Quantum chemical molecular dynamics under extreme conditions

Quantum chemistry has historically progressed from Hartree-Fock single point energy calculations for so-called “standard geometries” over the calculation of minimum energy reaction pathways to direct molecular dynamics simulations, where energies and gradients are calculated for (micro)canonical ensembles of particles “on the fly”. However, the time scale remains a problem since time cannot be efficiently parallelized on today’s supercomputers, and we therefore employ faster, approximate DFT and semiempirical methods. Such MD simulations may be augmented in a second step by complementary, automatized minimum energy reaction pathways using quenched trajectories, for which higher level calculations are feasible. It is important to note that especially in thermodynamically open systems, nonequilibrium dynamic effects play a central role without which static investigations would fail to capture essential processes. Furthermore, MD simulations of systems at high temperatures automatically include the important role of large entropy contributions, which may lead to large differences between the Gibbs free energy and electronic potential energy surfaces.

 

Quantum Chemical Molecular Dynamics (QM/MD) Simulations of Formation Mechanisms of Fullerenes, Carbon Onions, Carbon Nanotubes, and Metallofullerenes.

 MD under extreme condition

Shrinking hot giant road of fullerene formation (top), carbon onion formation from nanodiamonds (middle),
and carbon nanotube formation during Si evaporation from SiC (bottom).

Using the density functional tight binding (DFTB) method, we have been able to successfully investigate the formation mechanism of fullerene cages starting from ensembles of randomly oriented C2 molecules at high temperatures. Considering the open environment far from equilibrium conditions, we observed for the first time in accurate quantum chemical molecular dynamics that fullerene cages self-assemble naturally when periodically more C2 molecules are added in the simulations. Growth follows three steps: 1.) nucleation formation of a few connected pentagons and hexagons from interacting polyyne chains spontaneously created by the aggregation of C2 units, 2.) ring-condensation reactions by bond formation between atoms of the nucleus border and attached polyyne chains (dynamics of polyyne chains is important), pentagons and hexagons are created at similar ratio due to high temperature conditions, and 3.) cage closure by bridging/zipper type reactions between polyyne chains attached to the opening. The resulting fullerenes are typically giant fullerenes with more than 60 atoms. We found that vibrationally excited, hot giant fullerenes can shrink by carbon evaporation and attain more stable, rounder shapes down to the archetypical round and smallest IPR structure, the sockerball C60. We have therefore called this mechanism the “Shrinking Hot Giant road” of fullerene formation. The shrinking has recently been experimentally observed in HRTEM images (see Phys. Rev. Lett. 99, 175503 (2007) ). We are now extending our studies to the formation mechanism of metallofullerenes and single-walled carbon nanotubes (CNTs) by incorporating metal atoms into such simulations We already have successfully simulated the growth of carbon nanotubes on SiC in the high temperature vacuum evaporation method, carried out in the Kusunoki group at Nagoya University. This, as well as other nanocarbon related research, is carried out in part in collaboration with the group of Prof. emerit. Keiji Morokuma at the Fukui Institute for Fundamental Chemistry, Kyoto University.

Quantum Chemical Molecular Dynamics (QM/MD) of surface corrosion and erosion

 

researchaii
Self-assembly of C4H under low-energy hydrogenation of graphene. A 2×2 superstructure
of aromatic rings emerges during the addition of atomic hydrogen and the removal of hydrogen
atoms by H as H2 molecules.

In this area we are studying the functionlization, oxidation and hole formation of graphitic and related surfaces at high temperature conditions. Recent focus has been the hydrogenation of graphene and plasma-wall interaction studies. Our group is collaborating in this project with the National Institute for Fusion Science (NIFS) with Prof. Hiroaki Nakamura and Atsushi M. Ito, and with the Controlled Fusion Atomic Data Center at ORNL.

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