Abstract
We introduce quantum virtual-orbital fragmentation (Q-FVO), a systematic method for reducing the largest active space in correlated quantum-chemistry calculations. The complete occupied space is retained, the localized virtual space is partitioned into chemically motivated fragments, and the correlation energy is recovered through an inclusion-exclusion many-body expansion. Across six molecular benchmarks, the largest one-body Q-FVO calculations reduce the qubit requirement by 46 to 66 percent, while two-body calculations reduce it by approximately 31 to 42 percent relative to the corresponding unfragmented spaces. Two-body expansions recover most of the correlation energy, with errors of 0.9 to 7.5 kcal/mol; three-body expansions are below 1 kcal/mol for all CCSD tests and remain below 2 kcal/mol at CCSD(T). Illustrative statevector UCCSD calculations also reduce implementation-reported circuit depth while retaining accuracy below 1 kcal/mol. Q-FVO can be nested inside Q-EFMO real-space fragmentation and, in turn, the resulting cluster can be embedded in a Q-EFP environment. The hierarchy therefore reduces quantum-resource growth along three complementary dimensions: environment, molecular fragments, and virtual-orbital space.