An ab initio anharmonic approach to study vibrational spectra of small ammonia clusters.

Fermi resonance between the N-H stretching (ν and ν) and the overtone of N-H bending (2ν) in ammonia has hindered the interpretation and assignments of experimental spectra of small ammonia clusters. In this work, we carried out anharmonic vibrational calculations using MP2/aug-cc-pVDZ to examine the vibrational spectra of (NH) with a focus on the size evolution. The enhancement of hydrogen bond strength due to cooperative effects will cause ν and ν to red-shift and blue-shift, respectively, when the size of the cluster increases.