Introduction Quantum chemical ab-initio programs attempt to use the principles of quantum mechanics to obtain molecular properties of interest to a chemist or physicist. Such calculations can obtain molecular geometries, vibrational frequencies and barriers to rotation, ionization potentials and electron affinities, and many other properties. Quantum chemistry calculations provide a rapid and inexpensive alternative to experimental techniques for obtaining such properties. They are widely used both as a predictive tool and to confirm or explain experimental results. Quantum chemistry calculations are applicable to a wide range of chemical problems, including enzymology, thermochemistry of combustion, homogeneous catalysis, cluster science, and organic reaction mechanisms.
Many packages are available with varying capabilities, ease of use, and performance.
These programs are often quite large and complex. State of the art calculations using a typical workstation can require as much as a hundred or more megabytes of RAM, a gigabyte or more disk space, and take several days to several weeks to complete. Even larger calculations can be done with more powerful computers or clusters of workstations. Parallelization of these codes is an important step in increasing the capabilities and performance of the programs. In addition, the molecular visualization programs MolDen, RasMol, and XMol and the utility program Babel are available.