Analysis method for comparison of two similar molecules using natural orbitals of the difference density matrix: application to substituent effects

Abstract

In this paper, we propose an analysis method to compare the electronic states of two similar molecules using natural orbitals (NOs) of the difference density matrix. In this method, inert and unchanged orbital components of the electronic states can be excluded from the analysis. The difference can then be interpreted as being due to electron transfers between NO-pairs, whose eigenvalues have the same absolute values but opposite signs. As a demonstration, we show an application to the substituent effects on nitrobenzene by comparing nitrobenzene and benzene molecules. By investigating a small number of orbital pairs, our analysis method successfully and separately detects the inductive and the resonance effects.

Keywords:

• natural orbitals of difference density matrix
• substituent effect
• inductive effects
• resonance (mesomeric) effects
• nitrobenzene

Acknowledgements

Some of the results were obtained using the K computer at the RIKEN Advanced Institute for Computational Science (Proposal numbers: hp130037 and hp140237).