**Quantum mechanics based force field for carbon (QMFF-Cx) validated to
reproduce the mechanical and thermodynamics properties of graphite**

TA Pascal and N Karasawa and WA Goddard, JOURNAL OF CHEMICAL PHYSICS, 133, 134114 (2010).

DOI: 10.1063/1.3456543

As assemblies of graphene sheets, carbon nanotubes, and fullerenes
become components of new nanotechnologies, it is important to be able to
predict the structures and properties of these systems. A problem has
been that the level of quantum mechanics practical for such systems
(density functional theory at the PBE level) cannot describe the London
dispersion forces responsible for interaction of the graphene planes
(thus graphite falls apart into graphene sheets). To provide a basis for
describing these London interactions, we derive the quantum mechanics
based force field for carbon (QMFF-Cx) by fitting to results from
density functional theory calculations at the M06-2X level, which
demonstrates accuracies for a broad class of molecules at short and
medium range intermolecular distances. We carried out calculations on
the dehydrogenated coronene (C24) dimer, emphasizing two geometries:
parallel-displaced X (close to the observed structure in graphite
crystal) and PD-Y (the lowest energy transition state for sliding
graphene sheets with respect to each other). A third, eclipsed geometry
is calculated to be much higher in energy. The QMFF-Cx force field leads
to accurate predictions of available experimental mechanical and
thermodynamics data of graphite (lattice vibrations, elastic constants,
Poisson ratios, lattice modes, phonon dispersion curves, specific heat,
and thermal expansion). This validates the use of M06-2X as a practical
method for development of new first principles based generations of QMFF
force fields. (C) 2010 American Institute of Physics.
**doi:10.1063/1.3456543**

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