Gaussian 09 is the latest in the Gaussian series of programs. It provides state-of-the-art capabilities for electronic structure modeling. Gaussian 09 is licensed for a wide variety of computer systems. All versions of Gaussian 09 contain every scientific/modeling feature, and none imposes any artifical limitations on calculations other than your computing resources and patience. The Gaussian 09 versions for Windows computers and Power-PC-based Mac OS X computers are known as Gaussian 09W and Gaussian 09M (respectively). Gaussian 09 for Intel-based Mac OS X computers is generally licensed in the same way as other Linux/UNIX versions. A single-CPU 32-bit version is also available as a shrink-wrap licensed product which is known as Gaussian 09IM. All Linux/UNIX versions of Gaussian 09 can run on single CPU systems and in parallel on shared-memory multiprocessor systems. Gaussian 09W is available in separate single CPU and multiprocessor versions. Gaussian 09M is available in a single-CPU version only. For cluster and network parallel execution, the Linda parallel computing environment software must also be licensed. An updated version of Linda is required for all versions of G09.