The MD simulations were performed with Atomic Simulation Environment (ASE). For the direct NVE scheme, unless otherwise specified, 50 2 ps simulations were performed for spectra computation. To initialize each trajectory, the starting geometry was the optimized geometry and a kinetic energy E = k B T was added to each mode with random positive or negative velocity directions. For the direct NVT scheme, following ref 33, we employed the Nosé-Hoover chain thermostat which has been implemented in an in-house version of ASE. First a 10 ps simulation was performed for the system to reach equilibrium, then another 10 ps simulation was performed for the production run. It is known that the Nosé-Hoover chain thermostat may suffer from ergodicity issues in small or strongly harmonic systems, therefore for the NVT-NVE scheme, we used the Andersen thermostat 74 for NVT sampling.