Our group is mostly interested in the phenomenon of intermolecular interactions. It is remarkable how broad a range of physical, chemical, and biological phenomena originates from such interactions (also called van der Waals interactions), i.e., the interactions that do not involve forming a chemical bond between the interacting species. We have developed perturbative methods describing intermolecular interactions, the so-called symmetry-adapted perturbation theory (SAPT). These methods solve the quantum mechanical Schroedinger equation using a sequence of gradually improving approximations. Practical calculations are now possible for large molecules (containing up to about thirty atoms). The SAPT computer code written by our group is currently used by nearly 400 research groups worldwide.

Another area of interest are ultra-accurate calculations for small molecules. The simplest description of an atom or molecule ignores correlations between the electrons. To describe electron correlation effects, one uses many-body perturbation theory and coupled cluster methods. Most implementations of these methods expand wave functions into products of orbitals, which severely limits accuracy of predictions. To overcome this problem, we use explicitly-correlated functions, i.e., functions depending on interelectronic distances in the coupled cluster expansions of the molecular wave functions. For small molecules, this method produces benchmark correlation energies that serve as accuracy standards for other theoretical methods and provide the best possible agreement with experiment.