Moleculix is a high performance, robustly designed library written in C/C++. The API can be accessed using a thin Python wrapper, Moleculix for Python.
In addition, a few (Win32) applications using the Moleculix API are available; 1) Moleculix – Desktop and 2) Moleculix – Library Manager.
More information about the Moleculix API, and applications, can be found at the Moleculix webpage