The ground-state electronic and structural properties, and the electronic excitations of the lowest energy isomers of the Ag8 cluster are calculated using density functional theory (DFT) and timedependent DFT (TDDFT) in real time and real space scheme, respectively. The optical spectra provided by TDDFT predict that the D 2d dodecahedron isomer is the structural minimum of Ag8 cluster. Indeed, it is borne out by the experimental findings.PACS numbers: 36.40.Mr In latter years, a new field has emerged from the understanding, control and manipulation of objects at nanoscale level (nano-objects). It is commonly known as nanoscience. This field involves physics, chemistry and even engineering, and addresses a huge number of important issues starting from basic science and ending in a large variety of technological applications [1]. Among the nano-objects, the small clusters or nanoclusters play a very important role, since they are the bricks of nanoscience. Therefore, the study of small clusters deserves a special attention. In this respect, the steps to follow for a complete description of a cluster can be summarized in the following three questions: what is the lowest energy structure?, what is the effect of increasing or decreasing the temperature on the structural properties of a cluster? and the last step deals with the kinetic effects in the formation of the nanocluster. Here, we are only concerned about the first question for the silver octamer, leaving the other two questions open for future investigations.From the theoretical point of view, first-principles methods give an enormous advantage for understanding, projecting and inventing new materials that is reflected in the huge number of articles published in the field of materials science. Likewise, density functional theory (DFT) has emerged as a new and promising tool for ab initio electronic structure calculations and gives valuable information about the geometry of nanoscale systems [2] but unfortunately it not always predict the correct structure of the cluster under consideration. In this regard, the silver octamer belongs to the group of the controversial systems for which the lowest energy structure is unresolved by DFT.Recently, P. Radcliffe et al.[3] have proposed Ag 8 clusters embedded in helium droplets as a suitable system for light amplification based on an optically accessible long-living excited state (E*) and thereby, from the theoretical point of view, the determination of the structure becomes a key point as the first step for identifying and controlling the levels that populate E*. Up to now, it has not been possible to make a reliable theoretical prediction of the most stable structure of Ag 8 . A review of the literature reveals that there are two competing geometries in eight-atom clusters of s-electron elements, having T d and D 2d symmetry. In fact, different levels of theories favor different geometries: DFT in its local density approximation (LDA) [4], multireference configuration interaction method [5], a tight-binding appr...