Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Ghemical is a computational chemistry software package written in C++ and released under the GNU GPL. The program has GUI based on GTK+2 and supports quantum mechanical and molecular mechanic models, with geometry optimization, molecular dynamics, and a large set of visualization tools. Ghemical relies on external code to provide the quantum-mechanical calculations - MOPAC provides the semi-empirical MNDO, MINDO, AM1, and PM3 methods, and MPQC methods based on Hartree-Fock calculations. The chemical expert system is based on OpenBabel, which provides basis functionality like atom typing, rotamer generation and import/export of chemical file formats.