Despite numerous experiments and static electronic structure calculations, the nature of hydrated-electron clusters, (H ₂ O) _n ⁻ , remains poorly understood. Here, we introduce a hybrid ab initio molecular dynamics scheme, balancing accuracy against feasibility, to simulate vibrational and photoelectron spectra of (H ₂ O) _n ⁻ , treating all electrons quantum-mechanically. This methodology provides a computational tool for understanding the spectra of weakly bound and supramolecular anions and for elucidating the fingerprint of dynamics in these spectra. Simulations of (H ₂ O) ₄ ⁻ provide quantitative agreement with experimental spectra and furnish direct evidence of the nonequilibrium nature of the cluster ensemble that is probed experimentally. The simulations also provide an estimate of the cluster temperature ( T ≈ 150–200 K) that is not available from experiment alone. The “double acceptor” electron-binding motif is found to be highly stable with respect to thermal fluctuations, even at T = 300 K, whereas the extra electron stabilizes what would otherwise be unfavorable water configurations.