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.
"Dispersion of single walled carbon nanotubes in amidine solvents"
Soumendra N. Barman, Ding Pan, Michael Vosgueritchian, Arjan P. Zoombelt, Giulia Galli and Zhenan Bao
Nanotechnology23, 344011 (2012)
"Selective dispersion of high purity semiconducting single walled
carbon nanotubes with regioregular poly(3-alkylthiophene)s"
H. W. Lee, Y. Yoon, S. Park, J. H. Oh, S. H. Hong, L. S. Liyanage, H. Wang, S. Morishita,
N. Patil, Y. J. Park, J. J. Park, A. Spakowitz, G. Galli, F. Gygi, P. Wong, J. B.-H. Tok,
J. M. Kim, Z. Bao,
Nature Commun.2, 541 (2011)
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.
"Dispersion Interactions and Vibrational Effects in Ice as a Function of Pressure: A First Principles Study",
Eamonn D. Murray and Giulia Galli,
Phys. Rev. Lett.108, 105502 (2012)
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.
"Power series expansion of the RPA correlation energy: The role of the third- and higher-order contributions",
D. Lu, H-V Nguyen and G. Galli,
J. Chem. Phys.133, 154110 (2010)
"A first-principles study of weakly bound molecules using exact exchange and the Random Phase Approximation ",
H-V Nguyen and G. Galli,
J. Chem .Phys132, 044109 (2010)
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.