**1. Introduction**

A new physical theory—dozy-chaos mechanics or quantum-classical mechanics [1–4]—is designed to describe elementary physico-chemical processes, taking into account the chaotic dynamics of their transient state. The simplest version of quantum-classical mechanics is the quantum-classical mechanics of elementary electron transfers in condensed media [5,6]. This theory arose about twenty years ago [5,6] and proved its efficiency in explaining the optical spectra of polymethine dyes and their aggregates [3–11] and other physico-chemical phenomena [2,12–14]. The very first attempts to create it [15–18], which later turned out to be its particular cases [2,4,5,7–9], were undertaken more than thirty years ago. Quantum-classical mechanics can be considered as a kind of "generalization" of quantum mechanics, in which a new property of the electron is revealed [1,2,19]. This new property arises for an electron when it forms chemical bonds between atoms and consists in the appearance as a result of this ability to provoke chaos in the vibrational motion of nuclei in the process of molecular quantum transitions. The theoretical discovery of this unique ability of the electron made it possible to find

out the reason for the reorganization of the structure of the nuclear subsystem of the molecule and molecular systems as a result of electronic transitions in them. In other words, the discovery of the ability of an electron to create chaos in the motion of nuclei in a transient molecular state made it possible to explain how a light electron manages to shift the equilibrium positions of vibrations of very heavy nuclei, which occurs as a result of the redistribution of the electron charge during molecular "quantum" transitions. This chaos is called dozy chaos [7,8,20], since it occurs only in a transient molecular state and is absent in the initial and final adiabatic molecular states. As a result of the appearance of dozy chaos, the energy spectrum of electrons and nuclei in the transient state becomes continuous, which indicates the classical nature of the motion of electrons and nuclei in this state, while the initial and final states are quantum states that differ sharply from each other in the electronic and nuclear structure. For this reason, dozy-chaos mechanics can also be called quantum-classical mechanics [1–3,19], and the electron itself, which creates chaos in the transient state, can be called a quantum-classical electron [19]. Consequently, the molecular "quantum" transition can be called the quantum-classical molecular transition.

Formally, dozy chaos arises, in theory, as a result of replacing the infinitesimal imaginary addition *i*γ (γ > 0) in the energy denominator of the spectral representation of the full Green's function of an electron-nuclear system with a finite value [5–8,20]. This procedure of changing the quantity γ is forced and is associated with the elimination of an essential singularity that exists in the rates of molecular transitions if their dynamics are considered beyond the Born–Oppenheimer adiabatic approximation and the Franck–Condon principle [21–26]. The quantity γ can be considered as the width of the electron-nuclear energy levels in the transient molecular state, which ensures the exchange of energy and motion between electrons and nuclei in the transient state. However, as the comparison of theoretical results with experimental data on the optical spectra of polymethine dyes and their aggregates shows, the value of γ turns out to be much larger than the value of the vibrational quantum ω of nuclei: γ >> ω [1–9,19,20]. This circumstance points to the fact that the exchange of energy and motion between electrons and nuclei is so intense that it leads to chaos in their joint motion in a transient state. This chaos is the dozy chaos that we discussed above, and the quantity γ is called dozy-chaos energy [7,8,20].

Note that the well-known imaginary, damping gamma terms in the standard theory of radiation–matter interactions [27,28] are related to removing resonance singularities in perturbation theory. In quantum-classical mechanics, we are talking about the elimination of an essential singularity in the rates of electron-nuclear(-vibrational) transitions, which arises when taking into account the full-fledged electron-nuclear motion in the transient state, that is, when considering the electron-nuclear motion beyond the Born–Oppenheimer adiabatic approximation and the Franck–Condon principle. This motion is singular due to the incommensurability of the masses of light electrons and heavy nuclei and regardless of whether it is resonant or non-resonant. This is the fundamental novelty of our problem and our approach to its solution, where it becomes necessary to damp the singular dynamics in molecular systems, in comparison with the standard theory of radiation–matter interactions, where it becomes necessary to damp only resonances in atomic systems. Moreover, our imaginary gamma term already exists in the energy denominator of the total electron-nuclear(-vibrational) Green's function, by definition, as an infinitely small quantity. To eliminate the singularity in the rates of molecular transitions, which, as indicated above, exists within the framework of quantum mechanics, this gamma-term is simply assumed not to be infinitely small but finite, and thus becomes the dozy-chaos energy γ. For details of the discussion of this issue, see [2–4,7,8].

Dozy chaos is a mix of chaotic motions of the electronic charge, nuclear reorganization, and the electromagnetic field (dozy-chaos radiation) via which electrons and nuclei interact in the transient state. Apparently, the main mechanism for the occurrence of dozy chaos is associated with the interaction of an electron with optical phonons (see more details in Section 3 in [1]).

The emergence of chaos in dynamical systems is usually associated with the presence of any nonlinear interactions in them. In quantum-classical mechanics [2], the electron–phonon interaction in

the original Hamiltonian is assumed to be linear (see term <sup>κ</sup> *V*κ(**r**)*q*<sup>κ</sup> in Equation (1), Section 2) and has the same form as in the standard theory of many-phonon transitions [29], on the basis of which it was built. The condition γ >> ω arising in the complete Green's function of the system (see above) leads to its modification and, therefore, takes the whole theory beyond the scope of quantum mechanics. Therefore, it presents a challenge to solve the inverse problem, namely, using the modified Green's function or/and the general result for the rate constant of quantum-classical transitions (see Section 3), to find the form of the original non-Hermitian Hamiltonian [1] that corresponds to such a modified Green function or/and our overall result for the rate constant. In this non-Hermitian Hamiltonian obtained from the solution of the inverse problem, the electron–phonon interaction can turn out to be nonlinear [1]. Thus, the successful solution of the inverse problem will make it possible to clarify, in more detail, the nature of dozy chaos. On the other hand, it is also a challenge to register dozy chaos in an experiment, for example, using X-ray free-electron lasers [2,20].

The quantum-classical electron that provokes dozy chaos can be considered as some organizing physical principle in nature [19], and quantum-classical mechanics itself, and in this particular case, the quantum-classical mechanics of elementary electron transfers in condensed media, can be considered as the physical theory in which this organizing principle was discovered in science.

In any fundamental physical theory, as a rule, some kind of symmetry laws arises. Dozy-chaos mechanics, or in other words, quantum-classical mechanics, is no exception. The purpose of this concept review of the dozy-chaos mechanics of elementary charged particle (electron or proton) transfers in condensed media is to draw attention to a certain set of symmetries that arise in theory and are associated with various features and modes of charge-transfer dynamics. We call this set of symmetries dynamic symmetry in dozy-chaos mechanics.
