BEBOP, a new reactive potential using bond-energy/bond-order relationships
BEBOP is a reactive potential that has been developed for evaluating bond energies based on computed bond orders and populations without explicit consideration of the geometry of the molecule. The use of molecular dynamics to describe chemical reactivity has been limited to relatively small, well-defined chemical systems due to the geometric basis of the current reactive force fields, but applications such as development of fire-safe polymers require reactive force fields that are more accurate and can describe large, amorphous systems. In BEBOP, bond orders are quickly evaluated using an approximate electronic-structure method. Then, using a relationship between bond energy and bond order, the bond energies can be evaluated taking the electronic structure of the molecule into account.
More specifically, bond energies were assumed proportional to bond orders obtained using Mulliken population analysis from generalized valence-bond (GVB) wavefunctions. However, a semiempirical scheme based on GVB ab initio calculations would be impractical. As an alternative, the GVB bond orders have been accurately reproduced by convolution of Hartree-Fock bond orders with a Fermi-Dirac-like distribution mapped onto the interval from RAB = 0 to RAB = ∞. The resulting bond-order to bond-energy "density functional" relationship is: [special characters omitted] where BAB, β, and RF are empirical parameters adjusted to fit ab initio energies.
In BEBOP, this functional is currently applied to bond orders obtained from LSDA (Local Spin Density Approximation) wavefunctions. Additional terms to account for hybridization, short-range repulsion, electron transfer and electrostatic interactions are included based on the population analysis.
BEBOP(LSDA) was parameterized for H, C, N and O and successfully used to predict the equilibrium energies of seventeen hydrocarbon molecules ranging in atomization energies from 100-2000 kcal/mol and including highly strained systems, radicals and aromatic rings. Application to heavily distorted configurations of methane and ethane demonstrates the advantages of the BEBOP(LSDA) approach over geometric based force fields. Finally, BEBOP(LSDA) was used to predict the energies along the reaction path of model homolytic scission, radical addition, radical abstraction and rearrangement reactions commonly found during polymer decomposition.