Theoretical modeling of the formic acid dimer infrared spectrum: Shaping the O-H stretch band

The potential energy surface of the formic acid dimer is investigated in the vicinity of the minima. The linear absorption IR spectrum in the region of the O-H antisymmetric stretch is analyzed using a combination of a second-order perturbative treatment and non-perturbative calculations in reduced dimensionality. The importance of particular coupling mechanisms is studied systematically and it has been shown that the red-shift of the νOH(Bu) center frequency is almost exclusively caused by the Davydov coupling to the symmetric O-H stretching vibration. We find that anharmonic couplings to low frequency dimer stretching and rocking modes, and Fermi resonances to mid-infrared modes contribute to the width, but only in a minor extent to the frequency of the νOH(Bu) band maximum.