Oxygen vacancies (OVs) provide a powerful route for tuning the performance of oxide photocatalysts; however, a quantitative understanding of how vacancy concentration governs the structure–property landscape of \({\text{Bi}}_{{2}} {\text{WO}}_{{6}}\) . remains incomplete. In this work, first-principles density functional theory calculations were used to systematically investigate pristine and oxygen-vacancy-containing orthorhombic \({\text{Bi}}_{{2}} {\text{WO}}_{{6}}\) \(\left( {{\text{Pca2}}_{1} } \right)\) . Vacancy configurations were generated by removing oxygen atoms from inequivalent lattice sites. Formation-energy analysis identified O2 and O3 as the most favorable vacancy sites, and a series of models with OV concentrations of approximately 4.2–25%, corresponding to 1–6 oxygen vacancies in the 36-atom \({\text{Bi}}_{{8}} {\text{W}}_{4} {\text{O}}_{24}\) periodic cell, was constructed. Progressive vacancy introduction induced pronounced local coordination changes around Bi and non-monotonic lattice responses. In the selected 4-OV arrangement, the strong expansion of the b parameter and unit-cell volume was attributed to a configuration-dependent finite-cell effect arising from the ordered vacancy geometry, rather than to an intrinsic concentration threshold. Electronically, OVs introduced donor states near the conduction-band minimum, shifted the Fermi level into the conduction band, induced n-type character, and reduced the band gap from 2.326 eV in pristine \({\text{Bi}}_{{2}} {\text{WO}}_{{6}}\) to 0.9 eV for the three-vacancy model, with further narrowing at higher vacancy concentrations. Density-of-states and electron-density-difference analyses revealed Bi-6p-dominated defect states, vacancy-centered F-center-like electron localization, and progressive charge accumulation on Bi atoms. Optically, vacancy engineering produced a red-shifted absorption edge, emergent absorption features in the 1–4 eV range, enhanced low-energy dielectric responses, and an increased static dielectric constant.
