Bond breaking in stretched molecules: multi-reference methods versus density functional theory

Abstract

Several quantum chemistry methods were compared for modeling the breaking of bonds in small molecules subjected to extreme strain. This provides a rigorous test of quantum mechanical methods because a high degree of dynamical and non-dynamical correlation is required to accurately model bond breaking in a strained molecule. The methods tested included multi-reference methods, unrestricted Kohn–Sham density functional theory (DFT) using several functionals, and unrestricted coupled-cluster singles and doubles. It is challenging to employ the multi-reference method in a balanced way for the molecules considered due to the computational cost. While the DFT methods are less costly and provide balanced correlation, they do not have enough static correlation to properly model the bond-breaking curve to dissociation. Despite this, for the N12 DFT method the artifacts due to spin contamination of the unrestricted Kohn–Sham method were the least severe and tolerable. Given this, and the low computational cost, the N12 method was chosen for subsequent dynamical simulations for modeling fracture inception in polymers under extreme strain. The physical characteristics of the bond-breaking process are discussed as well as the influence of secondary conjugation on the process.