Reducing the vibrational coupling network in N-methylacetamide as a model for ab initio infrared spectra computations of peptides

The N-methylacetamide (NMA) molecule is studied as a model to develop scalable algorithms for the simulation of multi-dimensional infrared (IR) spectra of peptides and proteins. Anharmonic vibrational frequencies for the fundamental transitions of NMA in the transct conformation are computed using the vibrational self-consistent field (VSCF) method based on potential energy points from second-order Møller-Plesset (MP2) ab initio computations. Dual level schemes, employing coupling potentials evaluated at a lower level of theory, are successfully applied to reduce the total computational cost. Especially, the semi-empirical PM3 method is found to perform well for the evaluation of coupling terms. New selection schemes are introduced that reduce the number of coupling potentials needed in the VSCF procedure without significant loss in accuracy for the frequencies of the spectroscopically relevant amide-II, amide-I, and NH stretch (amide-A,B) modes.