We develop and validate a density functional, XYG3, based on the adiabatic connection formalism and the Gö rling-Levy couplingconstant perturbation expansion to the second order (PT2). XYG3 is a doubly hybrid functional, containing 3 mixing parameters. It has a nonlocal orbital-dependent component in the exchange term (exact exchange) plus information about the unoccupied KohnSham orbitals in the correlation part (PT2 double excitation). XYG3 is remarkably accurate for thermochemistry, reaction barrier heights, and nonbond interactions of main group molecules. In addition, the accuracy remains nearly constant with system size.Becke 3-parameter hybrid functional combined with Lee-Yang-Parr correlation functional ͉ density functional theory ͉ generalized gradient approximation ͉ local density approximation ͉ mean absolute deviation D ensity functional theory (DFT) has revolutionized the role of theory by providing accurate first-principles predictions of critical properties for applications in physics, chemistry, biology, and materials science (1). Despite dramatic successes, there remain serious deficiencies, for example, in describing weak interactions (London dispersion), which are so important to the packing of molecules into solids, the binding of drug molecules to proteins, and the magnitude of reaction barriers. We propose here a DFT functional that dramatically improves the accuracy for these properties by including the role of the virtual (unoccupied) states.Solution of the Schrödinger equation leads to the wavefunction, (r 1 , r 2 , …, r N ) (2), which depends on the 3N space coordinates and N spin coordinates of N-electrons in the system. Solving for such a wavefunction usually starts with the HartreeFock (HF) mean field description involving N self-consistent 1-particle spin-orbitals (in a Slater determinant), which is then used as the basis for expanding the wavefunction in a hierarchy of excited N-electron configurations, by using methods referred to as Møller-Plesset theory (e.g., MP2, MP3, MP4), couplecluster theory (e.g., CCSD(T)), and quadratic configuration interaction theory (e.g., QCISD(T)), etc. These methods are ab initio but suffer from problems of slow convergence with the size of the basis sets and the configuration expansion lengths, preventing scaling to large systems.In contrast, DFT is formulated in terms of the 1-particle density, (r), depending on only 3 spatial coordinates rather than 3N, as the fundamental quantity (3, 4). This dramatically simplifies the process of calculating the structures and properties. However, the exact form of the functional, whose solution will lead to the correct density, is not known. Even so, there has been an evolution of successively better approximations to this functional, that has already provided quite good accuracy for many problems (5-15).Perdew (16) has formulated the hierarchy of DFT approximations as a ''Jacob's ladder'' rising from the ''earth of Hartree'' to the ''heaven of chemical accuracy.'' The first rung of this ladder is the local (s...