Provides quick minimization for smaller molecules, using OpenBabel. Probably
reasonable for some few hundred atoms. Supports the following forcefields:
1. GAFF
2. MMFF94 & MMFF94s (which is optimized for minimization)
3. Ghemical
4. UFF
The default is the "best available" forcefield, which tries them in the order given
above until it finds one that can handle the molecule.