The IPOPT package is available from COIN-OR (http://www.coin-or.org) under the EPL (Eclipse Public License) open-source license and includes the source code for IPOPT. This means, it is available free of charge, also for commercial purposes. However, if you give away software including IPOPT code (in source code or binary form) and you made changes to the IPOPT source code, you are required to make those changes public and to clearly indicate which modifications you made. After all, the goal of open source software is the continuous development and improvement of software. For details, please refer to the Eclipse Public License.

Also, if you are using IPOPT to obtain results for a publication, we politely ask you to point out in your paper that you used IPOPT, and to cite the publication [13]. Writing high-quality numerical software takes a lot of time and effort, and does usually not translate into a large number of publications, therefore we believe this request is only fair :). We also have space at the IPOPT project home page where we list publications, projects, etc., in which IPOPT has been used. We would be very happy to hear about your experiences.