Novel structures of clusters of semiconductors and metals obtained from ab initio calculations Vijay Kumar Dr. Vijay Kumar Foundation, 1969 Sector 4, Gurgaon 122001 Properties of matter at the nanoscale are often vastly different from bulk and depend on size and shape in addition to the well known factors of composition and thermodynamic variables. Therefore exciting new opportunities have emerged to design materials with fine tuned properties and new applications are becoming possible using matter at the nanoscale. Great efforts are being made to explore new materials for miniature devices, novel catalysts for industrial applications, fuel cells for hydrogen economy, new processes for developing materials as well as to understand biological systems whose length scale overlaps with that of nanomaterials. Nanoparticles, nanowires, nanotubes, and nanobelts are some of the forms that have attracted great attention in recent years. As large fraction of atoms in these systems lie on the surface, there is often a reconstruction that generally leads to new structure of nanoparticles particularly in the range of size of ~ 1 nm. Computer simulations are playing a key role in understanding the structure-property relation of such systems as well as in designing new nanomaterials. In this talk I will discuss a few selected examples where ab initio calculations have helped (1) to resolve long standing experimental data on nanomaterials such as magnetic clusters of non-magnetic bulk Rh  which have been found to grow in simple cubic structure and semiconductor nanoparticles of CdSe  for which Cd34Se34 clusters have been found to be magic, and (2) to predict novel forms of silicon - the silicon fullerenes  and nanotubes  with metal encapsulation that have later received experimental support . More recently we have obtained  novel growth behavior of Pt clusters with diameter up to 3 nm. Some recent results on silicon nanowires  and MoS nanowires  will be also discussed.  Y.-C. Bae, V. Kumar, H. Osanai, and Y. Kawazoe, PRB72, 125427 (2005).  A. Kasuya et al., Nature Materials 3, 99 (2004) and to be published.  V. Kumar and Y. Kawazoe, PRL 87, 045503 (2001); ibid. 90, 055502 (2003); V. Kumar, Comput. Mater. Sci. 36, 1 (2006); A.K. Singh et al., PRB 71, 115429 (2005); V. Kumar in “Nanosilicon”, Elsevier (2008).  A. K. Singh et al., Nano Lett. 2, 1243 (2002); PRL 91, 146802 (2003).  K. Koyasu et al., JACS 127, 4998 (2005); Saranin et al., Nano Lett. 4, 1469 (2004).  V. Kumar and Y. Kawazoe, Phys. Rev. B77, 205418 (2008).  A. K. Singh et al. Nano Lett. 5, 2302 (2005); ibid. Nano Lett. 6, (2006).  P. Murugan, Nano Lett. 7 (2007); Appl. Phys. Lett. (in press).