Abstract
An assembly of a few to thousands of atoms or molecules is referred to as a
cluster. This chapter exclusively deals with atomic clusters and their classifications
based on the nature of bonding and the number of constituents. The size effect, surface
phenomena, and variation of properties with the size of atomic clusters have also been
discussed. The difficulties in cluster experiments have been highlighted. Various
theoretical methods to study the clusters have been mentioned, with the main focus on
the density functional theory (DFT). The accuracy of various DFT methods and
choosing appropriate methods have been particularly discussed. Finally, the way to
perform DFT-based studies on clusters is also explored. This chapter provides a brief
introduction to what the book is all about.