M.S., University of Warsaw, Poland, 1999
Ph.D., University of Warsaw, Poland, 2004
Postdoctoral Fellow, University of Delaware, 2003-2010

Theoretical and computational study of weak intermolecular interactions

Intermolecular interactions are everywhere. A network of intermolecular hydrogen bonds determines the properties of liquid water and aqueous solutions. Hydrogen bonds as well as other effects (like stacking interactions of aromatic side chains) govern the structure of proteins and the catalytic abilities of enzymes. Even the least reactive species like rare gas atoms exhibit weak mutual attraction due to a phenomenon called dispersion. In fact, this attraction is the very reason why rare gases form liquids and solids at low temperatures.
In a computational study of the interaction between two atoms or molecules, the key quantity is the potential energy surface (PES). This surface represents the electronic interaction energy of the dimer as a function of the intermonomer distance and of the relative orientation of monomers. Once the PES is known, we can calculate various quantities (spectra, virial coefficients, liquid and crystal parameters) that can be compared to experimental predictions.
The interaction energy at a given geometry is usually calculated in the so-called supermolecular manner: you pick your favorite quantum-chemistry electronic-structure theory, compute the energy of the dimer AB, and subtract the energies of monomers A and B. This subtraction can be tricky - the result is orders of magnitude smaller than the energies you subtract (it is like weighing a person by placing her on a large, fully loaded ship, and weighing the ship with and without her). Moreover, the result of the subtraction (a single number) does not really tell much about the underlying physics of the interaction. A useful alternative to the supermolecular approach is provided by symmetry-adapted perturbation theory (SAPT) where the interaction energy is calculated directly (no subtraction) as a sum of low-order terms in a perturbation series. Each of these terms has a clear physical interpretation and corresponds to an electrostatic, induction, dispersion, or exchange effect. The supermolecular and SAPT methods nicely complement each other and it is often useful to employ both approaches (as well as several different electronic-structure methods) to construct and interpret accurate PESs. Obviously, the larger the dimer, the more approximate methods have to be used. More accurate approaches tend to scale steeply with the system size and require a lot of CPU time and memory even on powerful supercomputers.
Specific topics addressed by the research in our group include
- further development of the SAPT methodology and software, in particular, application of SAPT to interactions of open-shell systems like radicals or transition-metal atoms and complexes,
- computational studies of simple molecules that are physisorbed (that is, bound by intermolecular forces, primarily dispersion) on metallic and graphite/graphene surfaces,
- development and applications of ultra-accurate PESs for weakly interacting atoms or small molecules.

Smith, D. G. A.; Patkowski, K.; Trinh, D.; Balakrishnan, N.; Lee, T.-G.; Forrey, R. C.; Yang, B. H.; Stancil, P. C. "Highly correlated electronic structure calculations of the He-C_{3} van der Waals complex and collision induced rotational transitions of C_{3}" J. Phys. Chem. A (accepted).