Interpretation of the nonlinear optical susceptibility of borate crystals from first principles

First-principles calculations of the second-order optical susceptibility ar e successfully performed for practical berate crystals, namely, LiB3O5, CsB 3O5 (CBQ), and BaB2O4, using the linearized augmented plane-wave band metho d. Spectral and spatial decomposition are proposed to elucidate the role of different ions and different electron states. It is shown, with CBO as an example, that transitions from the high-lying part of occupied O 2p bands, as the initial states, to the conduction band states around the three-coord inated B site dominate in the nonlinear response. [S0163-1829(98)05346-6].