Computing excited OH stretch states of water dimer in 12D using contracted intermolecular and intramolecular basis functions

Abstract

Due to the ubiquity and importance of water, water dimer has been intensively studied.Computing the (ro-)vibrational spectrum of water dimer is challenging. The potential has 8 wells separated by low barriers which makes harmonic approximations of limited utility. A variational approach is imperative, but difficult because there are 12 coupled vibrational coordinate. In this paper, we use a product contracted basis, whose functions are products of intramolecular and intermolecular functions computed using an iterative eigensolver. An intermediate matrix $\bs{F}$ facilitates calculating matrix elements. Using $\bs{F}$, it is possible to do calculations on a general potential without storing the potential on the full quadrature grid. We find that surprisingly many intermolecular functions are required. This is due to the importance of coupling between inter- and intra-molecular coordinates. The full $G_{16}$ symmetry of water dimer is exploited. We calculate, for the first time, monomer excited stretch states and compare $P(1)$ transition frequencies with their experimental counterparts. We also compare with experimental vibrational shifts and tunnelling splittings. Surprisingly, we find that the the largest tunnelling splitting, which does not involve interchange of the two monomers, is smaller in the asymmetric stretch excited state than in the ground state. Differences between levels we compute and those obtained with a [6+6] adiabatic approximation [Leforestier et al. J. Chem. Phys. {\bf{137}} 014305 (2012) ] are $\sim 0.6$ cm$^{-1}$ for states without monomer excitation, $\sim 4$ cm$^{-1}$ for monomer excited bend states, and as large as $\sim 10$ cm$^{-1}$ for monomer excited stretch states.

Quantum Chemical Investigation of the Cold Water Dimer Spectrum in the First OH-Stretching Overtone Region Provides a New Interpretation

“…The tunneling splittings for K a = 1 are much smaller than for K a = 0, and our calculated ground state tunneling splitting agrees well with earlier calculations . The differences in the calculated K a = 0 tunneling splittings are either due to the approximate nature of our reduced-dimensional calculations or due to differences between the MB-pol PES employed here and the CCpol-8sf PES employed in ref . We have recently illustrated relatively large differences for the coupling between intra- and intermolecular vibrations calculated with the MB-pol PES and the CCpol-8sf PESs, but we think that our reduced-dimensional treatment is the primary cause for the discrepancy.…”

The first HyDRA challenge for computational vibrational spectroscopy

“…Comparison is made to a recent full-dimensional anharmonic treatment 74 and the most recent experimental 22,[70][71][72][73] Albeit calculations with larger basis sets were available (and provide the basis for AC4), cutting the number of functions leads to an improved error compensation. The robustness is impressive given the amount of approximations involved.…”

“…As an example for the power of this collaborative approach, we suggested to the authors of the best submissions to compute the results for the OH donor vibration in the water dimer, which has been a rather controversial observable over the last few years. 22,[70][71][72][73] A well-converged full-dimensional variational treatment of the donor OH stretch band origin on a high quality analytical potential energy hypersurface is now available 74 yielding a value of 3599 cm À1 . This calculation matches the experimental consensus of about 3601 cm À1 well within the E10 cm À1 spread due to quantum tunneling of indistinguishable nuclei present in this floppy dimer.…”

Hydrogen sharing between two nitroxyl radicals in the gas phase and other microsolvation effects on the infrared spectrum of a bulky hydroxylamine

“…The latter may profit from inclusion of anharmonicity in reduced dimensionality. For the HAT process, semiclassical 42,43 and quantum dynamics 44 calculations would be particularly helpful to see whether the experimental constraint derived from the spectra is a tight or a loose one. This involves predictions for tunneling in vibrationally excited states of radical complexes 42,44,45 which are qualitatively expected to promote the process, but certainly challenging.…”