We present an analytic proof demonstrating the equivalence between the Random Phase Approximation (RPA) to the ground state correlation energy and a ring-diagram simplification of the Coupled Cluster Doubles (CCD) equations. In the CCD framework, the RPA equations can be solved in O(N 4 ) computational effort, where N is proportional to the number of basis functions.There has recently been a revival of interest in RPA in the quantum chemistry community. The RPA is popular for calculations of excitation energies both in finite systems 1,2 and in solids, 3,4 and is related to time-dependent density functional theory. 5,6,7 As a technique for describing electronic correlations, RPA has significant advantages, particularly for those interested in density functional theory. It describes dispersion and van der Waals interactions correctly, 8,9 and is exact for long-range correlations. 10 Left-right static correlations seem to be properly described by RPA, 11 and RPA fixes the pathologies of nonlocal Hartree-Fock-type exchange in metallic systems. Readers interested in details about RPA for ground state correlation can refer to the recent paper by Furche 12 where he discusses an interesting simplification to reduce the computational cost of RPA correlation and provides ample background information about RPA. Note that his work focuses on direct RPA, in which the exchange terms are neglected; as discussed later in this communication, this is the form of RPA most useful in the context of density functional theory.A connection between the RPA correlation energy and a ring diagram approximation to CCD was first mentioned by Freeman in his 1977 paper. 13 Very recently, A. Grüneis and G. Kresse reproduced this evidence and found numerical proof of the equivalence between these two approaches. 14 Here, we offer an analytic proof that these two problems yield identical correlation energies. To the best of our knowledge, no such formal proof has been given before.As a method for calculating electronic excitation spectra, RPA requires the solution ofThe matrices A, B, X, and Y are all ov × ov, where o and v are respectively the number of occupied and unoccupied spin-orbitals. The eigenvalue problem above can be completed by noting that if Xi Yi is an eigenvector with eigenvalue ω i , then Yi Xi is also an eigenvector, with eigenvalue −ω i . In the (real) canonical spin-orbital basis we use throughout this letter, we haveHere, ǫ p is a diagonal element of the Fock operator. Indices i, j, k, and l indicate occupied spin-orbitals, while a, b, c, d indicate unoccupied spin-orbitals. For arbitrary spin-orbitals p, q, r, and s, the two-electron integral pq rs is defined bywhere x is a combined space and spin electron coordinate. The RPA correlation energy can be obtained by considering two harmonic excitation energy problems: 12,15 RPA and the Tamm-Dancoff approximation (TDA) thereto, which sets B = 0 and thus solvesIn the quantum chemistry community, TDA is also known as configuration interaction singles (CIS). While TDA includes onl...