The dynamic second-order hyperpolarizabilities gamma of Si-3 and Si-4 clusters have been first calculated using the highly accurate coupled cluster cubic response theory. At omega = 0.43 au (1064 nm), the dynamic gamma values of the Si-3 and Si-4 clusters are 46.26 x 10(36) and 92.50 x 10(36) esu, respectively. The dynamic gamma values of the Si-3 and Si-4 clusters exhibit wide non-resonant optical region (omega < 0.04 au). The magnitude of macroscopic second-order hyperpolarizabilities chi((3))(-3 omega; omega, omega, omega) is of the order of 10(12) esu at omega < 0.04 au. Small Si clusters might be good candidates for nonlinear optical materials in infrared range. (C) 2008 Elsevier B.V. All rights reserved.