Calculating vibrational excited state absorptions with excited state constrained minimized energy surfaces

The modeling and interpretation of vibrational spectra are crucial for studying reaction dynamics using vibrational spectroscopy. Previous theoretical developments have mainly focused on fundamental vibrational transitions. In this study, we present a new method that uses excited state constrained minimized energy surfaces (CMES) to describe vibrational excited state absorptions. The excited state CMESs are obtained similarly to the previous ground state CMES development in our group but with additional wave function orthogonality constraints. Using a series of model systems, including the harmonic oscillator, Morse potential, double-well potential, and quartic potential, we demonstrate that this new procedure provides good estimations of the transition frequencies for vibrational excited state absorptions. The results are significantly better than those obtained from harmonic approximations using conventional potential energy surfaces.