The dipole polarizability, static first hyperpolarizability and UV-Vis spectrum of the recently identified nano-sized tetrahedral cluster of Au-20 have been investigated by using time-dependent density functional response theory. We have discovered that the An(20) cluster possesses remarkably large molecular second-order optical nonlinearity with the first hype rpol arizabilty (beta(xyz)) calculated to be 14.3 x 10(-30) esu. The analysis of the low-energy absorption band suggests that the charge transfer from the edged gold atoms to the vertex ones plays the key role in nonlinear optical (NLO) response of Au-20. (C) 2004 Elsevier B.V. All rights reserved.