Research Projects

Recent studies on IR spectroscopy have highlighted the need for accurate descriptions of vibrational structures of medium and large cyclic conjugated interstellar molecules and their ions. Accurate potential energy surface (PEs) computation with an accurate electronic structure theory is crucial for solving vibrational structure problems. However, computational difficulties persist in both parts. This proposal aims to develop a new DFT functional to calculate PEs of cyclic compounds. Wavefunction-based electron correlation methods are not applicable for computing PEs of cyclic conjugated molecules with an affordable basis set, and existing DFT methods produce significantly blue-shifted IR spectrum when computed with vibrational structure methods like VPT2, VCCM, etc. An alternative method is proposed, scaling force constants from DFT methods by finding suitable scaling factors from least square fitting of computed data against experimental frequencies. A variational approach for the excited states is more suitable for describing IR band structure. Two approaches are proposed: the vibrational coupled cluster method (VCCM) and an extended vibrational random phase approximation (VRPA). Two types of interstellar compounds will be studied: nitrogen-containing conjugated cyclic compounds and their ions, and large-size polycyclic aromatic hydrocarbons and their cations.