Parallel Quantum Monte Carlo for Molecules
W. A. Lester, Jr., Lawrence Berkeley National Laboratory and University of California, Berkeley
Research Objectives
Accurate computation of energies, reaction pathways, and other properties of high combustion systems.
Computational Approach
The quantum Monte Carlo method is used in both the variational Monte Carlo and diffusion Monte Carlo forms. The method as implemented in this project uses trial functions constructed using basis-set ab initio techniques to define the nodes of the many-body system.
Accomplishments
The reaction pathway for ground-state oxygen atom, O(3P), reacting with cyclopentadiene, C5H6, to form 2- and 3-cyclopentenone was determined. These calculations included the accurate determination of molecular species at each of the critical points along the reaction pathway: reactants, diradical intermediates, transition states, and products.
Our calculations show that the reaction for the formation of 2-cyclopentenone and 3-cyclopentenone arising from the electrophilic addition of O(3P) to a double bond of cyclopentadiene proceeds through an initially formed triplet diradical, undergoes triplet-singlet intersystem crossing to an open-shell diradical singlet, and progresses along the singlet manifold through transition states to the appropriate products.
As O(3P) approaches C5H6, two stable triplet-diradical states can be formed, depending on which carbon atom of the double bond the oxygen atom attaches. The O attachment site determines the remainder of the reaction path (and therefore the final product). The illustration shows these two configurations, labeled D-2CP for the triplet diradical precursor of 2-cyclopentenone, and D-3CP for the triplet diradical precursor of 3-cyclopentenone.
Significance
Cyclopentadiene is a constituent of diesel fuel. The present reaction provides a substantive example to demonstrate the capability of the diffusion Monte Carlo method to yield results for a mid-size system of comparable high accuracy to that obtained earlier for small systems. It is essential to be able to compute combustion energetics of sufficient accuracy to resolve ambiguities in complicated combustion mechanisms that may be dominated by selected elementary steps.
The triplet diradical precursor of 2-cyclopentenone (D-2CP) and the triplet diradical precursor of 3-cyclopentenone (D-3CP). In addition to atom positions, the orbitals of the triplet electrons are shown. In each case, one of these electrons is clearly localized on the O atom. For D-2CP, the molecular orbital for the second electron is localized on the carbon atom of the double bond that was broken. In the case of D-3CP, the second electron is partially delocalized across the three carbon atoms indicated, with more charge on the outer atoms than the center one.
Publications
J. C. Grossman, W. A. Lester, Jr., and S. G. Louie, "Cyclopentadiene stability: Quantum Monte Carlo, coupled cluster, and density function theory determinations," Mol. Phys (in press, 1998).
J. C. Grossman, W. A. Lester, Jr., and S. G. Louie, "Quantum Monte Carlo and density functional theory characterization of 2-cyclopentenone and 3-cyclopentenone formation from O(3P) + cyclopentadiene," J. Am. Chem. Soc. (submitted, 1998).