On the basis of the ZINDO program, we have designed a program to calculate the second-order nonlinear polarizabilitiesβlike, β0 andβµaccording to the sum-over-states (SOS) expression. A series of new 4-(dicyanomethylene)-2-methyl-6-(p-dithyla-mino-styryl)-4H-pyran (DCM) derivatives were designed and their electron spectra and nonlinear optical properties were studied. It is proposed that these compounds possess two important excited states close to each other in energy, both contri-buting to hyperpolarizability in an additive fashion; 4-(dicyanomethylene)-2,6-bis-(p-donor-styryl)-4H-pyran derivatives aremore nonlinear than 4-(dicyanomethylene)-2,6-bis-(p-donor-phenyl)-azo-4H-pyran derivatives. The high nonlinearities, good thermal stability and high transparency make them attractive candidates for second-order nonlinear applications such as electro-optic modulators and frequency doublers. © 2001 Elsevier Science B.V. All rights reserved.