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Progress of Theoretical Physics

Reordering into Normal Products, the Commutation Relation and the Particle-Hole Relation in the Many-Body Shell Model Bases

Masao Nomura

Published Feb 1, 1978• 6 citations
Publisher

Abstract

Treatment of a general operator in the shell model bases is investigated as a reformulation and a slight generalization of a previous work. First, reordering of an operator into normal products is presented by the use of matrix elements in a few-body configurations instead of the use of the Wick theorem. Second, the commutation relation and the particle-hole relation in the many-body bases are presented each of which is equivalent to the cfp relation arising from the completeness of basic functions. Next, the operator whose matrix element fulfills particle-hole symmetry is constructed from an arbitrary operator. In turn, the operator is expressed as a sum of particle-hole symmetrized operators.

DOI
10.1143/ptp.59.451

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The Journal of Chemical Physics•Jul 8, 1997•301 citations
Treatment of a general operator in the shell model bases is investigated as a reformulation and a slight generalization of a previous work. First, reordering of an operator into normal products is presented by the use of matrix elements in a few-body configurations instead of the use of the Wick theorem. Second, the commutation relation and the particle-hole relation in the many-body bases are presented each of which is equivalent to the cfp relation arising from the completeness of basic functions. Next, the operator whose matrix element fulfills particle-hole symmetry is constructed from an arbitrary operator. In turn, the operator is expressed as a sum of particle-hole symmetrized operators.
Matrix elements for fermion and boson operators describing the creation or annihilation of particles with multiple angular-momentum quantum numbers can be factored into products of matrix elements of generalized operators, one for each quantum number. The generalized operators have nonzero matrix elements between states of arbitrary permutational symmetry. This factorization greatly increases the efficiency of shell-model calculations. We provide a physical interpretation for these generalized operators and introduce a simple operator-reordering procedure.
Reordering of operators in the second quantization formalism is described by using state operators as basic operators, where the state operator is either a cluster of creation operators or that of annihilation operators specified by a set of quantum numbers of any representation. The commutation relation between state operators is simply expressed in terms of the coefficient of fractional parentage (cfp), the factor specific to state operator (de) composition. Wick’s theorem, prevalently applied to a string of creation and annihilation operators, is extended to a string of state operators. Similar extension is done for the contraction theorem.
Graphical methods of spin algebras are used to derive the expressions for the matrix elements of the total particle-number-conserving operators in the basis of hole-particle states, adapted to the chain $\mathrm{U}({n}^{\ensuremath{'}}+{n}^{\ensuremath{'}\ensuremath{'}})\ensuremath{\supset}\mathrm{U}({n}^{\ensuremath{'}})\ensuremath{\bigotimes}\mathrm{U}({n}^{\ensuremath{'}\ensuremath{'}})$, where ${n}^{\ensuremath{'}}$ and ${n}^{\ensuremath{'}\ensuremath{'}}$ designate dimensions of particle and hole subspaces, respectively. The matrix elements are expressed as a product of segment values, each associated with one orbital level as in the particle formalism, and of an additional segment value, representing a linkage of the hole and particle subspaces. It is shown that the particle formalism segment values can be used throughout except for the link segment, whose possible values are derived. An example of hole-particle bases and of their graphical representations is given and the advantages of the hole-particle formalism in shell-model calculations are outlined. An extension of this formalism to particle-number-nonconserving operators, needed in applications involving the $m\mathrm{p}\ensuremath{-}n\mathrm{h}$ propagators with $m\ensuremath{\ne}n$, mmm, n=0n=0n=0, 1, and 2, is discussed.
A generalization of normal ordering and of Wick’s theorem with respect to an arbitrary reference function Φ as some generalized “physical vacuum” is formulated in a different (but essentially equivalent) way than that suggested previously by one of the present authors. Guiding principles are that normal order operators with respect to any reference state must be expressible as linear combinations of those with respect to the genuine vacuum, that the vacuum expectation value of a normal order operator must vanish (with respect to the vacuum to which it is in normal order), and that the well-known formalism for a single Slater determinant as physical vacuum must be contained as a special case. The derivation is largely based on the concepts of “Quantum Chemistry in Fock space,” which means that particle-number-conserving operators (excitation operators) play a central role. Nevertheless, the contraction rules in the frame of a generalized Wick theorem are derived, that hold for non-particle-number-conserving operators as well. The contraction rules are formulated and illustrated in terms of diagrams. The contractions involve the “residual n-particle density matrices” λ, which are the irreducible (non-factorizable) parts of the conventional n-particle density matrices γ, in the sense of a cumulant expansion for the density. A spinfree formulation is presented as well. The expression of the Hamiltonian in normal order with respect to a multiconfiguration reference function leads to a natural definition of a generalized Fock operator. MC-SCF-theory is easily worked out in this context. The paper concludes with a discussion of the excited configurations and the first-order interacting space, that underlies a perturbative coupled cluster type correction to the MCSCF function for an arbitrary reference function, and with general implications of the new formalism, that is related to “internally contracted multireference configuration interaction.” The present generalization of normal ordering is not only valid for arbitrary reference functions, but also if the reference state is an ensemble state.