where Fj is the stepsize of an external electric field parallel to the coordinate axis and was set to be 0.003 a.u. in this work, which has been repeatedly confirmed to be very suitable for FF calculation. The electronic excitations of C 18 and (BN) n C (18-2n) (n = 1-9) were studied at the ωB97XD/6-311+G(2d) level of time-dependent DFT (TD-DFT) and the electronic absorption spectra were plotted based on the calculated results. We applied the hole-electron analysis developed by Lu to deeply explore and quantitatively study the characteristics of electron excitation. The hole and electron density distributions are defined as