William Lester, Alan Aspuru-Guzik, and Ouafae El Akramine, University of California, Berkeley
Alexander Kollias, Lawrence Berkeley National Laboratory
Xenophon Krokidis, Accelrys, Inc., Orsay Cedex, France
John Harkless, National Institute of Standards and Technology

This figure displays the electron localization function (ELF) for the interaction of CO with two chromium (Cr) atoms at a separation corresponding to the lattice spacing of Cr(110). The isosurfaces in panels (a) and (b) correspond to different values of ELF, eta = 0.65 and 0.56, respectively. The Cr1-Cr2 and C-O bonds are represented by green basins; the carbon, oxygen and chromium valence basins are red; while the cores are magenta. Panel (a) shows the localization domains of a two-Cr cluster before interaction with CO. Panel (b) displays the perpendicular binding mode of CO adsorbed on the surface. (Figure by O. El Akramine, X. Krokidis, and A. Kollias)

Research Objectives
This research is directed principally toward high-accuracy studies to enable the characterization of the reaction pathways (1) leading to the formation of the first aromatic ring in high-temperature environments and subsequent reactions ultimately leading to soot formation, and (b) governing combustion reactions of small organic alcohols.

Computational Approach
Our dominant computational technique is the quantum Monte Carlo (QMC) method in the diffusion Monte Carlo (DMC) variant. Our version of DMC employs effective core potentials to minimize computational effort. Variational Monte Carlo computations are carried out to test trial functions for DMC constructed as products of independent particle wave functions and correlation functions that depend on interparticle distances. The Schmidt-Moskowitz correlation function based on a functional form introduced by Boys and Handy is used in this work, along with a recently developed wave function optimization method.

Accomplishments
The addition of C₃H₃ and C₂H₂ to form cyclopentadienyl (cpd) radical was investigated using QMC and several density functional theory (DFT) methods. The kinetics of the reaction system were studied using time-dependent solution of the energy-transfer master equations. The computed heat of formation of the cpd radical and the rate of its thermal decomposition compare favorably with available experimental data.

We carried out QMC and DFT computations to identify and quantitatively characterize the ground-state reaction pathways for completion of chlorine plus methanol. Prior to the present calculations, only the reaction leading to methoxy radical had been identified. Using DFT and Fukui function procedures, we were able to identify a previously unknown direct reaction pathway leading to the formation of hydroxymethyl radical, which explained recent experimental data.

Elucidation of reaction pathways and associated rates for the formation of aromatics in high-temperature pyrolysis and oxidation of hydrocarbons is one of the most active areas of research in gas phase chemical kinetics. The primary focus is on the formation of the first aromatic ring from small aliphatics, because this step is perceived to be rate-limiting in the reaction sequence leading to larger aromatics and eventually soot. A critical aspect of the associated analysis is the stability of the normal (n) and iso (i) isomers at high temperature. QMC calculations of the energy differences in C₄H₃ and C₄H₅ isomers have determined greater stability for both types of isomers and enhanced stability of the n isomers relative to the i isomers.

Significance
With elementary reactions determined to ~1 kcal/mol, uncertainties in reaction paths can be resolved, removing ambiguity in mechanisms for the formation of successively larger precursors to soot formation. The ultimate goal is full characterization of the mechanism of soot formation, which will provide valuable insight on how to reduce a major pollution source. The methods developed in this research will also be applicable to molecular systems important in catalysis and other application areas.

Publications
I. Ovcharenko, A. Aspuru-Guzik, and W. A. Lester, Jr., "Soft pseudopotentials for efficient quantum Monte Carlo calculations: From Be to Ne and Al to Ar," J. Chem. Phys. 114, 7790 (2001).

R. N. Barnett, Z. Sun, and W. A. Lester, Jr., "Improved trial wave functions in quantum Monte Carlo: Application to acetylene and its dissociation fragments," J. Chem. Phys. 114, 2013 (2001).

W. A. Lester, Jr. and J. C. Grossman, "Quantum Monte Carlo for the electronic structure of combustion systems," in Recent Advances in Quantum Monte Carlo Methods: Part II, S. Rothstein, W. A. Lester, Jr., and S. Tanaka, eds. (World Scientific Publications, Singapore, in press).