In this work, diffusion Monte Carlo (DMC) methodology is expanded for efficient application to rovibrational states of molecules that experience large amplitude vibrations in their ground state. The vibrations and rotations of floppy molecules, as this class of molecules is termed, are strongly coupled, resulting in qualitatively incorrect predictions to be made of their associated energy level structure by treating the vibrations harmonically or the rotations rigidly. By constructing a method to accurately predict the rovibrational energy level structure, it is possible to properly assign experimental spectra taken of these molecules. Using these assignments with Boltzmann statistics, radio astronomers can determine amounts and temperatures of molecules in the interstellar medium; fundamental questions about the chemistry taking place in space can then be answered. For this research, different methods are explored for expressing the vibrational and rotational wavefunction within the framework of DMC. For DMC in general, the total wavefunction is expanded in a set of Dirac delta-functions called walkers. The walkers evolve in imaginary time according to the split operator approximation of the quantum propagator, where the kinetic energy operator is simulated by diffusion and the potential energy is given by a potential energy surface. As the simulation time approaches infinity, the energy and wavefunction reach their ground state values. The rotational portion of the wavefunction is expanded in a symmetric top basis set, where each walker is given a rotational state vector containing coefficients for all states of interest. The coefficients in the state vector evolve according to time-dependent perturbation theory. Vibrationally excited states are obtained by inserting nodes in the wavefunction. Results for H2D+ and CH5+ will be reported.

Exploring rovibrational states of floppy molecules using diffusion Monte Carlo

249th National Meeting (2015)

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