Essential vs\nRemovable Bath Anharmonicity: Path Integral\nResults with Model Electronic-Vibrational Baths

Fully quantum mechanical real-time\npath integral calculations\non\na two-level system (TLS) interacting with a simple or composite anharmonic\nbath are presented. The bath consists of a few or many units, each\ncomprising two or three electronic states coupled to secondary harmonic\nvibrational baths. The primitive harmonic bath approximation leads\nto qualitatively wrong system dynamics at high temperatures, while\nthe effective harmonic bath mapping becomes exact in the macroscopic\nbath limit. Significant deviations are observed when the number of\nbath units is small, including skewing and blue shifts in the population\noscillations and enhanced or suppressed coherence. These effects reflect\nthe essential anharmonic character of the bath and cannot be captured\nby harmonic bath treatments.