where erf(x) is the error function and ω c is a parameter for the cutoff frequency and we choose ω c = 200 cm -1 as k B T is about 200 cm -1 at the ambient temperature. The parameter σ determines how quickly the high-frequency modes are turned off, and is taken to be 2 cm -1 . With the low-frequency spectral density, J LF (ω), we compute the absorption spectrum and reorganization energy (λ LF ) from the low-frequency nuclear motions in the MD trajectory. The high-frequency spectral density is determined by applying an error function with the same cutoff frequency,