Graduate Student Coleman Howard will Present a Seminar to the Department
Ab initio wave function methods offer the most reliable way to calculate molecular properties of small chemical systems. This presentation examines efforts to reproduce experimental vibrational frequencies of the hydrogen-bonded dimers of water and hydrogen fluoride. Utilizing a high level of electron correlation with the coupled-cluster singles and doubles method, along with a perturbative approximation to connected triple excitations, the “gold-standard” CCSD(T) method is employed in conjunction with large, flexible atomic orbital basis sets to compute harmonic vibrational frequencies and dissociation energies expected to lie near the complete basis set (CBS) limit. Accounting for anharmonicity with 2nd order vibrational perturbation theory (VPT2), fundamental vibrational transitions predicted from CCSD(T) computations are in excellent agreement with experiment, within a few cm-1 of experimental values for intramonomer vibrational modes of (H2O)2 and (HF)2. In addition, the importance of sufficient electron correlation is demonstrated by examining inaccuracies of the lower-scaling MP2 method in comparison with CCSD(T). Lastly, ongoing work with VPT2 in conjunction with the CCSD(T) method is shown to demonstrate how these computations may aid experimentalists in the assignment of vibrational modes in spectra.