Effects of spin contamination on estimating bond dissociation energies of polycyclic aromatic hydrocarbons

Abstract

The objective of this work is to investigate the impact of spin contamination on the prediction of the enthalpies of formation of Polycyclic Aromatic Hydrocarbon (PAH) radicals and of the bond dissociation energies of their precursor molecule. These PAH radicals play a major role in the mass growth of soot precursors leading ultimately to the first soot particles. In this work, we highlights the errors due to spin contamination by comparing spin-unrestricted open-shell calculations (UHF, UMP2, and Quadratic CI singles and doubles [QCISD(T)]) with spin-restricted open-shell calculations (ROHF, ROMP2, and ROCCSD(T)). The results suggest that one should be very careful using any of the spin-unrestricted methods (even QCISD (T)) unless the hS 2 i values are extremely close to the theoretical value. Following these observations, we propose a new set of best-estimates for the enthalpies of formation of these critical PAH radicals using spin-restricted open-shell ROMP2 and RCCSD(T) calculations.

Group additive modeling of substituent effects in monocyclic aromatic hydrocarbon radicals

“…One of the issues frequently encountered in unrestricted Hartree‐Fock calculations for open‐shell systems is the contamination of the wavefunctions by higher spin multiplicity states. This may lead to electronic energies that are unrealistically low . High < 2 > values are noted for several MARs under investigation, particularly for some substituted benzoyl radicals.…”

“…This may lead to electronic energies that are unrealistically low. [58][59][60][61][62] High <Ŝ 2 > values are noted for several MARs under investigation, particularly for some substituted benzoyl radicals. Extensive validation against reported experimental data indicates that the final set of G4/BAC enthalpies of formation is reliable despite these spin contaminations.…”

Soot Prediction in a Model Aero-Engine Combustor using a Quadrature-based Method of Moments

“…The one-dimensional flames are calculated with the kinetic mechanism from Blanquart and Pitsch [37] and Narayanaswamy et al [38] assuming unity Lewis number. The mechanism contains detailed PAH chemistry up to C 16 and C 18 species (pyrene, etc.)…”

Development of Mutation++: Multicomponent Thermodynamic and Transport Properties for Ionized Plasmas written in C++

“…Mixture thermodynamic quantities are derived as sums of pure species properties, weighted by the composition of the mixture. Thermodynamic data for pure species can be found in several references [9][10][11][12][13][14][15][16][17]. Differences exist between each database, such as their format, temperature range of applicability, or degree of nonequilibrium supported.…”