A Bond-Energy/Bond-Order and Populations Relationship

We report an analytical bond energy from bond orders
and populations
(BEBOP) model that provides intramolecular bond energy decompositions
for chemical insight into the thermochemistry of molecules. The implementation
reported here employs a minimum basis set Mulliken population analysis
on well-conditioned Hartree–Fock orbitals to decompose total
electronic energies into physically interpretable contributions. The
model’s parametrization scheme is based on atom-specific parameters
for hybridization and atom pair-specific parameters for short-range
repulsion and extended Hückel-type bond energy term fitted
to reproduce CBS-QB3 thermochemistry data. The current implementation
is suitable for molecules involving H, Li, Be, B, C, N, O, and F atoms,
and it can be used to analyze intramolecular bond energies of molecular
structures at optimized stationary points found from other computational
methods. This first-generation model brings the computational cost
of a Hartree–Fock calculation using a large triple-ζ
basis set, and its atomization energies are comparable to those from
widely used hybrid Kohn–Sham density functional theory (DFT,
as benchmarked to 109 species from the G2/97 test set and an additional
83 reference species). This model should be useful for the community
by interpreting overall ab initio molecular energies
in terms of physically insightful bond energy contributions, e.g.,
bond dissociation energies, resonance energies, molecular strain energies,
and qualitative energetic contributions to the activation barrier
in chemical reaction mechanisms. This work reports a critical benchmarking
of this method as well as discussions of its strengths and weaknesses
compared to hybrid DFT (i.e., B3LYP, M062X, PBE0, and APF methods),
and other cost-effective approximate Hamiltonian semiempirical quantum
methods (i.e., AM1, PM6, PM7, and DFTB3).