Wang, X.; Carrington, T. NUMERICALLY EXACT CALCULATION OF ROVIBRATIONAL LEVELS OF Cl−H2O. Proceedings of the International Symposium on Molecular Spectroscopy, Urbana, IL, June 16-21, 2014.
Large amplitude vibrations of Van der Waals clusters are important because they
reveal large regions of a potential energy surface (PES). To calculate spectra of
Van der Waals clusters it is common to use an adiabatic approximation. When coupling between
intra- and inter-molecular coordinates is important non-adiabatic coupling cannot be neglected and it is
therefore critical to develop and test theoretical methods that couple both types of coordinates.
We have developed new product basis and contracted basis Lanczos methods for Van der Waals complexes and tested
them by computing rovibrational energy levels of Cl$^-$H$_2$O.
The new product basis is made of
functions of the inter-monomer distance,
Wigner functions that depend on Euler angles specifying the orientation of H$_2$O
with respect to a frame attached to the inter-monomer Jacobi vector,
basis functions for H$_2$O vibration, and
Wigner functions that depend on Euler angles specifying the orientation of
the inter-monomer Jacobi vector with respect to a space-fixed frame.
%tc why not include the angles of the Jacobi vector?
An advantage of this product basis is that it can be used to make an efficient contracted basis
by replacing the vibrational basis functions for the monomer
with monomer vibrational wavefunctions.
Due to weak coupling between intra- and inter-molecular coordinates, only a few tens of monomer vibrational wavefunctions are necessary.
The validity of the two new methods is established by comparing energy levels with benchmark rovibrational levels
obtained with polyspherical coordinates and spherical harmonic type basis functions.
%tc i add
For all bases, product structure is exploited to calculate eigenvalues with the Lanczos algorithm.
%
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For Cl$^-$H$_2$O, we are able, for the first time, to compute accurate splittings due to
tunnelling between the two equivalent $C_s$ minima. We use the
PES of Rheinecker and Bowman (RB).\footnote{ J. Rheinecker and J. M. Bowman,
\newblock J. Chem. Phys. {\bf 124} 131102 (2006); J. Rheinecker and J. M. Bowman,
\newblock J. Chem. Phys. {\bf 125} 133206 (2006)}
Our results are in good agreement with experiment for the five
fundamental bands observed.\footnote{S. Horvath, A. B. McCoy, B. M. Elliott, G. H. Weddle, J. R. Roscioli, and M. A. Johnson
\newblock J. Phys. Chem. A {\bf 114} 1556 (2010)}