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Publications of the Research Institute for Mathematical Sciences

Galilei Invariant Molecular Dynamics

Christian D. Jäkel, Günther Hörmann

Published Oct 31, 1995• 1 citation
Publisher

Abstract

We construct a model for a chemical reaction in the Heisenberg representation. The time evolution exists in the thermodynamic limit, independent of the particle density and the initial state. In mathematical terms, we establish a C*-dynamical system. Thus we can benefit from general results in algebraic quantum statistical mechanics, showing, for example, that equilibrium states exist. Galilei invariance of our nonrelativistic model is demonstrated by defining it directly on the Galilean space-time manifold, without reference to any coordinate system. PACS 05.30.Fk, 03.70.+ k, ll.10.Cd, 11.15.Tk

DOI
10.2977/prims/1195163719

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We construct a model for a chemical reaction in the Heisenberg representation. The time evolution exists in the thermodynamic limit, independent of the particle density and the initial state. In mathematical terms, we establish a C*-dynamical system. Thus we can benefit from general results in algebraic quantum statistical mechanics, showing, for example, that equilibrium states exist. Galilei invariance of our nonrelativistic model is demonstrated by defining it directly on the Galilean space-time manifold, without reference to any coordinate system. PACS 05.30.Fk, 03.70.+ k, ll.10.Cd, 11.15.Tk
Abstract We begin the discussion of nonrelativistic quaternionic quantum mechanics by analyzing the allowed form of H for a single-particle system. We will assume, as is done in complex nonrelativistic quantum mechanics, that H is invariant under Galilean transformations and that the kinetic or noninteracting part of H is rotation and translation invariant. In analyzing the implications of these assumptions, we follow the method used in Sec. 13-4 of Jauch (1968a) in the complex quantum mechanics case [see also Mackey (1968), and Feynman as described in Dyson (1990)], and so work in the coordinate representation (cf. Sec. 2.4), and use the Heisenberg picture (cf. Sec. 3.3) to describe time evolution. For brevity of notation, we omit the subscript H for Heisenberg picture operators and denote the coordinate representation Hamiltonian H(x) as simply H2.
We use an algebraic method to model the molecular collision dynamics of a collinear triatomic system. Beginning with a forced oscillator, we develop a mathematical framework upon which inelastic and reactive collisions are modeled. The model is considered algebraic because it takes advantage of the properties of a Lie algebra in the derivation of a time-evolution operator. The time-evolution operator is shown to generate both phase-space and quantum dynamics of a forced oscillator simultaneously. The model is considered semi-classical because only the molecule's internal degrees-of-freedom are quantized. The relative translation between the colliding atom and molecule in an exchange reaction (AB+C ->A+BC) contains no bound states and any possible tunneling is neglected so the relative translation is treated classically. The purpose of this dissertation is to develop a working model for the quantum dynamics of a collinear reactive collision. After a reliable model is developed we apply statistical mechanics principles by averaging collisions with molecules in a thermal bath. The initial Boltzmann distribution is of the oscillator energies. The relative velocities of the colliding particles is considered a thermal average. Results are shown of quantum transition probabilities around the transition state that are highly dynamic due to the coupling between the translational and transverse coordinate.
A compact description of the evolution of a many-body quantum system, e.g., a dilute gas, is provided by the generalization of the usual reaction U matrix to a system U matrix. Using this tool, it is demonstrated that in such systems (i) time-reversal-invariant states are unstable and cannot exist in nature or be ``constructed in the laboratory;'' (ii) the time-reversal invariance of the Hamiltonian leads inexorably to relaxation towards thermal equilibrium of arbitrary nonequilibrium states, i.e., to dissipation, which thus allows the definition of a quantum arrow of time; (iii) the second law of thermodynamics, and hence the complete field of thermodynamics, is a consequence of quantum physics. These results have at their basis the hermiticity of the Hamiltonian and the Heisenberg uncertainty relations.
This paper is devoted to the study of the Galilei group and its representations. The Galilei group presents a certain number of essential differences with respect to the Poincaré group. As Bargmann showed, its physical representations, here explicitly constructed, are not true representations but only up-to-a-factor ones. Consequently, in nonrelativistic quantum mechanics, the mass has a very special role, and in fact, gives rise to a superselection rule which prevents the existence of unstable particles. The internal energy of a nonrelativistic system is known to be an arbitrary parameter; this is shown to come also from Galilean invariance, because of a nontrivial concept of equivalence between physical representations. On the contrary, the behavior of an elementary system with respect to rotations, is very similar to the relativistic case. We show here, in particular, how the number of polarization states reduces to two for the zero-mass case (though in fact there are no physical zero-mass systems in nonrelativistic mechanics). Finally, we study the two-particle system, where the orbital angular momenta quite naturally introduce themselves through the decomposition of the tensor product of two physical representations.
The applicability of the time-dependent Schrödinger equation to molecular reaction dynamics is discussed from several perspectives. First, with reference to Schrödinger's original paper, we point out that he himself stated that what is now called the time-dependent Schrödinger equation is insufficient to solve the problems of non-conservative systems like chemical reactions. Second, the detonation velocity of some explosives has been demonstrated as a new parameter that directly illustrates the timescales of elementary chemical reactions. The calculated values of ~30 fs are in good agreement with the timescale of 10–100 fs for chemical reactions, as experimentally demonstrated by the Nobel laureate Zewail. Third, we consider the state of an electron in a box when the length of the box suddenly increases from L to 2L, which is often found in textbooks as the simplest example of the application of the time-dependent Schrödinger equation to chemical reactions, and highlight the inconsistency between this model and an actual chemical reaction. Finally, we re-iterate that only quantum steady states can be accurately described by the Schrödinger equation and that it is the most natural to connect changes between adjacent quantum steady states by the variational principle.
The nonlinear equation of motion that accomplishes a self-consistent unification of quantum mechanics (QM) and thermodynamics conceptually different from the (von Neumann) foundations of quantum statistical mechanics (QSM) and (Jaynes) quantum information theory (QIT), but which reduces to the same mathematics for the thermodynamic equilibrium (TE) states, and contains standard QM in that it reduces to the time-dependent Schrodinger equation for zero entropy states is discussed in full mathematical detail. By restricting the discussion to a strictly isolated system (noninteracting, disentangled and uncorrelated), we show how the theory departs from the conventional QSM/QIT rationalization of the second law of thermodynamics, which instead emerges in QT (quantum thermodynamics) as a theorem of existence and uniqueness of a stable equilibrium state for each set of mean values of the energy and the number of constituent particles. To achieve this, the theory assumes Trρ lnρ B −k for the physical entropy and is designed to implement two fundamental ansatzs: (1) that in addition to the standard QM states described by idempotent density operators (zero entropy), a strictly isolated and uncorrelated system admits also states that must be described by non-idempotent density operators (nonzero entropy); (2) that for such additional states the law of causal evolution is determined by the simultaneous action of a Schrodinger-von Neumann-type Hamiltonian generator and a nonlinear dissipative generator which conserves the mean values of the energy and the number of constituent particles, and in forward time drives the density operator in the 'direction' of steepest entropy ascent (maximal entropy generation). The resulting dynamics is well defined for all non-equilibrium states, no matter how far from TE. Existence and uniqueness of solutions of the Cauchy initial value problem for all density operators implies that the equation of motion can be solved not only in forward time, to describe relaxation towards TE, but also in backward time, to reconstruct the 'ancestral' or primordial lowest entropy state or limit cycle from which the system originates. Zero entropy states as well as a well defined family of non-dissipative states evolve unitarily according to pure Hamiltonian dynamics and can be viewed as unstable limit cycles of the general nonlinear dynamics.