Extension of the composite CBS-QB3 method to singlet diradical calculations

TL;DR

This paper extends the CBS-QB3 computational method to accurately calculate energies of singlet diradicals, addressing previous limitations related to spin-contamination corrections, and demonstrates its effectiveness on various hydrocarbons and heteroatom systems.

Contribution

A new correction term is introduced to improve CBS-QB3 accuracy for singlet diradicals, enhancing its applicability to a broader range of radical species.

Findings

CBS-QB3 energies closely match experimental data

New correction improves singlet diradical calculations

Method effective for hydrocarbons and heteroatom systems

Abstract

The composite CBS-QB3 method is widely used to obtain accurate energies of molecules and radicals although its use in the case of singlet diradicals gives rise to some difficulties. The problem is related to the parameterized correction this method introduces to account for spin-contamination. We report a new term specifically designed to describe singlet diradicals separated by at least one CH2 unit. As a test case, we have computed the formation enthalpy of a series of diradicals that includes hydrocarbons as well as systems involving heteroatoms (nitrogen, oxygen). The resulting CBS-QB3 energies are very close to experiment.