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we know the solution here, just the quadratic formula x= p 2 4ac 2a: (31.4) But suppose we didn’t have/remember this. The idea behind perturbation theory is to attempt to solve (31.3), given the The ket $$|n^i \rangle$$ is multiplied by $$\lambda^i$$ and is therefore of order $$(H^1/H^o)^i$$. These series are then fed into Equation $$\ref{7.4.2}$$, and terms of the same order of magnitude in $$\hat{H}^1/\hat{H}^o$$ on the two sides are set equal. \infty & x< 0 \; and\; x> L \end{cases} \nonumber\]. Have questions or comments? Bibliography First order perturbation theory will give quite accurate answers if the energy shiftscalculated are (nonzero and) … We know that the unperturbed harmonic oscillator wavefunctions $$\{|n^{0}\} \rangle$$ alternate between even (when $$v$$ is even) and odd (when $$v$$ is odd) as shown previously. Perturbation Theory is developed to deal with small corrections to problems which wehave solved exactly, like the harmonic oscillator and the hydrogen atom. Beneficial to both beginning students and researchers, Asymptotic Analysis and Perturbation Theory immediately introduces asymptotic notation and then applies this tool to familiar problems, including limits, inverse functions, and integrals. Problems and Solutions Exercises, Problems, and Solutions Section 1 Exercises, Problems, and Solutions Review Exercises 1. \infty & x< 0 \;\text{and} \; x> L \end{cases} \nonumber\]. Taking the inner product of both sides with $$\langle n^o |$$: $\langle n^o | \hat{H}^o | n^1 \rangle + \langle n^o | \hat{H}^1 | n^o \rangle = \langle n^o | E_n^o| n^1 \rangle + \langle n^o | E_n^1 | n^o \rangle \label{7.4.14}$, since operating the zero-order Hamiltonian on the bra wavefunction (this is just the Schrödinger equation; Equation $$\ref{Zero}$$) is, $\langle n^o | \hat{H}^o = \langle n^o | E_n^o \label{7.4.15}$, and via the orthonormality of the unperturbed $$| n^o \rangle$$ wavefunctions both, $\langle n^o | n^o \rangle = 1 \label{7.4.16}$, and Equation $$\ref{7.4.8}$$ can be simplified, $\bcancel{E_n^o \langle n^o | n^1 \rangle} + \langle n^o | H^1 | n^o \rangle = \bcancel{ E_n^o \langle n^o | n^1 \rangle} + E_n^1 \cancelto{1}{\langle n^o | n^o} \rangle \label{7.4.14new}$, since the unperturbed set of eigenstates are orthogonal (Equation \ref{7.4.16}) and we can cancel the other term on each side of the equation, we find that, $E_n^1 = \langle n^o | \hat{H}^1 | n^o \rangle \label{7.4.17}$. {E=\frac{1}{2} h v+\gamma \frac{3}{4 a^2}} Periodic Perturbation. For this system, the unperturbed Hamilonian and solutions is the particle in an infiinitely high box and the perturbation is a shift of the potential within the box by $$V_o$$. Watch the recordings here on Youtube! At this stage, the integrals have to be manually calculated using the defined wavefuctions above, which is left as an exercise. The first step in any perturbation problem is to write the Hamiltonian in terms of a unperturbed component that the solutions (both eigenstates and energy) are known and a perturbation component (Equation $$\ref{7.4.2}$$). \begin{array}{c} Newton's equation only allowed the mass of two bodies to be analyzed. Perturbation theory gives us a method for relating the problem that can be solved exactly to the one that cannot. Sudden Displacement of SHO. The gradually increasing accuracy of astronomical observations led to incremental demands in the accuracy of solutions to Newton's gravitational equations, which led several notable 18th and 19th century mathematicians, such as Lagrange and Laplace, to extend and generalize the methods of perturbation theory. E^{1} &=2 \gamma\left(\frac{a}{\pi}\right)^{\left(\frac{1}{2}\right)} \frac{1\cdot 3}{2^{3} a^2}\left(\frac{\pi}{a}\right)^{\frac{1}{2}}\end{aligned} \nonumber\]. It is truncating this series as a finite number of steps that is the approximation. The series does not converge. The equations thus generated are solved one by one to give progressively more accurate results. Introduction to perturbation theory 1.1 The goal of this class The goal is to teach you how to obtain approximate analytic solutions to applied-mathematical problems that can’t be solved “exactly”. The problem of the perturbation theory is to find eigenvalues and eigenfunctions of the perturbed potential, i.e. The basic principle is to find a solution to a problem that is similar to the one of interest and then to cast the solution to the target problem in terms of parameters related to the known solution. We say H(q;p;t) = H 0(q;p;t) + H … Semiclassical approximation. For example, the first order perturbation theory has the truncation at $$\lambda=1$$. Collecting the zero order terms in the expansion (black terms in Equation $$\ref{7.4.10}$$) results in just the Schrödinger Equation for the unperturbed system, $\hat{H}^o | n^o \rangle = E_n^o | n^o \rangle \label{Zero}$. This is essentially a step function. Another point to consider is that many of these matrix elements will equal zero depending on the symmetry of the $$\{| n^o \rangle \}$$ basis and $$H^1$$ (e.g., some $$\langle m^o | H^1| n^o \rangle$$ integrals in Equation $$\ref{7.4.24}$$ could be zero due to the integrand having an odd symmetry; see Example $$\PageIndex{3}$$). However, in this case, the first-order perturbation to any particle-in-the-box state can be easily derived. The Problem Book in Quantum Field Theory contains about 200 problems with solutions or hints that help students to improve their understanding and develop skills necessary for pursuing the subject. References: Grifﬁths, David J. Notice that each unperturbed wavefunction that can "mix" to generate the perturbed wavefunction will have a reciprocally decreasing contribution (w.r.t. In this chapter we will discuss time dependent perturbation theory in classical mechanics. Perturbation problems depend on a small positive parameter. Perturbation theory has been widely used in almost all areas of science. Knowledge of perturbation theory offers a twofold benefit: approximate solutions often reveal the exact solution's essential dependence on specified parameters; also, some problems resistant to numerical solutions may yield to perturbation methods. V_o & 0\leq x\leq L \\ Some texts and references on perturbation theory are , , and . A critical feature of the technique is a middle step that breaks the problem into "solvable" and "perturbation" parts. Many problems we have encountered yield equations of motion that cannot be solved ana-lytically. Legal. Suitable for those who have completed the standard calculus sequence, the book assumes no prior knowledge of differential equations. The harmonic oscillator wavefunctions are often written in terms of $$Q$$, the unscaled displacement coordinate: $| \Psi _v (x) \rangle = N_v'' H_v (\sqrt{\alpha} Q) e^{-\alpha Q^2/ 2} \nonumber$, $\alpha =1/\sqrt{\beta} = \sqrt{\dfrac{k \mu}{\hbar ^2}} \nonumber$, $N_v'' = \sqrt {\dfrac {1}{2^v v!}} Let's consider only the first six wavefunctions that use these Hermite polynomials $$H_v (x)$$: The first order perturbation to the ground-state wavefunction (Equation $$\ref{7.4.24}$$), \[ | 0^1 \rangle = \sum _{m \neq 0}^5 \dfrac{|m^o \rangle \langle m^o | H^1| 0^o \rangle }{E_0^o - E_m^o} \label{energy1}$. In this chapter, we describe the aims of perturbation theory in general terms, and give some simple illustrative examples of perturbation problems. The problem, as we have seen, is that solving (31.1) for all but the simplest potentials can be di cult. Approximate methods. As long as the perburbation is small compared to the unperturbed Hamiltonian, perturbation theory tells us how to correct the solutions to the unperturbed problem to approximately account for the influence of the perturbation. Fermi’s Golden Rule . One typically obtains a convergent expansion of the solution with respect to ", consisting of the unperturbed solution and higher- … More often one is faced with a potential or a Hamiltonian for which exact methods are unavailable and approximate solutions must be found. Berry's Phase. The problem of the perturbation theory is to find eigenvalues and eigenfunctions of the perturbed potential, i.e. We turn now to the problem of approximating solutions { our rst (and only, at this stage) tool will be perturbation theory. The basic idea here should be very familiar: perturbation theory simply means finding solutions to an otherwise intractable system by systematically expanding in some small parameter. As with Example $$\PageIndex{1}$$, we recognize that unperturbed component of the problem (Equation $$\ref{7.4.2}$$) is the particle in an infinitely high well. We can use symmetry of the perturbation and unperturbed wavefunctions to solve the integrals above. theory . $E_n^1 = \int_0^L \sqrt{\dfrac{2}{L}} \sin \left ( \dfrac {n \pi}{L} x \right) V_o \sqrt{\dfrac{2}{L}} \sin \left ( \dfrac {n \pi}{L} x \right) dx \nonumber$, or better yet, instead of evaluating this integrals we can simplify the expression, $E_n^1 = \langle n^o | H^1 | n^o \rangle = \langle n^o | V_o | n^o \rangle = V_o \langle n^o | n^o \rangle = V_o \nonumber$, so via Equation $$\ref{7.4.17.2}$$, the energy of each perturbed eigenstate is, \begin{align*} E_n &\approx E_n^o + E_n^1 \\[4pt] &\approx \dfrac{h^2}{8mL^2}n^2 + V_o \end{align*}. There exist only a handful of problems in quantum mechanics which can be solved exactly. Matching the terms that linear in $$\lambda$$ (red terms in Equation $$\ref{7.4.12}$$) and setting $$\lambda=1$$ on both sides of Equation $$\ref{7.4.12}$$: $\hat{H}^o | n^1 \rangle + \hat{H}^1 | n^o \rangle = E_n^o | n^1 \rangle + E_n^1 | n^o \rangle \label{7.4.13}$. Note that the zeroth-order term, of course, just gives back the unperturbed Schrödinger Equation (Equation $$\ref{7.4.1}$$). Use perturbation theory to approximate the wavefunctions of systems as a series of perturbation of a solved system. \nonumber \]. In quantum mechanics, there are large differences in how perturbations are handled depending on whether they are time-dependent or not. The first step when doing perturbation theory is to introduce the perturbation factor ϵ into our problem. This is, to some degree, an art, but the general rule to follow is this. Exercise $$\PageIndex{3}$$: Harmonic Oscillator with a Quartic Perturbation, Use perturbation theory to estimate the energy of the ground-state wavefunction associated with this Hamiltonian, $\hat{H} = \dfrac{-\hbar}{2m} \dfrac{d^2}{dx^2} + \dfrac{1}{2} kx^2 + \gamma x^4 \nonumber.$, The model that we are using is the harmonic oscillator model which has a Hamiltonian, $H^{0}=-\frac{\hbar}{2 m} \frac{d^2}{dx^2}+\dfrac{1}{2} k x^2 \nonumber$, To find the perturbed energy we approximate it using Equation \ref{7.4.17.2}, $E^{1}= \langle n^{0}\left|H^{1}\right| n^{0} \rangle \nonumber$, where is the wavefunction of the ground state harmonic oscillator, $n^{0}=\left(\frac{a}{\pi}\right)^{\left(\frac{1}{4}\right)} e^{-\frac{ax^2}{2}} \nonumber$, When we substitute in the Hamiltonian and the wavefunction we get, $E^{1}=\left\langle\left(\frac{a}{\pi}\right)^{\left(\frac{1}{4}\right)} e^{-\frac{ax^2}{2}}\right|\gamma x^{4}\left|\left(\frac{a}{\pi}\right)^{\left(\frac{1}{4}\right)} e^{-\frac{ax^2}{2}} \right \rangle \nonumber$. System Upgrade on Fri, Jun 26th, 2020 at 5pm (ET) During this period, our website will be offline for less than an hour but the E-commerce and registration of new users may not be available for up to 4 hours. Use perturbation theory to approximate the energies of systems as a series of perturbation of a solved system. Perturbation Theory In this chapter we will discuss time dependent perturbation theory in classical mechanics. We use cookies to help provide and enhance our service and tailor content and ads. Using Equation $$\ref{7.4.17}$$ for the first-order term in the energy of the ground-state, $E_n^1 = \langle n^o | H^1 | n^o \rangle \nonumber$, with the wavefunctions known from the particle in the box problem, $| n^o \rangle = \sqrt{\dfrac{2}{L}} \sin \left ( \dfrac {n \pi}{L} x \right) \nonumber$. Adding the full expansions for the eigenstate (Equation $$\ref{7.4.5}$$) and energies (Equation $$\ref{7.4.6}$$) into the Schrödinger equation for the perturbation Equation $$\ref{7.4.2}$$ in, $( \hat{H}^o + \lambda \hat{H}^1) | n \rangle = E_n| n \rangle \label{7.4.9}$, $(\hat{H}^o + \lambda \hat{H}^1) \left( \sum _ {i=0}^m \lambda ^i| n^i \rangle \right) = \left( \sum_{i=0}^m \lambda^i E_n^i \right) \left( \sum _ {i=0}^m \lambda ^i| n^i \rangle \right) \label{7.4.10}$. Perturbation theory allows one to find approximate solutions to the perturbed eigenvalue problem by beginning with the known exact solutions of the unperturbed problem and then making small corrections to it based on the new perturbing potential. The first step when doing perturbation theory is to introduce the perturbation factor $$\epsilon$$ into our problem. Neutron Magnetic Moment. 7.4: Perturbation Theory Expresses the Solutions in Terms of Solved Problems, [ "article:topic", "Perturbation Theory", "showtoc:no" ], 7.3: Trial Functions Can Be Linear Combinations of Functions That Also Contain Variational Parameters, First-Order Expression of Energy ($$\lambda=1$$), First-Order Expression of Wavefunction ($$\lambda=1$$), harmonic oscillator wavefunctions being even, information contact us at info@libretexts.org, status page at https://status.libretexts.org. This is, to some degree, an art, but the general rule to follow is this. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. A regular perturbation problem is one for which the perturbed problem for small, nonzero values of "is qualitatively the same as the unperturbed problem for "= 0. \left(\dfrac{\alpha}{\pi}\right)^{1/4} \nonumber\]. Dyson series 11.2.3 . However, the denominator argues that terms in this sum will be weighted by states that are of. Neutron in Rotating Magnetic Field. To leave a comment or report an error, please use the auxiliary blog. $$\hat{H}^{o}$$ is the Hamitonian for the standard Harmonic Oscillator with, $$\hat{H}^{1}$$ is the pertubtiation $\hat{H}^{1} = \epsilon x^3 \nonumber$. To find the 1st-order energy correction due to some perturbing potential, beginwith the unperturbed eigenvalue problem If some perturbing Hamiltonian is added to the unperturbed Hamiltonian, thetotal H… This method, termed perturbation theory, is the single most important method of solving problems in quantum mechanics and is widely used in atomic physics, condensed matter and particle physics. Because of the complexity of many physical problems, very few can be solved exactly (unless they involve only small Hilbert spaces). Therefore the energy shift on switching on the perturbation cannot be represented as a power series in $$\lambda$$, the strength of the perturbation. One example is planetary motion, which can be treated as a perturbation on a problem in which the planets … It is easier to compute the changes in the energy levels and wavefunctions with a scheme of successive corrections to the zero-field values. The summations in Equations $$\ref{7.4.5}$$, $$\ref{7.4.6}$$, and $$\ref{7.4.10}$$ can be truncated at any order of $$\lambda$$. Equation $$\ref{7.4.13}$$ is the key to finding the first-order change in energy $$E_n^1$$. Sudden Perturbation of Two-level Atom. – Local (or asymptotic) bounds. Electron Passing Through Magnetic Field. \end{array} Short lecture on an example application of perturbation theory. The first step in a perturbation theory problem is to identify the reference system with the known eigenstates and energies. V_o & 0\leq x\leq L/2 \\ The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. For this example, this is clearly the harmonic oscillator model. In this monograph we present basic concepts and tools in perturbation theory for the solution of computational problems in finite dimensional spaces. Chapter 7 Perturbation Theory. There exist only a handful of problems in quantum mechanics which can be solved exactly. There are higher energy terms in the expansion of Equation $$\ref{7.4.5}$$ (e.g., the blue terms in Equation $$\ref{7.4.12}$$), but are not discussed further here other than noting the whole perturbation process is an infinite series of corrections that ideally converge to the correct answer. to solve approximately the following equation: using the known solutions of the problem Perturbation theory is a useful method of approximation when a problem is very similar to one that has exact solutions. The task is to find how these eigenstates and eigenenergies change if a small term $$H^1$$ (an external field, for example) is added to the Hamiltonian, so: $( \hat{H}^0 + \hat{H}^1 ) | n \rangle = E_n | n \rangle \label{7.4.2}$. That is to say, on switching on $$\hat{H}^1$$ changes the wavefunctions: $\underbrace{ | n^o \rangle }_{\text{unperturbed}} \Rightarrow \underbrace{|n \rangle }_{\text{Perturbed}}\label{7.4.3}$, $\underbrace{ E_n^o }_{\text{unperturbed}} \Rightarrow \underbrace{E_n }_{\text{Perturbed}} \label{7.4.4}$. Calculating the first order perturbation to the wavefunctions (Equation $$\ref{7.4.24}$$) is more difficult than energy since multiple integrals must be evaluated (an infinite number if symmetry arguments are not applicable). This occurrence is more general than quantum mechanics {many problems in electromagnetic theory are handled by the techniques of perturbation theory. In fact, numerical and perturbation methods can be combined in a complementary way. For this case, we can rewrite the Hamiltonian as, The first order perturbation is given by Equation $$\ref{7.4.17}$$, which for this problem is, $E_n^1 = \langle n^o | \epsilon x^3 | n^o \rangle \nonumber$, Notice that the integrand has an odd symmetry (i.e., $$f(x)=-f(-x)$$) with the perturbation Hamiltonian being odd and the ground state harmonic oscillator wavefunctions being even. FIRST ORDER NON-DEGENERATE PERTURBATION THEORY Link to: physicspages home page. Time-independent perturbation theory Variational principles. 11.1 Time-independent perturbation . Excitation of Electron by Electric Field. However, changing the sign of $$\lambda$$ to give a repulsive potential there is no bound state, the lowest energy plane wave state stays at energy zero. So of the original five unperturbed wavefunctions, only $$|m=1\rangle$$, $$|m=3\rangle$$, and $$|m=5 \rangle$$ mix to make the first-order perturbed ground-state wavefunction so, $| 0^1 \rangle = \dfrac{ \langle 1^o | H^1| 0^o \rangle }{E_0^o - E_1^o} |1^o \rangle + \dfrac{ \langle 3^o | H^1| 0^o \rangle }{E_0^o - E_3^o} |3^o \rangle + \dfrac{ \langle 5^o | H^1| 0^o \rangle }{E_0^o - E_5^o} |5^o \rangle \nonumber$. Changing this into integral form, and combining the wavefunctions, \begin{align*} E^{1} &=\int_{-\infty}^{\infty}\left(\frac{a}{\pi}\right)^{\left(\frac{1}{2}\right)} e^{\frac{-ax^2}{2}} \gamma x^{4} dx \\[4pt] &=\gamma\left(\frac{a}{\pi}\right)^{\frac{1}{2}} \int_{-\infty}^{\infty} x^{4} e^{-a x^2} d x \end{align*}, $\int_{0}^{\infty} x^{2 \pi} e^{-a x^2} dx=\frac{1 \cdot 3 \cdot 5 \ldots (2 n-1)}{2^{m+1} a^{n}}\left(\frac{\pi}{a}\right)^{\frac{1}{2}} \nonumber$, Where we plug in $$\mathrm{n}=2$$ and $$\mathrm{a}=\alpha$$ for our integral, \begin{aligned}E^{1} &=2 \gamma\left(\frac{a}{\pi}\right)^{\left(\frac{1}{2}\right)} \int_{0}^{\infty} x^{4} e^{-a x^2} d x \\ Perturbation theory is a vast collection of mathematical methods used to obtain approximate solution to problems that have no closed-form analytical solution. It’s just there to keep track of the orders of magnitudes of the various terms. We’re now ready to match the two sides term by term in powers of $$\lambda$$. In this monograph we present basic concepts and tools in perturbation theory for the solution of computational problems in finite dimensional spaces. Review of interaction picture 11.2.2 . Perturbation theory is a very broad subject with applications in many areas of the physical sciences. More often one is faced with a potential or a Hamiltonian for which exact methods are unavailable and approximate solutions must be found. To leave a comment or report an error, please use the auxiliary blog. Expanding Box. Here, we will consider cases where the problem we want to solve with Hamiltonian H(q;p;t) is \close" to a problem with Hamiltonian H One such case is the one-dimensional problem of free particles perturbed by a localized potential of strength $$\lambda$$. Excitation of H-atom. A central theme in Perturbation Theory is to continue equilibriumand periodic solutionsto the perturbed system, applying the Implicit Function Theorem.Consider a system of differential equations Equilibriaare given by the equation Assuming that and thatthe Implicit Function Theorem guarantees existence of a l… Using Equation $$\ref{7.4.17}$$ for the first-order term in the energy of the any state, \[ \begin{align*} E_n^1 &= \langle n^o | H^1 | n^o \rangle \\[4pt] &= \int_0^{L/2} \sqrt{\dfrac{2}{L}} \sin \left ( \dfrac {n \pi}{L} x \right) V_o \sqrt{\dfrac{2}{L}} \sin \left ( \dfrac {n \pi}{L} x \right) dx + \int_{L/2}^L \sqrt{\dfrac{2}{L}} \sin \left ( \dfrac {n \pi}{L} x \right) 0 \sqrt{\dfrac{2}{L}} \sin \left ( \dfrac {n \pi}{L} x \right) dx \end{align*}, The second integral is zero and the first integral is simplified to, $E_n^1 = \dfrac{2}{L} \int_0^{L/2} V_o \sin^2 \left( \dfrac {n \pi}{L} x \right) dx \nonumber$, \begin{align*} E_n^1 &= \dfrac{2V_o}{L} \left[ \dfrac{-1}{2 \dfrac{\pi n}{a}} \cos \left( \dfrac {n \pi}{L} x \right) \sin \left( \dfrac {n \pi}{L} x \right) + \dfrac{x}{2} \right]_0^{L/2} \\[4pt] &= \dfrac{2V_o}{\cancel{L}} \dfrac{\cancel{L}}{4} = \dfrac{V_o}{2} \end{align*}, The energy of each perturbed eigenstate, via Equation $$\ref{7.4.17.2}$$, is, \begin{align*} E_n &\approx E_n^o + \dfrac{V_o}{2} \\[4pt] &\approx \dfrac{h^2}{8mL^2}n^2 + \dfrac{V_o}{2} \end{align*}. The technique is appropriate when we have a potential V(x) that is closely By continuing you agree to the use of cookies. At this stage we can do two problems independently (i.e., the ground-state with $$| 1 \rangle$$ and the first excited-state $$| 2 \rangle$$). It also happens frequently that a related problem can be solved exactly. {E=E^{0}+E^{1}} \\ (2005), Introduction to Quantum Mechan-ics, 2nd Edition; Pearson Education - Problem … Our intention is to use time-independent perturbation theory for the de-generate case. The general approach to perturbation theory applications is giving in the flowchart in Figure $$\PageIndex{1}$$. For instance, Newton's law of universal gravitation explained the gravitation between two astronomical bodies, but when a third body is added, the problem was, "How does each body pull on each?" The energy difference in the denominators goes to zero and the corrections are no longer small. where $$m$$ is how many terms in the expansion we are considering. The first-order change in the energy of a state resulting from adding a perturbing term $$\hat{H}^1$$ to the Hamiltonian is just the expectation value of $$\hat{H}^1$$ in the unperturbed wavefunctions. The degeneracy is 8: we have a degeneracy n2 = 4 without spin and then we take into account the two possible spin states (up and down) in the basis |L2,S2,L z,S zi. Singular perturbation problems are of common occurrence in all branches of applied mathematics and engineering. The strategy is to expand the true wavefunction and corresponding eigenenergy as series in $$\hat{H}^1/\hat{H}^o$$. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Example $$\PageIndex{2}$$: A Harmonic Oscillator with a Cubic Perturbation, Estimate the energy of the ground-state wavefunction associated with the Hamiltonian using perturbation theory, $\hat{H} = \dfrac{-\hbar}{2m} \dfrac{d^2}{dx^2} + \dfrac{1}{2} kx^2 + \epsilon x^3 \nonumber$. Many problems we have encountered yield equations of motion that cannot be solved ana-lytically. While this is the first order perturbation to the energy, it is also the exact value. In the following derivations, let it be assumed that all eigenenergies andeigenfunctions are normalized.