Development of ab-initio anharmonic algorithms and their applications to understand vibrational coupling
Vibrational motions of molecules are intrinsically "anharmonic," for example vibrational spectra of protonated species often consist of complex vibrational features that can not be accounted for by normal modes of the target molecule due to the vibrational coupling between proton modes and other dark states. Since proton stretch modes in protonated molecules often carry large oscillator strength which in term gives rise to a great chance to understand the vibrational coupling in these molecular systems via their vibrational spectra. We have recently developed two ab initio anharmonic algorithms. These algorithms have been applied to a range of protonated species to reveal simple pictures behind their complex vibrational feature. Some of the selected systems are solvated hydronium and a few proton-bound dimers. Complex vibrational feature of functional group containing N-H and C-H due to Fermi Resonance has also been resolved with the assistance of our ab initio anharmonic schemes.