We evaluate the possible formation of recently proposed PO 4-deficient FePO4 by calculating its structure and stability at various charge states using the DFT+U theory. Unpaired electrons resulting from PO4 deficiency tend to localize on undercoordinated Fe neighbors. The absence of a PO4 unit causes local lattice distortions which are found to be sensitive to the charge state. Our calculations show that neutral and negatively charged PO4 vacancies may coexist under intrinsic conditions. The PO4-deficient FePO4 matrix turns out to be substantially softened, which may contribute to enhanced Li diffusion and provide clues toward the design of high performance LiFePO4 cathode.