically relevant environment) and/or low temperature media (e.g. the cold surface of interstellar dust grains). Thanks to recent advances in quantum simulations of large systems with the MultiConfiguration Time-Dependent Hartree (MCTDH) method1, recent years have witnessed an ever growing interest in 'brute-force' approaches to tackle this kind of problems2. In these approaches one follows the energy-conserving dynamics of a (sub)system coupled to a finite-size bath and observes the (sub)system dissipative dynamics for times less than the Poincaré recurrence time. The main issue here is the size of the bath, and this prevents at present application of the strategy to realistic problems. In order to make progress, we have recently exploited some typical features of the system-bath interactions: (i) with the Local Coherent-State Approximation3 (LCSA) we introduced a simplified (Coherent-State) description of local bath states, thereby allowing the description of the most important (sub)system-bath quantum correlations at a very low computational cost; (ii) with an Effective “L” Mode Transformation recently developed4 we aim to greatly simplify the MCTDH description of the dissipative dynamics in general system-bath problems. In this contribution, the work done so far on LCSA will be critically summarized by comparison with exact MCTDH results on model problems, and recent results on the search of robust (symplectic) propagation schemes and effective mode transformations will be briefly reported.