Weakly Bonded Systems

Dispersion interactions play a key role in determining the physical and chemical properties of weakly bonded systems, e.g., molecular crystals, as well as in a number of fundamental phenomena, for example, adhesion, physical adsorption, and wetting of surfaces. The dispersion contribution to vdW interactions is difficult to describe with the available quantum simulation techniques, either based on density functional theory or on quantum chemistry methods.

We are investigating ways of computing dispersion forces using techniques based on exact exchange (EXX) and RPA (that is a description of the correlation energy within the random phase approximation), and on approximate, vdW functionals. We have also explored the applicability of quantum monte carlo.

Carbon Nanotubes-polymer composites

The ability to effectively disperse and separate high purity semiconducting single walled carbon nanotubes (SWNTs) is crucial for thin film electronics applications. The group of Prof. Z. Bao at Stanford University has shown that the polymer regioregular poly(3-alkylthiophene)s may be used to sort sc-SNWTs. Through rational selection of polymers, solvent and temperature, they achieved highly selective dispersion of sc-SNWTs. The approach also enables direct film preparation after a simple centrifugation step, thus omitting the need for tedious polymer removal. We used first principle calculations and a geometric model to build a structural model of the polymer wrapped around the nanotube and to rationalize Raman measurements.

Molecular crystals

Using EXX/RPA, we investigated intermolecular interactions in three different types of molecular systems: the benzene and methane crystals and self-assembled monolayers of phenylenediisocyanide, which involve aromatic rings, sp3 C-H bonds, and isocyanide triple bonds, respectively. The EXX/RPA perturbative approach yields structural properties in agreement with experiment, however binding energies tend to be underestimated. The dependence of results on input wavefunctions was investigated for specific system.

EXX/RPA and vdW-functionals

We investigated the connection, at the theoretical level, between EXX/ RPA and vdW-Density Functionals, as well as the importance of considering third order terms in perturbative expansions of correlation energies. Our study of rare gas and alkaline-earth dimers show that perturbative EXX/RPA gives reasonable results, however in some cases (e.g. Be2) one may need to include self-consistency. Work is in progress to derive a self-consistent EXX/RPA scheme.


The excitement generated by the ability to fabricate graphene layers has renewed interest in weak interactions in graphitic systems. Unfortunately, the nature and strength of binding in graphitic materials are poorly understood. For example, large uncertainties are associated with measurements of a fundamental physical quantity such as the strength of interlayer binding in graphite. We have computed the binding energy between graphite layers using quantum monte carlo. Our results are in good agreement with most recent experiments, and provide a benchmark value for future calculations and further experimental measurements. The interaction energy between planes varies, as a function of distance, with a power law close to that found for two semiconducting planes.