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I. Introduction 4227 II. State of the Art 4228 A. General Outline of the Supermolecular Approach 4228 B. Highly Correlated Treatments 4229 C. New Developments 4230 1. Density Functional Theory 4230 2. Local-Correlation Methods 4230 D. Basis-Set Issue 4231 1. Basis-Set Superposition Error 4231 2. Basis-Set Saturation 4231 E. State of the Art Example 4232 III. Challenges 4233 A. Inclusion of Intramolecular Degrees of Freedom 4233 1. Atom−Linear Molecule Case 4233 2. Diatom−Diatom Case 4235 3. Polyatomic Trimer Case 4236 B. Nonadditive Interactions 4236 1. General Outline 4236 2. Evaluation of Single and Triple Exchange Terms: Pseudodimer Approach 4237 3. Rare-Gas Dimer + Chromophore Clusters 4238 4. Ar 2 -HF: Empirical Model vs ab Initio Calculations 4238 5. Ar 2 −H 2 O 4240 6. Ar 2 CO 2 : Nonadditivity of the Red Shift 4241 7. Ar 2 Cl -4241 C. Open-Shell Clusters 4242 1. Nature of Interaction in Open-Shell Clusters 4242 2. Adiabatic vs Diabatic Solutions 4243 3. Treatment of BSSE in Open-Shell Clusters 4243 D. Model Calculations 4244 1. Ar−OH(X 2 Π): A Paradigm 4244 2. He−CH(X 2 Π): Incipient π Bond 4245 3. ArO -: Electron Photodetachement Spectrum 4. Prereactive Complex: Cl( 2 P) + HCl IV. Concluding Remarks V. Abbreviations VI. Acknowledgments VII. References

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Three-dimensional potential energy and dipole moment surfaces of the Cl−–H2 system are calculated ab initio by means of a coupled cluster method with single and double excitations and noniterative correction to triple excitations with augmented correlation consistent quadruple-zeta basis set supplemented with bond functions, and represented in analytical forms. Variational calculations of the energy levels up to the total angular momentum J=25 provide accurate estimations of the measured rotational spectroscopic constants of the ground van der Waals levels n=0 of the Cl−⋯H2/D2 complexes although they underestimate the red shifts of the mid-infrared spectra with v=0→v=1 vibrational excitation of the monomer. They also attest to the accuracy of effective radial interaction potentials extracted previously from experimental data using the rotational RKR procedure. Vibrational predissociation of the Cl−⋯H2/D2(v=1) complexes is shown to follow near-resonant vibrational-to-rotational energy transfer mechanism so that more than 97% of the product monomers are formed in the highest accessible rotational level. This mechanism explains the strong variation of the predissociation rate with isotopic content and nuclear spin form of the complex. Strong deviation of the observed relative abundances of ortho and para forms of the complexes from those of the monomers is qualitatively explained by the secondary ligand exchange reactions in the ionic beam, within the simple thermal equilibrium model. Positions and intensities of the hot v=0, n=1→v=1, n=1 and combination v=0, n=0→v=1, n=1 bands are predicted, and implications to the photoelectron spectroscopy of the complex are briefly discussed.

The potential energy surface of CH4-H2O is calculated through the fourth-order Mo/ller–Plesset perturbation theory. In an attempt to obtain basis-set saturated values of interaction energies the extended basis sets are augmented by bond functions which simulate the effects of high-symmetry polarization functions. The absolute minimum occurs for the configuration involving the C–H-O hydrogen-bond in which O-H points toward one of the faces of the CH4 tetrahedron. The equilibrium C–O separation is equal to 6.8 a0 which corresponds to the bond energy of 0.83 kcal/mol. Due to basis set unsaturation of the dispersion energy the bond energy may still be underestimated by about 0.05 kcal/mol. The secondary minimum involving the C-H–O hydrogen-bond is some 0.2 kcal/mol less stable, and the corresponding C–O distance is longer by 0.6 a0. The anisotropy of the potential energy surface is analyzed via the perturbation theory of intermolecular forces. The binding in CH4-H2O is chiefly due to the dispersion energy which sets the general trend for the anisotropy of the surface. A more detailed examination, however, indicates that the anisotropy of the surface results from a complex interplay of various factors, including electrostatics, exchange repulsion, and to a lesser degree, the deformation effects. Analysis of various exchangeless perturbation approximations to the deformation effect indicates that the neglect of exchange component of deformation may lead to an incorrect description of the van der Waals region. The analytical potential for the CH4-H2O interaction is provided.