This is the first modern open-source release of MOPAC, which is a direct continuation of MOPAC's closed-source commercial development ending in MOPAC2016. The source of the two versions has been synchronized for the last year, with the differences either being superficial (reorganization of source into directories & LGPL license tagging) or minor (redefined API to accommodate the switch to a shared library). The open-source version contains a CMake build system with packaging of self-contained, cross-platform installers based on the Qt Installer Framework.