Towards an effective potential for the monomer, dimer, hexamer, solid and liquid forms of hydrogen fluoride

We present an attempt to build up a new two-body effective potential for hydrogen fluoride, fitted to theoretical and experimental data relevant not only to the gas and liquid phases, but also to the crystal. The model is simple enough to be used in Molecular Dynamics and Monte Carlo simulations. The potential consists of: a) an intra-molecular contribution, allowing for variations of the molecular length, plus b) an inter-molecular part, with three charged sites on each monomer and a Buckingham "exp-6" interaction between fluorines. The model is able to reproduce a significant number of observables on the monomer, dimer, hexamer, solid and liquid forms of HF. The shortcomings of the model are pointed out and possible improvements are finally discussed.Comment: LaTeX, 24 pages, 2 figures. For related papers see also http://www.chim.unifi.it:8080/~valle

AN AB INITIO POTENTIAL ENERGY SURFACE AND RO-VIBRATIONAL CALCULATIONS FOR (HCl)2(HCl)_{2}

Author Institution: Herzberg Institute of Astrophysics, National Research Council of Canada; Institut f\""{u}r Theoretische Chemie, and Strahlenchemie, University of ViennaAn ab initio global potential energy surface has been computed for the dimer $(HCl)_{2}$ within the associated coupled pair functional (ACPF) framework using an extended polarized basis set. These 1058 points covering an energy range of up to $40000 cm^{-1}$ above the equilibrium have been fitted to a 6D analytical model containing 32 adjustable parameters with a weighted standard deviation of $23.5 cm^{-1}$. The global minimum energy path, which is significantly different from that for $(HF)_{2}$, and the stationary point geometries and barrier heights have been determined. With this ab intio model, rotational-vibrational calculations, including those using an one-dimensional semi-rigid bender hamiltonial have been performe

AN AB INITIO SEMIRIGID BENDER CALCULATION OF THE ROTATION AND TRANS-TUNNELLING SPECTRA OF (HF)2(HF)_{2} AND (DF)2(DF)_{2}

$^{1}$ P.R. Bunker et al., J. Chem. Phys. 89, 3002 (1988). $^{2}$W.J. Lafferty et al., J. Mol. Spectrosc. 123, 434 (1987). $^{3}$A.S. Pine et al., J. Chem. Phys. 81, 2939 (1984).Author Institution: Herzberg Institute of Astrophysics, National Research Council of Canada; Department of Chemistry, Amherst College; Institut fur Theoretische Chemie and Strahlenchemie der Universitat Wien, Wahringerstrasse 17, A-1090 Wien, Austria.Using a purely ab initio minimum energy $path^{1}$ for the trans-tunnelling motion in the HF dimer the energy levels for the K-type rotation and trans-tunnelling motion for $(HF)_{2}$ and $(DF)_{2}$ are calculated with a one-dimensional Semirigid Bender Hamiltonian and no adjustable parameters. The transition moments for rotation-tunnelling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate $\bar{B}$ values. The energy levels we obtain are in close agreement with $experiment;^{2}$ for example the K-O tunnelling splitting in s$(HF)_{2}$ is calculated as $0.65 cm^{-1}$ compared to the experimental value of $0.65869 cm^{-1}.$ As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the Semirigid Bender formalism is able to account quantitatively for the unusual K-dependence of the rotational energies resulting from the quasilinear $behaviour,^{3}$ and that the trans-tunnelling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the $\Delta K=0$ and $\pm 1$ subbands in the tunnelling spectra of $(HF)_{2}$ and $(DF)_{2}$, many of which have not yet been observed

VIBRATION-TUNNELING ENERGY LEVELS OF (HF)2(HF)_{2} FROM CLOSE-COUPLING CALCULATIONS ON AN AB INITIO POTENTIAL SURFACE

Author Institution: Department of Chemistry, Amherst College; Herzberg Institute of Astrophysics, National Research Council Canada; Institut F\""ur Theoretische Chemie und Strahlenchemie der Universit\""at, Wien, A-1090 Wien, W\""ahringerstrase 17, Austria.Using the analytical expression developed by Bunker, $et. al.^{1}$ for the ab initio potential enery surface of HF $dimer,^{2}$ close-coupling calculations are performed giving the energies of excited vibration and vibration-tunneling states. These results are compared with experiment when possible and with the results of approximate methods of calculation. The large number of channels required for convergence indicates that in some ways the HF dimer resembles a ``normal'' strongly-bound molecule more closely than a weakly-bound complex such as$(H_{2})_{2}.$ $^{1}$ P.R. Bunker, M. Kofranek, H. Lischka, and A. Karpfen, J. Chem. Phys. 89, 3002 (1988). $^{2}$M. Kofranek, H. Lischka, and A. Karpfen, Chem. Phys. 121, 137 (1988)