Top left: Ehrenfest Force F(r) ∇ρ(r)·σ(r) trajectory map for Li4. Top right: The {qFA,qFA′} path-packets are presented on the F(r) molecular graph. Bottom left and right: The {qF,qF′} and {qσHF,qσHF′} path-packets, using QTAIM on the F(r) molecular graph, the green circles indicate (BCPs).

The effect of a varying, directional E x, E y, and E z electric field on the ethene molecule was investigated using next‐generation quantum theory of atoms in molecules (QTAIM). Despite using low E‐field strengths that are within the realm of experiment and do not measurably alter the molecular geometry, significant changes to the QTAIM properties were observed. Using conventional QTAIM, the shifting of the C─C and C─H bond critical points (BCPs) demonstrates polarization through an interchange in the size of the atoms involved in a bond, since a BCP is located on the boundary between a pair of bonded atoms. Next‐generation QTAIM, however, demonstrates the polarization effect more directly with a change in morphology of the 3‐D envelope around the BCP. Modest increases of ≈ 2% in the ellipticity ε of the BCP were uncovered when the C─C bond was aligned parallel or anti‐parallel to the applied E x‐field. Significant asymmetries were found in the response of the next‐generation QTAIM 3‐D paths of the C─H bonds to the applied E‐field. When the E‐field coincided with the C─C bond, the BCP moved in response and was accompanied by the envelope constructed from 3‐D next‐generation paths. The response displayed a polarization effect that increased with increasing magnitude of the E x‐field parallel and anti‐parallel to the C─C bond. Our analysis demonstrates that next‐generation QTAIM is a useful tool for understanding the response of molecules to E‐fields, for example, for the screening of molecular wires for the design of molecular electronic devices. © 2019 Wiley Periodicals, Inc.

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The realization of technologically relevant functional systems from idealized photochromic compounds remains elusive due to the double requirement that such switches must possess both highly efficient photo-isomerization reactivity and extremely low fatigue over a large number of switching cycles. Nowadays, improvements of the switching properties in complex diarylethene structures are mainly attained on a "trial and error" basis through chemical substitutions aimed at tuning the chemical properties of the core of the diarylethene. Therefore, we present new guiding principles to analyze the first excited state reactivity of diarylethenes based on the quantum theory of atoms in molecules (QTAIM) including the stress tensor. This approach straightforwardly provides consistent theoretical justification to partner the already successful symmetric substitution patterns obtained from experiments. The guiding principles provided by QTAIM and stress tensor suggest more complex asymmetric patterns should be included for the systematic design of new technologically relevant functional compounds. The stress tensor trajectory T r (s) analysis is used to characterize the photochromism reaction as reusable and the fatigue reaction as irreversible and find candidate sites for alteration by future experiment. K E Y W O R D S diarylethenes, photochromism, QTAIM, stress tensor

The effect of an electric field on a recently proposed molecular switch based on a quinone analogue was investigated using nextgeneration quantum theory of atoms in molecules (QTAIM) methodology. The reversal of a homogenous external electric field was demonstrated to improve the "OFF" functioning of the switch. This was achieved by destabilization of the H atom participating in the tautomerization process along the hydrogen bond that defines the switch. The "ON" functioning of the switch, from the position of the tautomerization barrier, is also improved by the reversal of the homogenous external electric field: this result was previously inaccessible. The "ON" and "OFF" functioning of the switch was visualized in terms of the response of the most preferred directions of motion of the electronic charge density to the applied external field. All measures from QTAIM and the stress tensor provide consistent results for the factors affecting the "ON" and "OFF" switch performance. Our analysis therefore demonstrates use for future design of molecular electronic devices.

A vector‐based representation of the chemical bond is introduced, which we refer to as the bond‐path framework set B = {p, q, r}, where p, q, and r represent 3 eigenvector‐following paths with corresponding lengths H*, H, and the familiar quantum theory of atoms in molecules (QTAIM) bond‐path length (BPL). The intended application of B is for molecules subjected to various types of reactions and distortions, including photoisomerization reactions, applied torsions θ, or normal modes of vibration. The lengths H* and H of the eigenvector‐following paths are constructed using the e 1 and e 2 Hessian eigenvectors, respectively, along the bond path, these corresponding to the least and most preferred directions of charge density accumulation. In particular, the paths p and q provide a vector representation of the scalar QTAIM ellipticity ε. The bond‐path framework set B is applied to the excited state deactivation of fulvene that involves distortions along various intramolecular degrees of freedom, such as the bond stretching/compression of bond‐length alternation and bond torsion distortions. We find that the H* and H lengths can differentiate between the ground and excited electronic states, in contrast to the QTAIM BPL. Five unique paths were presented for B = {(p0,p1), (q0,q1), r} for the ground and first excited states where the profile of the scaling factor, the ellipticity ε, reveals a large unexpected asymmetry for the excited state.

A quantum theory of atoms in molecules (QTAIM) and stress tensor analysis was applied to analyze intramolecular interactions influencing the photoisomerization dynamics of a light-driven rotary molecular motor. For selected nonadiabatic molecular dynamics trajectories characterized by markedly different S state lifetimes, the electron densities were obtained using the ensemble density functional theory method. The analysis revealed that torsional motion of the molecular motor blades from the Franck-Condon point to the S energy minimum and the S/S conical intersection is controlled by two factors: greater numbers of intramolecular bonds before the hop-time and unusually strongly coupled bonds between the atoms of the rotor and the stator blades. This results in the effective stalling of the progress along the torsional path for an extended period of time. This finding suggests a possibility of chemical tuning of the speed of photoisomerization of molecular motors and related molecular switches by reshaping their molecular backbones to decrease or increase the degree of coupling and numbers of intramolecular bond critical points as revealed by the QTAIM/stress tensor analysis of the electron density. Additionally, the stress tensor scalar and vector analysis was found to provide new methods to follow the trajectories, and from this, new insight was gained into the behavior of the S state in the vicinity of the conical intersection.

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