There are many chemistry software packages in existence today encompassing a variety of theoretical methods and computational techniques. Many of the programs currently used are well established but evolving constantly and it is believed the time is right to revisit the state of the software in light of major developments in functionality and computer hardware. This book is a reference for computational software, not as a series of user manuals, but as an update on new developments in the major program packages highlighting what is unique to these packages and also fundamental techniques that are common to all.