Abstract A many-body variational calculation is carried out using a trial wave function which is a product of pair functions. An exact integro-differential equation for the pair function is derived, valid for any system of more than three Bose particles.
It is shown that, by means of a parametric representation, a variational calculation for a system of Bose particles can be carried through using explicit trial wave functions which describe a state with a fixed number of particles and include pair correlations. For the ground state the results of previous work are obtained as the first term in a steepest descent calculation. An effective Schrodinger equation is obtained for the relative motion of two particles. The application of the method to excited states is discussed; in particular for the case of a vortex line in the system it is shown that the method leads to an analysis which is similar to that for the ground state and could be carried through in the same way.
A trial solution constructed from two-particle functions is applied to the problem of the N-particle system with strong interactions. In a variational treatment based on this wave function the expectation values, which cannot be factored into single-particle integrals, are evaluated by a cluster development in powers of the particle density. The procedure is illustrated by a calculation of the pair distribution function and zero-point energy of the hard sphere gas, for Bose and Fermi statistics.
Under the assumption that the ground-state wave function is expressed as a product of pair functions, the expectation value of the ground-state energy is rigorously expressed in terms of the pair distribution function. An integro-differential equation is derived for the pair distribution function from the variational principle that the expectation value of the energy is minimum. It is pointed out that the expression for the energy and the integro-differential equation become identical with those derived by Abe if the hyper-netted chain (HNC) approximation is introduced. It is proved that the HNC approximation is compatible with the virial theorem. The nature of the formulas is investigated by applying them to some cases that have been investigated by other theories. It is suggested that the present theory may in general predict the phonon-like excitations.