The quantum mechanical hypervirial theorems (HVT) technique is used in treating two basic inequalities relating the ground-state mean square radius of the orbit of a particle in a central potential and its kinetic energy, respectively, to the spacing of the two lowest energy levels Δ E . For quite a wide class of those potentials, the parameters of which also lead to a sufficiently small dimensionless quantity s , the difference between the two sides of each inequality is of order s 3 and higher (while Δ E is of order s and higher), and thus it is expected in general to be quite small.
A method is presented for relating a ground-state density distribution to the effective single-particle potential which has this density for its ground state for a noninteracting system of particles. The method is useful for systems consisting of a small number of spinless fermions and in the case of two particles a second-order, nonlinear differential equation relates the density and the effective potential. The question of
v representability is addressed through examples of densities for one- and three-dimensional systems. It is found that densities of the form ${e}^{\ensuremath{-}ar}$ for two-particle systems are not ground-state
v representable. The added flexibility of three dimensions over one allows an effective potential which does not necessarily have the chosen density as its ground state, but as its lowest nondegenerate state.
A three-dimensional stochastic process which has the particle density as its probability density is constructed from Nelson's stochastic mechanics, yielding a particle trajectory picture for density-functional theory. This diffusion process enables simulating the detailed behavior of multiparticle quantum systems with substantial computational economy. A stochastic-mechanical form of the Heisenberg uncertainty principle holds in the new context. Moreover, a kinetic energy functional is defined and shown to be connected with other kinetic energy expressions, and in the case of a time-independent ground state a formal dynamics for the process is proposed. A related real-space version is developed of Bohm's quantum potential formalism.