Linked Diagram Expansion Research Articles

Linked-Diagram Expansion for the Equation of State of a Gas of Molecules

The method of the linked-diagram expansion is applied to the equilibrium statistical mechanics of a nondegenerate gas of molecules, taking into account the structure of the molecules, and using the Pauli principle for all the electrons. The equation of state is obtained in the form of a virial expansion analogous to that of Ursell and Mayer.

Sur la détermination des premiers états d'un système de fermions dans le cas dégénéré

The linked diagram expansion of the ground state energy and wave function of a system of fermions are extended to the case where the ground state of the unperturbed system is degenerate, as, for instance, in a nucleus with an incomplete shell. The state where all the complete shells are filled is taken as vacuum state. It is then shown, to all orders in the perturbation expansion, that the energy of the first levels is given by the sum of two terms. The first term, is the energy of the core alone and is given by a linked diagram expansion of the usual form. The second term is the energy of the particles outside the core. It is obtained by solving an eigenvalue problem in the space of the degenerate unperturbed wave functions. The corresponding eigen-functions are given by linked diagram expansions very similar with the usual ones. A few generalizations of the method, and its relation to the Brillouin-Wigner perturbation method are discussed.

Sur la détermination de l'état fondamental d'un système de particules

The linked diagram expansions of the ground state energy and wave function of a system of particles are derived by a method involving the use of the time-dependent as well as the time-independent formulations of perturbation theory. The essential step is the derivation, in the time-dependent formulation, of a simple exponential formula obtained by summation of the disconnected vacuum-vacuum diagrams.