The clusters Cs2B6O10, Li2 B6O10, and CsLiB6O10, consisting in crystalline states, are employed to model

the electronic structures and nonlinear optical properties of optical materials CBO(CsB3O5), LBO(LiB3O5),

and CLBO (CsLiB6O10). The electronic structures and dynamic second-order polarizabilities of these clusters

have been calculated using the intermediate neglect of different overlap with singlet excited configuration-interaction method and in combination with the sum-over-states method, respectively. Then, the second-order

susceptibilities of these three bulks have been estimated in terms of the microspecies number density and local-field correction factor at a low-frequency zone of photons. It is found that the top levels of the valence band derive from O 2p orbitals and the lowest edge of the conduction band derives from B 2p and O 2p orbitals. The calculated energy gap increases in the order of CsLiB6O10 (6.31 eV)<Cs2B6O10 (6.66 eV)<Li2B6O10 (7.26 eV) as compared to the observed optical gap of crystals, CLBO (180 nm)<CBO (170 nm)<LBO (160 nm), individually. The calculated results show that the excited-state charge transfers from O 2p to B value orbitals make the most significant contributions to the second-order susceptibility for all of these three crystals. It is also found that the natures of orbitals both at the upper valence band and at the lowest edge of the conduction band are the least changed among the three clusters M(B6O10) (M=Cs2 , Li2 , CsLi), and that the high ionicity of cation and anionic group interaction results in a large NLO coefficient. Both the ionicity and the calculated nonlinear optical coefficients decrease in the order of CBO>CLBO>LBO. This result will give a clue in designing a new nonlinear optical material with the same anionic group. [S0163-1829(99)04339-8]