ABSTRACT:Because real atoms have ground states, while relativistic Hamiltonians do not, there is a first-principles reason to treat relativistic problems by means of density functional theory (DFT). Here, such a relativistic DFT is presented in terms of difference equations, which arise in turn from discretized differential equations. Two explicit examples considered are: (i) harmonically confined independent Fermions filling an arbitrary number of closed shells, and (ii) hydrogen-like atomic ions for arbitrary atomic number Z. These findings are then compared and contrasted with wave function theories, going back at least to Wall. Finally, some proposals are put forward for future work, both theoretical and experimental, bearing on relativistic DFT formulated in terms of low-order difference equations. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem 105: 701-708, 2005 Key words: relativistic electron density; difference equations 1. Background and Outline S ome two decades ago, in an article on density functional theory (DFT) and its applications, Callaway and the author [1] drew attention to the fact that, whereas real atoms have ground states, relativistic Hamiltonians do not. Even at that early stage, it was tempting to build a relativistic DFT without appeal to such Hamiltonians. Subsequent work by Holas and March [2,3] therefore attempted to construct a relativistic theory directly in terms of the so-called density amplitude (r) 1/2 , where (r), as usual, is the ground-state electron density.Progress in nonrelativistic DFT, going back to the work of Lawes and March [4], is possible, at least for some currently important problems such as harmonic confinement, in terms of a linear differential equation to solve for the density (r) itself, rather than the nonlinear equation for the density amplitude referred to above. Specifically, for harmonically confined independent Fermions filling an arbitrary number of closed shells, the nonrelativistic ground-state Fermion density (r) satisfies in ddimensions the linear third-order homogeneous differential equation [5]:Here is the characteristic frequency associated with the harmonic confinement, and (M ϩ 1) de- We will return briefly to the nonrelativistic solution of Eq. (1) in three dimensions below. At this point, we record unorthodox developments in relativistic wave function theory, which we compare and contrast with the relativistic DFT approach, the focus of the present study (discussed in detail later in this work). These "unorthodox" wave function treatments go back, at least, to Wall [6], with related and independent work more than a decade later by Ruijsenaars [7] and by Ord and Mann [8]. Some details are recorded below, especially in Section 4.3 and Appendix A2. Suffice it to say here that Wall [6] took the Schrö dinger differential equation for free electrons and converted it into a "relativistic wave function difference equation" by introducing a finite interval, the scale of which was determined by the Compton wavelength ϭ h/m 0 c, where m 0 now denote...