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Physical Review

Relation between the Brueckner-Frohberg and Nosanow Cluster Expansions

Samuel B. Trickey

Published Feb 5, 1968• 8 citations
Publisher

Abstract

We demonstrate the existence of a relation between the partial sums of the cluster expansion derived by Brueckner and Frohberg in their treatment of quantum crystals, and the partial sums of the expansion obtained by Nosanow for the same problem. Such a relation was found for a particular special case by Hetherington, Mullin, and Nosanow, and we show that with certain refinements their result holds for the general term.

DOI
10.1103/physrev.166.177

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We demonstrate the existence of a relation between the partial sums of the cluster expansion derived by Brueckner and Frohberg in their treatment of quantum crystals, and the partial sums of the expansion obtained by Nosanow for the same problem. Such a relation was found for a particular special case by Hetherington, Mullin, and Nosanow, and we show that with certain refinements their result holds for the general term.
In order to go over from a perturbation expansion of the grand partition function (the unlinked-cluster expansion) to an expansion of the thermodynamic potential (the linked-cluster expansion) in powers of the interaction, it is necessary to treat carefully those terms in which creation (or annihilation) operators for the same state occur twice or more. The unlinked- and linked-cluster expansions for a system of fermions are here shown to be equivalent by a direct comparison of the terms which occur in each. The relation between the two expansions is illustrated by the example of a system of fermions interacting only with a single-particle potential.
We give a brief outline of the perturbative expansion and the cluster properties of nondiagonal matrix elements in correlated-basis theory. A structural study of the perturbative series shows the presence of systematic cancellations among the terms of the series. It is proved by virtue of these cancellations that the expansion is linked and closely resembles the perturbative expansion of Brueckner-Goldstone theory. It is found that the ladder diagrams can be summed up by solving a Bethe-Goldstone---type equation which has recently been derived in correlated coupled-cluster theory. A brief discussion of some properties of ring diagrams is also given.
An approximation method developed previously to deal with many particles in strong interaction is examined in further detail. It is shown that the series giving the interaction energy is a development in a sequence of linked or irreducible cluster terms each of which gives a contribution to the energy proportional to the total number of particles. Consequently the convergence of the expansion is independent of the total number of particles. The origin of this simple feature is illustrated by showing that a similar situation exists in the expansion of standard perturbation theory. The numerical convergence of the expansion is quantitatively discussed for the nuclear problem where it is shown that the correction arising from the first cluster term involving three particles is less than the leading term by a factor of about ${10}^{\ensuremath{-}4}$. The smallness of the correction is largely a result of the action of the exclusion principle.
The usual perturbation treatment of the many-body problem is modified by eliminating the two-body interaction and introducing in its place a ttt matrix. The Watson, Brueckner, and Bethe (W.B.B.) treatment of the many-body problem is shown to be a special case of the modified perturbation treatment. The modified perturbation treatment involves no assumption except that of supposing the convergence of the perturbation expansion. Thus to each order in the expansion the results must be meaningful and no unphysical "unlinked cluster" terms can appear. Within the framework of the modified perturbation treatment it is easy to interpret the concepts of the Pauli principle for intermediate states and the self-consistent choice of the comparison potential which play such an important role in the W.B.B. treatment. This work is similar to that of J. Goldstone in that we relate the W.B.B. treatment to a perturbation treatment of the many-body problem. We differ from Goldstone in that we use the usual time-independent perturbation theory instead of the time-dependent one. The time-independent approach permits a direct comparison with the W.B.B. treatment and facilitates discussion of the Pauli principle for intermediate states and of the self-consistent potential.
A perturbation theory is described which leads to two alternative methods for calculating the eigenvalues and normalized eigenstates of a perturbed Hamiltonian, the results being given to all orders in the perturbation. The expansions obtained are formulated in terms of two sets of partial sums which have simple, easily characterized forms, and they combine some of the advantages of both the Rayleigh-Schr\"odinger and Brillouin-Wigner perturbation methods while avoiding some of the difficulties of each. In particular, one form of the perturbation expansions given here will in most cases of interest have a larger radius of convergence than the Rayleigh-Schr\"odinger series. As is usual for infinite-order treatments, the explicit form of the final result applies only to nondegenerate levels. However, some of the intermediate relations obtained in the derivation are of general validity. Since certain classes of unlinked diagrams appear in the expansions, the method described here is not suitable for application to systems containing a large number of particles.