Quantum Invariances

For a Hamiltonian which is invariant under the space inversion operator P it has already been shown that 

Space inversion symmetry therefore yields a conservation law for the physical quantity P, called parity. If the state of the system at the given time, is an eigenstate of P belonging to the eigenvalue 1, i.e. its parity is +1 or —1, the system must maintain this parity at any later time. 

The effects of parity conservation are not relevant for classical mechanics. Classical systems are in fact arrangements of mixed parity, so that no new information is obtained by taking their mirror images. 

The behaviour of a composite quantum system under space inversion may be affected if its constituent particles have intrinsic parity. Consider a composite bound system with a Hamiltonian which is invariant under space inversion. Let ma be the mass of constituent particle a with internal wave function ipa, -e. 

Multiplying the expression by P, remembering that P2 — /, shows that k = 1. That is, tpa is an eigenvector of P with eigenvalues of 1, 

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Here the last inequality holds due to the Schrodinger-Robertson uncertainty relation [271,279,321,322]. The meaning of the parameter d as the universal quantum invariant has also been discussed [323]. The minimal invariant variance (100) can be expressed in terms of S and d as... 

Applications of quantum mechanics to chemistry invariably deal with systems (atoms and molecules) that contain more than one particle. Apart from the hydrogen atom, the stationary-state energies caimot be calculated exactly, and compromises must be made in order to estimate them. Perhaps the most useful and widely used approximation in chemistry is the independent-particle approximation, which can take several fomis. Conuiion to all of these is the assumption that the Hamiltonian operator for a system consisting of n particles is approximated by tlie sum... 

Figure Al.2.7. Trajectory of two coupled stretches, obtained by integrating Hamilton s equations for motion on a PES for the two modes. The system has stable anhamionic synmretric and antisyimnetric stretch modes, like those illustrated in figrne Al.2.6. In this trajectory, semiclassically there is one quantum of energy in each mode, so the trajectory corresponds to a combination state with quantum numbers nj = [1, 1]. The woven pattern shows that the trajectory is regular rather than chaotic, corresponding to motion in phase space on an invariant torus.

However, the reader may be wondering, what is the connection of all of these classical notions—stable nonnal modes, regular motion on an invariant toms—to the quantum spectmm of a molecule observed in a spectroscopic experiment Recall that in the hannonic nonnal modes approximation, the quantum levels are defined by the set of quantum numbers (Up. . Uyy) giving the number of quanta in each of the nonnal modes. 

At this point the reader may feel that we have done little in the way of explaining molecular synnnetry. All we have done is to state basic results, nonnally treated in introductory courses on quantum mechanics, connected with the fact that it is possible to find a complete set of simultaneous eigenfiinctions for two or more commuting operators. However, as we shall see in section Al.4.3.2. the fact that the molecular Hamiltonian //coimmites with and F is intimately coimected to the fact that //commutes with (or, equivalently, is invariant to) any rotation of the molecule about a space-fixed axis passing tlirough the centre of mass of the molecule. As stated above, an operation that leaves the Hamiltonian invariant is a symmetry operation of the Hamiltonian. The infinite set of all possible rotations of the... 

The limit equation governing limj -,o qc can be motivated by referring to the quantum adiabatic theorem which originates from work of Born and FOCK [4, 20] The classical position g influences the Hamiltonian very slowly compared to the time scale of oscillations of in fact, infinitely slowly in the limit e — 0. Thus, in analogy to the quantum adiabatic theorem, one would expect that the population of the energy levels remain invariant during the evolution ... 

The eleetrostatie potential is not invariant under rotations of the eleetron about the x or y axes (those perpendieular to the moleeular axis), so Lx and Ly do not eommute with the Hamiltonian. Therefore, only Lz provides a "good quantum number" in the sense that the operator Lz eommutes with the Hamiltonian. 

Equation (4.24) indicates that the quantum number of the transverse x-vibration is an adiabatic invariant of the trajectory. At T=0 becomes the instantaneous zero-point spread of the transverse vibration (2co,) in agreement with the uncertainty principle. 

Taxonomic classification invariantly have been first (and frequently the only until now) constructive influence of quantum theory on a given area in chemistry, and numerical approximations to wavefunctions have been remarkably unproductive compared with the enormous amount of mathematical quantities handled by modern computers (Jorgensen [1986]). 

In addition most of the more tractable approaches in density functional theory also involve a return to the use of atomic orbitals in carrying out quantum mechanical calculations since there is no known means of directly obtaining the functional that captures electron density exactly. The work almost invariably falls back on using basis sets of atomic orbitals which means that conceptually we are back to square one and that the promise of density functional methods to work with observable electron density, has not materialized. 

Among the usual advantages of such expressions as Eq. (7-80) and (7-81), one is salient they show forth the invariance of p and w with respect to the choice of the basis functions, u, in terms of which p, a, and P are expressed. The trace, as will be recalled, is invariant against unitary transformations, and the passage from one basis to another is performed by such transformations. The trace is also indifferent to an exchange of the two matrix factors, which is convenient in calculations. Finally, the statistical matrix lends itself to a certain generalization of states from pure cases to mixtures, required in quantum statistics and the theory of measurements we turn to this question in Section 7.9. 

It turns out that this choice is correct and that L and L" can be so chosen that (11-144) is satisfied. We shall return to this question in our discussion of the asymptotic condition in Section 11.5 of the present chapter. As preparation for these considerations, we turn in the next few seotions to a discussion of the invariance properties of quantum electrodynamics and their consequences. 

Invariance Properties.—Before delving into the mathematical formulation of the invariance properties of quantum electrodynamics, let us briefly state what is meant by an invariance principle in general. As we shall be primarily concerned with the formulation of invariance principles in the Heisenberg picture, it is useful to introduce the concept of the complete description of a physical system. By this is meant at the classical level a specification of the trajectories of all particles together with a full description of all fields at all points of space for all time. The equations of motion then allow one to determine whether the system could, in fact, have evolved in the way... 

In a quantum mechanical framework, Postulate 1 remains as stated. It implies that there exists a well-defined connection and correspondence between the labels attributed to the space-time points by each observer, between the state vectors each observer attributes to a given physical system, and between observables of the system. Postulate 2 is usually formulated in terms of transition probabilities, and requires that the transition probability be independent of the frame of reference. It should be stated explicitly at this point that we shall formulate the notion of invariance in terms of the concept of bodily identity, wherein a single physical system is viewed by two observers who, in general, will have different relations to the system. 

Now in quantum theory the description of a physical system in the Heisenberg picture for a given observer O is by means of operators Q, which satisfy certain equations of motion and commutation rules with respect to O s frame of reference (coordinate system x). The above notion of an invariance principle can be stated alternatively as follows If, when we change this coordinate frame of reference (i.e., for observer O ) we are able to find a new set of operators that obeys the same equations of motion and the same commutation rules with respect to the new frame of reference (coordinate system x ) we then say that these observers are equivalent and the theory invariant under the transformation x - x. The observable consequences of theory in the new frame (for observer O ) will then clearly be the same as those in the old frame. 

The statement that quantum electrodynamics is invariant under such a spatial inversion (parity operation) can be taken as the statement that there exist new field operators >p (x ) and A x ) expressible in terms of tji(x) and Au(x) which satisfy the same commutation rules and equations of motion in terms of s as do ift(x) and A x) written in terms of x. In fact one readily verifies that the operators... 

We must next consider more precisely the connection between the description of bodily identical states by the two observers (the requirements of Postulate 1). Quite in general, in fact, a physical theory, and quantum electrodynamics in particular, is fully defined only if the connection between the description of bodily identical states by (equivalent) observers is known for every state of the system and for every pair of observers. Since the observers are equivalent every state which can be described by 0 can also be described by O. Given a bodily state of the same system, observer 0 will ascribe to it a state vector Y0> in his Hilbert space and observer O will attribute to it a state vector T0.) in his Hilbert space. The above formulation of invariance means that there exists a one-to-one correspondence between the vectors Y0> and Y0.) used by observers 0 and O to describe bodily the same state.3 This correspondence guarantees that the two Hilbert spaces are in fact isomorphic. It is, therefore, possible for the two observers to agree to describe states of the system by vectors in the same Hilbert space. A similar statement can be made for the observables there exists a one-to-one correspondence between the operators Q0 and Q0>, which observers 0 and O attribute to observables. The consistency of the theory (Postulate 2) demands, however, that the two observers make the same prediction as the outcome of the same experiment performed on bodily the same system. This requires the relation... 

The discussion at the beginning of this section, when coupled with the fact that the observers 0 and O agree to describe bodily the same state by the same state vector, has exhibited the invariance of quantum electrodynamics under space inversion in the Heisenberg-type description. 

Let us next adopt the Schrodinger-type description. The statement that quantum electrodynamics is invariant under space inversion can now be translated into the statement that there exists a unitary operator U(it) such that... 

Consider next the relativistic invariance of quantum electrodynamics. Again, loosely speaking, we say that quantum electrodynamics is relativistically invariant if its observable consequences are the same in all frames connected by an inhomogeneous Lorentz transformation a,A ... 

Invariance of Quantum Electrodynamics under Discrete Transformations.—In the present section we consider the invariance of quantum electrodynamics under discrete symmetry operations, such as space-inversion, time-inversion, and charge conjugation. 

As indicated at the beginning of the last section, to say that quantum electrodynamics is invariant under space inversion (x = ijX) means that we can find new field operators tfi (x ),A v x ) expressible in terms of fj(x) and A nix) which satisfy the same equations of motion and commutation rules with respect to the primed coordinate system (a = igx) as did tf/(x) and Av(x) in terms of x. Since the commutation rules are to be the same for both sets of operators and the set of realizable states must be invariant, there must exist a unitary (or anti-unitary) transformation connecting these two sets of operators if the theory is invariant. For the case of space inversions, such a unitary operator is... 

Particle-Antipartide Conjugation.—If quantum electrodynamics is invariant under space inversion, then it does not matter whether we employ a right- or left-handed coordinate system in the description of ptnely electrodynamio phenomena. To speak of right and left is, an arbitrary convention in a worlcl ip which only electrodynamics operates. 

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