Written for scientists and students in the areas of computational chemistry, computational biology, materials science, and pharmacology, this reference is a guide to the pitfalls and special considerations required to properly perform large-scale calculations of molecular structure and properties. It also provides a rigorous comparison of large-scale results from the most important approximate methods commonly used today. The first to examine in detail the differences between approximate methods and traditional electronic structure calculations, the book includes a CD with sample scripts, data sets, and worked examples.