The electronic structure, nonlinear electronic polarization induced by a static external electric field, and frequency dependent second-harmonic susceptibility tensor of the borophosphate BPO4 are studied by a first-principles calculation based on density-functional theory. Our calculated results show that the borophosphate BPO4 has a large band gap similar to 10.4 eV, which is larger than the band gap of the widely used nonlinear optical crystal KBe2BO3F2. However, BPO4 also has a nonlinear coefficient d(36) = 0.92 pm/V at static limit, which also is larger than the nonlinear coefficient d(11) = 0.47 pm/V of KBe2BO3F2. The unexpected larger nonlinear coefficient of BPO4 can be interpreted by the relatively strong s-p hybridization in BPO4, which can enhance the inter-band Berry connections, while the O 2p orbitals dominating valence bands in KBe2BO3F2 are very flat, resulting from weak s-p hybridization.

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