  Employing extensive quantum-chemical calculations at the DFT/B3LYP and MP2
level, a quartic force field of isolated N-methylacetamide is constructed.
Taking into account 24 vibrational degrees of freedom, the model is employed to
perform numerically exact vibrational configuration interaction calculations of
the vibrational energy relaxation of the amide I mode. It is found that the
energy transfer pathways may sensitively depend on details of the theoretical
description. Moreover, the exact reference calculations were used to study the
applicability and accuracy of (i) the quasiclassical trajectory method, (ii)
time-dependent second-order perturbation theory, and (iii) the instantaneous
normal mode description of frequency fluctuations. Based on the results,
several strategies to describe vibrational energy relaxation in biomolecular
systems are discussed.
