TAILIEUCHUNG - Ideas of Quantum Chemistry P101

Ideas of Quantum Chemistry P101 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 966 G. VECTOR AND SCALAR POTENTIALS Fig. . How do we understand the arbitrariness of the vector potential A Figs. a b c represent schematically three physically equivalent vector potentials A. Fig. a shows a section in the plane z 0 axis z protrudes towards the reader from the xy plane of the vector field A 2 H x r with H 0 0 H and H 0. We see that vectors A become longer and longer when we leave the origin where A 0 they rotate counter-clockwise. Such A therefore determines H directed perpendicularly to the page and oriented towards the reader. By the way note that any shift of the potential obtained should give the same magnetic field perpendicular to the drawing Fig. b . This is what we get Fig. b after adding according to eq. the gradient of function f ax by c to potential A because A Vf A ia jb A R A where R ia jb const. The transformation is only one of the possibilities. If we took an arbitrary smooth function f x y . with many maxima minima and saddle points as in the mountains we would deform Fig. b by expanding or shrinking it like a pancake. In this way we might obtain the situation shown in Fig. c . All these situations a b c are physically indistinguishable on condition that the scalar potential u is changed appropriately . G. VECTOR AND SCALAR POTENTIALS 967 2 V exp V exp - lt-X h c 7 h2 - exp 2m I -t xJA W - h c iq h c -21 expf- 1- X Vx 2 h c h c exp -- XjAx h c 7 - 2 V 2m j- exp - x Vx h c h c 7 Vexp - xh- h c Dividing the Schrodinger equation by exp - x we obtain - A-X l W- l- - f- l- - VX 2 Ax 2 V - l- - vX 2m L h c L h c J h c EW r . Let us define a vector field A r using function x r A r V x r . Hence we have h2 T iq -----A l - 2m L h c f-21 a2 v a h c 2 V VW E r and introducing the momentum operator p -ihV we obtain p2W f- A2 - f- p4 2m c c - 2 pW - A c VW VW E r or finally p--a VW E 2m c which is the equation corresponding to the particle moving in electromagnetic field with vector potential A see p. 654. Indeed the last .

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