A theoretical study on the second harmonic generation properties of molecules and crystals of 4,5-bis(2',4'-dinitrophenylthio)-1,3-dithiole-2-one(BNPT-DTO) and 4,5-bis(2',4'-dinitrophenylthio)-1,3-dithiole-2-thione(BNPT-DTT)

On the basis of ZINDO method,the program for the calculation of nonlinear second-order optical susceptibilities beta(ijk) has been devised according to the formula of sum-over-states and the values of molecules of BNPT-DTO and BNPT-DTT have been calculated under condition of second harmonic generation. The relation of nonlinear second-order optical susceptibilities between molecules and crystals of BNPT-DTO, BNPT-DTT is analyzed at the molecular level,the computation results are in better agreement with the experimental results.