Uniform electron gases Peter M. W. Gill∗ and Pierre-Franc¸ois Loos† Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia (Dated: February 7, 2012) We show that the traditional concept of the uniform electron gas (UEG) — a homogeneous system of finite density, consisting of an infinite number of electrons in an infinite volume — is inadequate to model the UEGs that arise in finite systems. We argue that, in general, a UEG is characterized by at least two parameters, viz. the usual one-electron density parameter r and a new two-electron parameter h. We outline a systematic strategy to determine a new density functional E(r;h) across the spectrum of possible r and h values. PACS numbers: 31.15.E-, 71.15.Mb, 71.10.Ca, 73.20.-r Keywords: Uniform electron gas; Homogeneous electron gas; jellium; density functional theory I. INTRODUCTION For example, significant improvements in accuracy can be achieved by using both the density r(r) and its gradient ∇r(r), The year 2012 is notable for both the journal and one of us an approach called gradient-corrected DFT [30]. Even better for, in the months ahead, both TCA and PMWG will achieve results can be achieved by including a fraction of Hartree- their half-centuries. It is inevitable and desirable that such Fock exchange (yielding hybrid methods [31, 32]) or higher occasions lead to retrospection, for it is often by looking back- derivatives of r(r) (leading to meta-GGAs [33]). wards that we can perceive most clearly the way ahead. Thus, However, notwithstanding the impressive progress since the as we ruminate on the things that we ought not to have done, 1970s, modern DFT approximations still exhibit fundamen- we also dream of the things that we ought now to do. tal deficiencies in large systems [34], conjugated molecules The final decade of the 20th century witnessed a major rev- [35], charge-transfer excited states [36], dispersion-stabilized olution in quantum chemistry, as the subject progressed from systems [37], systems with fractional spin or charge [38], an esoteric instrument of an erudite cognoscenti to a common- isodesmic reactions [39] and elsewhere. Because DFT is in place tool of the chemical proletariat. The fuel for this revo- principle an exact theory, many of these problems can be traced lution was the advent of density functional theory (DFT) [1] ultimately to the use of jellium as a reference system and the models and software that were sufficiently accurate and user- ad hoc corrections that its use subsequently necessitates. It is friendly to save the experimental chemist some time. These not a good idea to build one’s house on sand! days, DFT so dominates the popular perception of molecular In an attempt to avoid some of the weaknesses of jellium- orbital calculations that many non-specialists now regard the based DFT, we have invented and explored an alternative two as synonymous. paradigm called Intracule Functional Theory (IFT) [40–42]. In In principle, DFT is founded in the Hohenberg-Kohn theo- this approach, the one-electron density r(r) is abandoned in rem [2] but, in practice, much of its success can be traced to favour of two-electron variables (such as the interelectronic the similarity between the electron density in a molecule and distance r12) and we have discovered that the latter offer an the electron density in a hypothetical substance known as the efficient and accurate route to the calculation of molecular uniform electron gas (UEG) or jellium [3–18]. The idea — the energies [43–48]. Nonetheless, IFT is not perfect and has Local Density Approximation (LDA) — is attractively simple: shortcomings that are complementary to those of DFT. As a if we know the properties of jellium, we can understand the result, one should seek to combine the best features of each, electron cloud in a molecule by dividing it into tiny chunks of to obtain an approach superior to both. That is the goal of density and treating each as a piece of jellium. the present work and we will use atomic units throughout this The good news is that the properties of jellium are known article. from near-exact Quantum Monte Carlo calculations [19–29]. Such calculations are possible because jellium is characterized by just a single parameter r, the electron density. II. ELECTRONS ON SPHERES The bad news is that jellium has an infinite number of elec- arXiv:1202.1092v1 [physics.chem-ph] 6 Feb 2012 trons in an infinite volume and this unboundedness renders it, In recent research, we were led to consider the behavior of in some respects, a poor model for the electrons in molecules. electrons that are confined to the surface of a ball. This work Indeed, the simple LDA described above predicts bond ener- yielded a number of unexpected discoveries [49–57] but the gies that are much too large and this led many chemists in the one of relevance here is that such systems provide a beautiful 1970s to dismiss DFT as a quantitatively worthless theory. new family of uniform electron gases (see also [58]). Most of the progress since those dark days has resulted from concocting ingenious corrections for jellium’s deficiencies. A. Spherium atoms ∗ Corresponding author; [email protected] The surface of a three-dimensional ball is called a 2-sphere † [email protected] (for it is two-dimensional) and its free-particle orbitals (Table 2 TABLE I. The lowest free-particle orbitals on a 2-sphere TABLE III. The lowest free-particle orbitals on a glome (i.e. a 3- p sphere) Name l m 4p Ylm(q;f) Name l m n p Ylmn(c;q;f) s 0 0 1 1s 0 0 0 2−1=2 1=2 p0 1 0 3 cosq 1=2 2s 1 0 0 21=2 cos c p+1 1 +1 (3=2) sinq exp(+if) 1=2 2p 1 1 0 21=2 sin c cosq p−1 1 –1 (3=2) sinq exp(−if) 0 2p+1 1 1 +1 sin c sinq exp(+if) 1=2 2 d0 2 0 (5=4) (3cos q − 1) 2p−1 1 1 –1 sin c sinq exp(−if) 1=2 d+1 2 +1 (15=2) sinq cosq exp(+if) 1=2 3s 2 0 0 2−1=2(4cos2 c − 1) d−1 2 –1 (15=2) sinq cosq exp(−if) 1=2 2 3p 2 1 0 121=2 sin c cos c cosq d+2 2 +2 (15=8) sin q exp(+2if) 0 1=2 2 3p 2 1 +1 61=2 sin c cos c sinq exp(+if) d−2 2 –2 (15=8) sin q exp(−2if) +1 1=2 3p−1 2 1 –1 6 sin c cos c sinq exp(−if) 2 2 3d0 2 2 0 sin c (3cos q − 1) TABLE II. Number of electrons in L-spherium and L-glomium atoms 1=2 2 3d+1 2 2 +1 6 sin c sinq cosq exp(+if) 1=2 2 L 0 1 2 3 4 5 6 7 3d−1 2 2 –1 6 sin c sinq cosq exp(−if) 1=2 2 2 3d+2 2 2 +2 (3=2) sin c sin q exp(+2if) L-spherium 2 8 18 32 50 72 98 128 1=2 2 2 3d−2 2 2 –2 (3=2) sin c sin q exp(−2if) L-glomium 2 10 28 60 110 182 280 408 C. Exactly solvable systems I) are the spherical harmonics Ylm(q;f). It is known that l 2l + 1 One of the most exciting features of the two-electron atoms jY (q;f)j2 = (1) ∑ lm 4p 0-spherium and 0-glomium is that, for certain values of the m=−l radius R, their Schrodinger¨ equations are exactly solvable [51]. and doubly occupying all the orbitals with 0 ≤ l ≤ L thus yields The basic theory is as follows. a uniform electron gas. We call this system L-spherium and The Hamiltonian for two electrons on a sphere is will compare it to two-dimensional jellium [18]. ∇2 ∇2 1 The number of electrons (Table II) in L-spherium is H = − 1 − 2 + (7) 2 2 u n = 2(L + 1)2 (2) where u is the interelectronic distance r ≡ jr − r j. If we 2 12 1 2 the volume of a 2-sphere is V = 4pR and, therefore, assume that the Hamiltonian possesses eigenfunctions that (L + 1)2 depend only on u, it becomes r = 2 (3) 2pR u2 d2 (2D − 1)u D − 1 d 1 H = − 1 + − + (8) 4R2 du2 4R2 u du u B. Glomium atoms where D is the dimensionality of the sphere. Three years ago, we discovered that H has polynomial eigenfunctions, but only The surface of a four-dimensional ball is a 3-sphere (or for particular values of R. (This is analogous to the discovery “glome” [59]) and its free-particle orbitals (Table III) are the that hookium[? ] has closed-form wavefunctions, but only for hyperspherical harmonics Y (c;q;f). It is known [60] that particular harmonic force constants [62, 63].) We showed that lmn there exist b(n+1)=2c nth-degree polynomials of this type and l m 2 2 (l + 1) that the associated energies and radii satisfy jYlmn(c;q;f)j = (4) ∑ ∑ 2p2 2 m=0 n=−m 4Rn;mEn;m = n(n + 2D − 2) (9) and doubly occupying all the orbitals with 0 ≤ l ≤ L thus yields where the index m = 1;:::;b(n + 1)=2c.