Potential energy surfaces, quasiadiabatic channels, rovibrational spectra, and intramolecular dynamics of (HFh and its isotopomers from quantum Monte Carlo calculations

Abstract

Abstract The hydrogen fluoride dimer (HF)2 is the simplest molecule exhibiting the phenomenon of a hydrogen bond. In this case, the hydrogen fluoride molecules are loosely held together by the relatively weak hydrogen bond to produce a floppy molecule with anharmonic vibrations not easily characterized, but producing an interesting and complex spectroscopy. In this paper, Quack and Suhm report the use of the diffusion QMC method in analyzing the behavior of the dimer and its isotopomers, and the successful prediction and assignment of spectral bands observed in their earlier experiments.a The DQMC calculations were carried out for nuclear motion on two different six-dimensional potential energy surfaces adjusted to reproduce experimentally measured spectra including anharmonic interactions between all vibrational modes. Two new QMC methods for calculating excited rotational and vibrational states were introduced: a clamped-coordinate quasiadiabatic channel and a centrifugal-energy approximation scheme. In general, quantitative agreement with experimental observations was obtained, and the success of DQMC methods in the prediction and analysis of the spectra of hydrogen-bonded systems was clearly demonstrated. The large differences in electron and proton masses led to extensive computation requirements due to the slow equilibration and serial correlation induced by the heavier protons. The calculations were executed on one of the first massively parallel computers, a Thinking Machines CM-2 with 65,536 processors. Frequent communication among the processors was required to balance the number of walkers treated in each.