Efficient computation of density-functional orbitally resolved reactivity indices

An algorithm for computation of density-functional-based orbital reactivity indices, such as orbitally resolved hardness and softness tensors, total hardness and softness, and the Fukui indices, has been extended for systems with degenerate electronic states and implemented in the deMon computer code. The method treats explicitly degenerate orbitals, thus avoiding spurious numerical errors in hardness tensor computations. Benchmark calculations for a series of small molecules and some larger, highly degenerate systems reveal good computational performance and numerical stability. The influence of the choice of auxiliary basis functions on the results is also examined and discussed. The predicted orbital and total reactivity index values are found to be insensitive to the applied exchange-correlation functionals. Applications of the orbital Fukui indices to a series of (4n + 2) π-electron [n]-annulenes show the possibility of rationalizing the reactivity of individual molecular orbital contributions.