I think GDIS accomplishes your conditions. As it is an interface (like Material Studio) you will need to install some packages to run the tasks. As you are interested in using molecular mechanics (with UFF), I recommend to install also the GULP package, specialized in molecular dynamics and in lattice dynamics.
GDIS is a GTK based program for the display and manipulation of
isolated molecules and periodic systems. It is in development, but is
nonetheless fairly functional.
It has the following features:
- Support for several file types (CIF, BIOSYM, XYZ, XTL, MARVIN, and GULP)
- OpenGL rendering (requires gtkglarea)
- Assorted tools for visualization (measurements, ribbons, polyhedral display)
- Useful manipulation tools, including matrix transformations and periodic image display.
- Powerful surface generation and crystal morphology tools.
- Animation of BIOSYM and GULP trajectory files
GDIS also allows you to perform the following functions through other packages:
- Model rendering (courtesy of POVRay)
- Energy minimization (courtesy of GULP)
- Morphology calculation (courtesy of cdd)
- Space group processing (courtesy of SgInfo)
- View the Periodic Table (courtesy of GPeriodic)
- Load additional filetypes, such as PDB (courtesy of Babel)