  The size-dependent structures and optical properties of CdSeS nanoclusters in
water medium are investigated. The stability of different size-dependent
Cd$_n$Se$_m$S$_p$ nanoclusters (up to n=6) is studied using density functional
theory/time-dependent density functional theory (DFT/TDDFT). The computed
results for ground (S$_0$) and excited (S$_1$, S$_2$, S$_3$) states are
experimentally verified through UV-Vis spectroscopy. Computed ab initio results
suggest that CdSeS clusters are significantly more hyperpolarizable compared to
CdX (X= S, Se, Te) clusters. Structure dependent response properties are also
observed, especially for n>3. Larger hyperpolarizabilities ($\beta$
and$\gamma$), charge variation and orbital analysis establish Cd$_4$Se$_m$S$_p$
clusters, as nonlinear optically active quantum dots (QDs).
