Читать книгу Quantum Mechanics, Volume 3 - Claude Cohen-Tannoudji - Страница 174

Let us write the Hartree-Fock equations (87) in terms of wave functions: these equations are strictly equivalent to (87), written in terms of operators and kets, but their form is sometimes easier to use, in particular for numerical calculations.

Assuming the particles have a spin, we shall note the wave functions φν(r), with:

(91)

where the spin quantum number ν can take (2S + 1) values; according to the nature of the particles, the possible spins S are S = 0, S = 1/2, S = 1 etc. As in Complement E_XV (§ 2-d), we introduce a complete basis for the individual state space, built from kets that are all eigenvectors of the spin component along the quantization axis, with eigenvalue νk. For each value of n, the spin index ν takes on a given value νn and is not, therefore, an independent index. As for the potentials, we assume here again that V₁ is diagonal in ν, but that its diagonal elements may depend on ν. The interaction potential, however, is described by a function W₂ (r, r′) that only depends on r – r′, but does not act on the spins.

To obtain the matrix elements of in the representation {|r, ν)}, we use (85) after replacing the |θ〉 by the |φ〉 (we showed in § 3 that this was possible). We now multiply both sides by and , and sum over the subscripts k and l; we recognize in both sides the closure relations:

(92)

This leads to:

(93)

As in § C-5 of Chapter XV, we get the sum of a direct term (the term 1 in the central bracket) and an exchange term (the term in ηP_ex). This expression contains the same matrix element as relation (87) of Complement E_xv, the only difference being the presence of a coefficient in each term of the sum (plus the fact that the summation index goes to infinity).

(i) For the direct term, as we did in that complement, we insert a closure relation on the particle 2 position:

(94)

Since the interaction operator is diagonal in the position representation, the part of the matrix element of (93) that does not contain the exchange operator becomes:

(95)

The direct term of (93) is then written:

(96)

which is equivalent to relation (91) of Complement E_XV.

(ii) The exchange term is obtained by permutation of the two particles in the ket appearing on the right-hand side of (93); the diagonal character of W₂(1,2) in the position representation leads to the expression:

(97)

For the first scalar product to be non-zero, the subscript j must be such that νj = ν; in the same way, for the second product to be non-zero, we must have νj = ν′. For both conditions to be satisfied, we must impose ν = ν′, and the exchange term (93) is equal to:

(98)

where the summation is on all the values of j such that νj = ν: this term only exists if the two interacting particles are totally indistinguishable, which requires that they be in the same spin state (see the discussion in Complement E_XV).

We now define the direct and exchange potentials by:

(99)

The equalities (87) then lead to the Hartree-Fock equations in the position representation:

(100)

The general discussion of § 3-b can be applied here without any changes. These equations are both nonlinear and self-consistent, as the direct and exchange potentials are themselves functions of the solutions of the eigenvalue equations (100). This situation is reminiscent of the zero-temperature case, and we can, once again, look for solutions using iterative methods. The number of equations to be solved, however, is infinite and no longer equal to the finite number N, as already pointed out in § 3-c. The set of solutions must span the entire individual state space. Along the same line, in the definitions (99) of the direct and exchange potentials, the summations over j are not limited to N states, but go to infinity. However, even though the number of these wave functions is in principle infinite, it is limited in practice (for numerical calculations) to a high but finite number. As for the initial conditions to start the iteration process, one can choose for example the states and energies of a free fermion gas, but any other conjecture is equally possible.