Diffusion Monte Carlo approaches for investigating the structure and vibrational spectra of fluxional systems

Abstract

Recent advances in diffusion Monte Carlo (DMC) are reviewed within the context of the vibrational motions of systems that undergo large amplitude motions. Specifically, the authors describe the DMC approach for obtaining the ground state wave function and zero-point energy (ZPE) of the system of interest, as well as extensions to the method for evaluating probability amplitudes, rotational constants, vibrationally excited states and methods for obtaining vibrational spectra. The discussion is framed in terms of the properties of several systems of current experimental and theoretical interest, specifically complexes of neon atoms with OH or SH, , , and . The results of the DMC simulations provide the information necessary to characterize the extent of delocalization of the probability amplitudes, even in the ground vibrational states. Methods for evaluating expectation values and vibrationally excited states are explored, and, when possible, the results are compared with those from other approaches. Finally, the methods for evaluating intensities are described and existing and future challenges for the approach are reviewed. Contents PAGE 1. Introduction 78 2. Systems 79 2.1. Nen · XH complexes 79 2.2. and 80 2.3. 83 3. Ground state wave functions and energies 84 3.1. Theory 84 3.2. Results 86 4. Obtaining properties from DMC 89 4.1. Averaging by Pair Counting (AVPC) 89 4.2. Adiabatic DMC 89 4.3. Descendent weighting 91 4.4. Example 1 – Bond lengths in 91 4.5. Example 2 – Bond length distributions for and 91 4.6. Example 3 – Rotational constants 93 5. Excited states 96 5.1. Example 4 – The stretch fundamental in Ar3 99 5.2. Example 5 – The fundamentals in Ne2XH 100 5.3. Example 6 – Fundamental vibrations in and 102 6. Using DMC to interpret spectra 103 7. Summary and future prospects 104 Acknowledgements 105 References 105