Expectation values

If electronic wave packets are going to behave as particles, their wave packets must have properties like velocity and acceleration. A wave packet is not necessarily an eigenfunction to the operator related to a certain property or observable, for example, angular momentum or kinetic energy. A distribution of values is observed. The expectation value is defined as a kind of average value for the region of space where the electron can be found  

The number calculated is a real value, called the ppectation value of Q, since Q is a Hermitean operator. If T is an eigenfunction of Q, the expectation value is equal to the eigenvalue, as is easily seen in Equation 1.38. 

If the function wave T is normalized, the denominator in Equation 1.38 may be left out. Plane waves extended over the whole space cannot be normalized, of course. A set of functions TJ is said to be orthogonal or non-overlapping if 

---

If the whole range is to be represented by just one value (which of course gives no indication of the range of uncertainty), then the expectation value is used ... 

Within this work [7] a method and model to determine the optical transfer function (OTF) for the detector chain without detailed knowledge of the internal detector and camera characteristics was developed. The expected value of the signal S0.2 is calculated with... 

The general type of approach, that is, the comparison of an experimental heat of immersion with the expected value per square centimeter, has been discussed and implemented by numerous authors [21,22]. It is possible, for example, to estimate sv - sl from adsorption data or from the so-called isosteric heat of adsorption (see Section XVII-12B). In many cases where approximate relative areas only are desired, as with coals or other natural products, the heat of immersion method has much to recommend it. In the case of microporous adsorbents surface areas from heats of immersion can be larger than those from adsorption studies [23], but the former are the more correct [24]. 

Close inspection of equation (A 1.1.45) reveals that, under very special circumstances, the expectation value does not change with time for any system properties that correspond to fixed (static) operator representations. Specifically, if tlie spatial part of the time-dependent wavefiinction is the exact eigenfiinction ). of the Hamiltonian, then Cj(0) = 1 (the zero of time can be chosen arbitrarily) and all other (O) = 0. The second tenn clearly vanishes in these cases, which are known as stationary states. As the name implies, all observable properties of these states do not vary with time. In a stationary state, the energy of the system has a precise value (the corresponding eigenvalue of //) as do observables that are associated with operators that connmite with ft. For all other properties (such as the position and momentum). 

These moments are related to many physical properties. The Thomas-Kulm-Reiche sum rule says that. S (0) equals the number of electrons in the molecule. Other sum rules [36] relate S(2),, S (1) and. S (-l) to ground state expectation values. The mean static dipole polarizability is md = e-S(-2)/m,.J Q Cauchy expansion... 

Consider, at t = 0, some non-equilibrium ensemble density P g(P. q°) on the constant energy hypersurface S, such that it is nonnalized to one. By Liouville s theorem, at a later time t the ensemble density becomes ((t) t(p. q)), where q) is die function that takes die current phase coordinates (p, q) to their initial values time (0 ago the fimctioii ( ) is uniquely detemiined by the equations of motion. The expectation value of any dynamical variable ilat time t is therefore... 

Let B denote an observable value. Its expectation value at time t is given by... 

Because this current is given by a conmuitator, its equilibrium expectation value is zero. Using die first expression in (A3.2.43). the response function is given by... 

One can show that the expectation value of the Hamiltonian operator for the wavepacket in equation (A3.11.71 is ... 

The difference compared to equation B 1.13.2 or equation B 1.13.3 is the occurrence of the expectation value of the operator (the two-spin order), characterized by its own decay rate pjg and coupled to the one-spin longitudinal operators by the tenus 8j aud 5. We shall come back to the physical origin of these tenus below. 

The power spectmm of the image PS(k) is tiien given by tlie expectation value (jg) I k)) nomially... 

In NMR, the magnetization in the xy plane is detected, so it is the expectation value of the operator that is measured. This is just the unweighted sum of all the operators for the individual spins . It may be a fimction of several time variables (multi-dimensional experiments), including the tune during the acquisition. 

For such a function, variational optimization of the spin orbitals to make the expectation value ( F // T ) stationary produces [30] the canonical FIF equations... 

In this approach [51], the expectation value ( T // T ) / ( T ) is treated variationally and made stationary with respect to variations in the C and. coeflScients. The energy fiinctional is a quadratic function of the Cj coefficients, and so one can express the stationary conditions for these variables in the secular fonu... 

In this approach [ ], the LCAO-MO coefficients are detemiined first via a smgle-configuration SCF calculation or an MCSCF calculation using a small number of CSFs. The Cj coefficients are subsequently detemiined by making the expectation value ( P // T ) / ( FIT ) stationary. 

In contrast to variational metliods, perturbation tlieory and CC methods achieve their energies by projecting the Scln-ddinger equation against a reference fiinction (transition formula (size-extensive energies. 

QMC teclmiques provide highly accurate calculations of many-electron systems. In variational QMC (VMC) [112, 113 and 114], the total energy of the many-electron system is calculated as the expectation value of the Hamiltonian. Parameters in a trial wavefiinction are optimized so as to find the lowest-energy state (modem... 

The result of this approximation is that each mode is subject to an effective average potential created by all the expectation values of the other modes. Usually the modes are propagated self-consistently. The effective potentials governing die evolution of the mean-field modes will change in time as the system evolves. The advantage of this method is that a multi-dimensional problem is reduced to several one-dimensional problems. 

A quantum mechanical treatment of molecular systems usually starts with the Bom-Oppenlieimer approximation, i.e., the separation of the electronic and nuclear degrees of freedom. This is a very good approximation for well separated electronic states. The expectation value of the total energy in this case is a fiinction of the nuclear coordinates and the parameters in the electronic wavefunction, e.g., orbital coefficients. The wavefiinction parameters are most often detennined by tire variation theorem the electronic energy is made stationary (in the most important ground-state case it is minimized) with respect to them. The... 

---