Perturbation theory weak intermolecular interaction calculations

Since the symmetry-adapted perturbation theory provides the basis for the understanding of weak intermolecular interactions, it is useful to discuss the convergence properties of the sapt expansion. High-order calculations performed for model one-electron (Hj) (30), two-electron (H2) (14, 15), and four-electron (He and He-Hz) (31) systems show that the sapt series converges rapidly. In fact, already the second-order calculation reproduces the exact variational interaction energies with errors smaller than 4%. Several recent applications strongly indicate that this optimistic result holds for larger systems as well. 

In this paper we have given a short overview of the application of symmetry-adapted perturbation theory to intermolecular potentials and interaction-induced properties, and of the methods going directly from these intermolecular properties to experimentally measurable quantities such as collision-induced Raman spectra, rovibrational spectra, and second (pressure and dielectric) virial coefficients. The results presented in this paper show that intermolecular potentials and dipole/polarizability surfaces obtained from ab initio SAPT calculations can be used to correctly describe the spectroscopic and dynamical processes involving weakly bound complexes. 

Weakly bound systems from van der Waals to strong hydrogen bonds are treated theoretically in ECC in the article by Bogumil Jeziorski Intermolecular Interactions by Perturbation Theory). Related articles are those of Tucker Carrington Vibrational Energy Level Calculations), Wolfgang Domcke Vibronic Dynamics in Polyatomic Molecules), and of Martin Quack Multiphoton Excitation). 

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