The vibrational spectra of the zinc cadmium thiocyanate (ZCTC) single crystal are studied by Raman spectroscopy and ab initio calculation with the molecular orbital (MO) theory using a GAUSSIAN 98 program. Agreement is achieved between the vibrational wavenumbers predicted by the theory and those observed in Raman scattering experiments. The results show that the characteristic vibrational modes of ZCTC crystal arise mainly from the internal vibrations of the SCN, Zn(NCS)(4), and Cd(SCN)(4) clusters. The three-dimensional network structure of ZCTC crystal is identified, and the high optical nonlinearity of the crystal is chiefly ascribed to the conjugated charge-transfer (-Zn-NCS-Cd-) bridges that connect all the distorted Zn(NCS), and Cd(SCN), tetrahedra together. (C) 2002 Elsevier Science B.V. All rights reserved.