Based on the ionic group theory, the SHG coefficients of KB5 crystal have been calculated, which leads to the following values d31=2.61×1010 esu, d32=0.07×1010 esu and d33=3.26×1010 esu. They agree fairly well with the expeimental ones. Our calculation results indicate that the hydrogen bond of (B5O10) ionic group has a large effect on the coefficients of KB5 crystal, we are therefore able to determine to how large an extent the H belongs to the ionic group. The probability of the H belonging to the ionic group is about 40-60%. Furthermore, the reasons for too small SHG coefficient of KB5 have been analysed. In addition to the small microscopic SHG coefficients of (B5O10) group, the space arrangement of (B5O10) group is in an unfavourable manner so that the largest microscopic SHG coefficients, x123, are cancelled each other. Finally, some structural requirments are proposed to search for the new type non-linear optical crystals among the B-O compounds, composed of six-membered rings, each having three borons in trigonal and tetrahedral coordination.
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