Beyond energetic and scalar measures: Next generation quantum theory of atoms in molecules

Abstract Next generation quantum theory of atoms in molecules (NG‐QTAIM) is currently the only vector‐based quantum chemical theory as all other quantum chemical theories are scalar‐based. NG‐QTAIM can, for instance, be used to distinguish enantiomers, isotopomers undergoing normal modes of vibration, predict ring‐opening reaction products, ground and excited states at a conical intersection and predict reaction pathways of permutation‐inversion isomers. As a consequence, NG‐QTAIM can uniquely be used to investigate iso‐energetic phenomena where the reliance on differences in geometric measures is removed. NG‐QTAIM has already been used to determine molecular chirality consistent with the Cahn–Ingold–Prelog (CIP) classifications for chiral molecules in addition to the reactant and product of a chiral S N 2 reaction. The CIP classifications however, cannot quantify the chirality of intermediate reaction structures due to their formally achiral nature. We identify the presence of chirality by quantifying a helical response of the bond electrons consistent with photoexcitation circular dichroism experiments that utilize the helical motion of the bound electrons for chiral discrimination. NG‐QTAIM can also be used to provide nongeometric based reasoning as to why there are stronger hydrogen‐bonds in cyclic water clusters compared with compact water clusters. Additionally, the effects of electric‐fields and laser irradiation in conditions not amenable to explanation using scalar measures are quantified. The QuantVec software is introduced, which coordinates external programs and efficiently computes the NG‐QTAIM analyses using the wavefunctions from a wide range of ground and excited states computed with DFT, coupled‐cluster, complete active space SCF, configuration interaction, or quantum dynamics methods. This article is categorized under: Structure and Mechanism > Molecular Structures Structure and Mechanism > Reaction Mechanisms and Catalysis Software > Quantum Chemistry