Water potential-energy surfaces

The case of water is particularly convenient because the required high Ka states may be detected in the solar absorption spectrum. However, it is difficult to observe the necessary high vibrational angular momentum states in molecules, which can only be probed by dispersed fluorescence or stimulated emission techniques. On the other hand, it is now possible to perform converged variational calculations on accurate potential energy surfaces, from which one could hope to verify the quantum monodromy and assess the extent to which it is disturbed by perturbations with other modes. Examples of such computed monodromy are seen for H2O in Fig. 2 and LiCN in Fig. 12. 

Hobza, P., Bludsky, O. Suhai, S., 1999, Reliable Theoretical Treatment of Molecular Clusters Counterpoise-Corrected Potential Energy Surface and Anharmonic Vibrational Frequencies of the Water Dimer , Phys. Chem. Chem. Phys., 1, 3073. 

Goldman, N. Leforestier, C. Saykally, R. J., A firstprinciples potential energy surface for liquid water from VRT spectroscopy of water clusters, Philos. Trans. R. Soc. A 2005, 1-16. doi 10.1098/rsta.2004.1504... 

Figure 5.30 Alternative water pentamer isomers having partial anticooperativity, (a)-(c), or higher coordination and ring strain, (d). Labels in (a)-(c) correspond to clockwise monomer numbering from the top (see the text). (Species (a)-(c) have been optimized under the constraint of planar equilateral skeletal geometry to prevent rearrangement to Wsc [Fig. 5.29(a)] and are therefore only near-stationary points on the potential-energy surface.)...

For a nonlinear molecule consisting of N atoms, the potential energy surface depends on 3N - 6 independent coordinates and depicts how the PE changes as relative coordinates of the atomic nuclei involved in the chemical reaction are varied. For example, consider the water molecule (Fig. 9.10). 

The potential energy of water molecule is a function of three parameters, i.e. ru r2 and angle 0. A plot of the complete potential energy surface of a water molecule, therefore, will be four dimensional. Since we are limited to three dimensions for plotting functions, we cannot draw the entire potential... 

Even a molecularly smooth single-crystal face represents a potential energy surface that depends on the lateral position x, y) of the water molecule in addition to the dependence on the normal distance z. One simple way to introduce this surface corrugation is by adding the lattice periodicity. An example of this approach is given by Berkowitz and co-workers for the interaction between water and the 100 and 111 faces of the Pt crystal. In this case, the full (x, y, z) dependent potential was determined by a fit to the full atomistic model of Heinzinger and co-workers (see later discussion). 

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