Triply differential cross sections are calculated for one-photon double ionization of neon and argon at various photon energies and electron energy sharings by using a frozen-core treatment to represent the remaining electrons of the residual ion. Angular distributions agree well with all existing experimental data, showing that in spite of its simplicity the method can treat the double ionization of complex targets reliably. A comparison of the cross sections for helium, neon, and argon into the same final state symmetry at the same relative excess energies reveals a distinctive signature of the role of electron correlation in each target. DOI: 10.1103/PhysRevLett.110.173001 PACS numbers: 32.80.Fb, 34.10.+x One-photon double photoionization (DPI) of atomic targets is a fundamental problem that has been the subject of significant investigation because of the explicitly correlated nature of the process. The simplest target where DPI can occur is helium, for which experiments using coincidence measurements of the resulting Coulomb fragments allow description of the fully differential electron dynamics. In parallel, ab initio theoretical treatments of helium DPI have produced results in excellent agreement with experiment: Double ionization of helium can be considered a solved problem (see, e.g., Refs. [1][2][3][4][5], and references therein). In contrast, for heavier rare-gas atoms, the volume of experimental measurements of one-photon DPI greatly outweighs the theoretical data, which have been confined to model treatments [6] and parameterization of the process [7,8]. At the same time, significant interest in studying rare-gas targets in laser-atom interactions has evolved beyond the single-photon perturbative limit of DPI. Processes such as high-harmonic generation [9], attosecond streaking [10], and multiphoton ionization [11] with coherent light sources represent some of the most recent examples of the evolving scope for investigating multielectron dynamics in rare gases. Application of precise and kinematically complete experimental techniques has outpaced the capacity for absolute theoretical treatments to fully represent the total correlated dynamics in rare-gas targets beyond helium. For single-photon DPI of neon or argon, no accurate fully ab initio description of this process has been achieved.Here we report the results of a method that makes use of a fully correlated grid-based expansion to describe the electrons ejected into the double continuum in the presence of the remaining bound electrons that advances our theoretical capacity to describe DPI in many-electron atomic targets. The key to this framework is the construction of an atomic orbital basis that represents the bound N À 2 electrons, which will remain frozen throughout the double ionization process [12]. Thus the final state represents two indistinguishable continuum electrons that will share the excess photon energy in the presence of those electrons remaining bound to the parent ion. The proper description of the angular distributions a...