  We investigate the dynamic second-order hyperpolarizabilities
{\gamma}(-3{\omega}; {\omega}, {\omega}, {\omega}) (indicated by {\gamma}THG)
of the Si2C and Si3C clusters using the highly accurate coupled cluster
singles-and-doubles (CCSD) response approach. The static {\gamma} values of the
Si2C and Si3C clusters are 1.99 \times 10-35 and 3.16 \times 10-35 esu,
respectively. Similar to the {\alpha} values, the {\gamma} values of the Si2C
and Si3C clusters are smaller than those of the Si3 and Si4 clusters,
respectively, which is related to much smaller static {\gamma} value of the C
atom than the Si atom.
