Integer electron-count cuts in variational second-order reduced-density-matrix theory. II. A convergent hierarchy of rigorous relaxations for real-space pair-density representability

Variational second-order reduced-density-matrix calculations can dissociate molecules into fractionally charged fragments; Part I largely removed this failure with integer electron-count conditions on fixed partitions. Building on the realizability theory of point processes, we place these conditions in a convergent hierarchy of rigorous relaxations, defined by finite occupation linear programs (LPs) on spatial partitions, for real-space pair densities at fixed one-particle density. Every level contains all representable pair densities, the levels tighten under nested refinement, and their intersection characterizes pair densities admitting a symmetric N-particle probability-measure extension. In a fixed orbital basis the levels give monotonically increasing lower bounds to the full configuration-interaction energy, which they need not reach, even for the Coulomb repulsion. The spin-summed real-space cuts of Part I are its two-cell members. The LP optimum is also a measure: at fixed one-particle density it equals the L¹ distance of the coarse-grained candidate from representability and increases under refinement to that of the candidate, with a vanishing coarse-graining error; its primal law constructs a representable approximation, and its dual a violated valid inequality, used as a cut in molecular calculations. The count conditions are not implied by the usual positivity conditions: moment data that satisfy all three-particle positivity conditions, including T₁ and T₂, give through Gaussian orbitals a smooth pair density that violates an integer count. For atoms at fractional electron number, ensemble relaxations with the integer-count condition lie closer to the exact piecewise-linear energy than the zero-variance fixed-N form.