Hydrogen Bonds and Weakly Bound Systems

49 A priori approaches have also been developed, see, Valiron, Vibok, and Mayer, 1993, as well as Paizs and 

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The cluster energies are obtained from the solution of the nonrelativistic Schrodinger equation for each system. The expansion of the trial many-electron wave function delineates the level of theory (description of electron correlation), whereas the description of the constituent orbitals is associated with the choice of the orbital basis set. A recent review (Dunning 2000) outlines a path, which is based on hierarchical approaches in this double expansion in order to ensure convergence of both the correlation and basis set problems. It also describes the application of these hierarchical approaches to various chemical systems that are associated with very diverse bonding characteristics, such as covalent bonds, hydrogen bonds and weakly bound clusters. 

The quantitative evaluation of the corresponding interaction energies had to await the development of computers and ab initio systems in the 1960s. By the early 1970s it was apparent that self-consistent field (SCF) theory provides a reasonably accurate description of hydrogen-bonded complexes like (H O) while theories that explicitly account for electron correlation " must be used for systems which are predominantly bound by dispersion forces, such as He-H2 and Hcj. Rapid developments in both hardware and software have since taken place and ab initio calculations on weakly bound systems are now routinely being carried out. Useful information is gathered in this way and the potential surfaces obtained find application in simulation studies of liquids, solids and various solvation problems. ... 

In general, second-order Moller-Plesset perturbation theory (a specific case of second-order many-body perturbation theory) is the workhorse of electronic structure techniques for weakly bound systems because the method tends to provide a reliable description of a wide range of weak interactions. For most hydrogen-bonding scenarios, MP2 energetics are extremely accurate and nearly identical to those from CCSD(T) computations with the same basis set. In fact, a recent study revealed that MP2 interaction energies obtained with an appropriate triple- basis set agree favorably with CCSD(T) CBS... 

This definition is still valid today. However, what is considered an independent system today is very different than in 1960. Through the development of a wide range of sophisticated synthetic techniques and sensitive measurement technologies, experimental physical chemists have prepared and studied a wide range of weakly bound molecular complexes, ranging from hydrogen-bonded dimers that are bound by a few kcal mol to weakly bound complexes such as Ar-HX (X = F, Cl) that are bound by a few tenths of a kcal mol , to, recently, the most weakly bound system of all, He2. 

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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