Kirkwood superposition approximation

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The Kirkwood superposition approximation takes its name from John G. Kirkwood (see Eq. 40 Ref. 1, Eq. 5.6 Ref. 2)

{\rm g}_N^{(3)}({\mathbf r}_1,{\mathbf r}_2,{\mathbf r}_3)={\rm g}_N^{(2)}({\mathbf r}_1,{\mathbf r}_2){\rm g}_N^{(2)}({\mathbf r}_2,{\mathbf r}_3){\rm g}_N^{(2)}({\mathbf r}_3,{\mathbf r}_1)

It appears that this was used as a basis of a closure for the Kirkwood integral equation (Ref. 1) and the Yvon, and Born-Green (Ref. 2) until the work of Morita and Hiroike (Ref. 3). It was pointed out in Ref.s 4 and 5, that there is an inconsistency between the pressure and the compressibility equation if this superposition approximation is used to generate g(r). This approximation is rigorously correct for one-dimensional systems, and is only true in three-dimensions in the limit of zero density.


  1. John G. Kirkwood, "Statistical Mechanics of Fluid Mixtures", Journal of Chemical Physics 3 pp. 300-313 (1935)
  2. M. Born and H. S. Green "A General Kinetic Theory of Liquids. I. The Molecular Distribution Functions" Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences 188 pp. 10-18 (1946)
  3. Tohru Morita and Kazuo Hiroike "A New Approach to the Theory of Classical Fluids. I" Progress of Theoretical Physics 23 pp. 1003-1027 (1960)
  4. B. R. A. Nijboer and L. Van Hove "Radial Distribution Function of a Gas of Hard Spheres and the Superposition Approximation", Physical Review 85 pp. 777 - 783 (1952)
  5. G. S. Rushbrooke and H. I. Scoins "On the Theory of Fluids", Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, 216 pp. 203-218 (1953)
  6. Ryuzo Abe "On the Kirkwood Superposition Approximation", Progress of Theoretical Physics 21 pp. 421-430 (1959)
  7. Russell V. Cochran and L. H. Lund "On the Kirkwood Superposition Approximation", Journal of Chemical Physics 41 pp. 3499-3504 (1964)
  8. G. H. A. Cole "Classical fluids and the superposition approximation", Reports on Progress in Physics 31 pp. 419-470 (1968)