Atom typing behaviorΒΆ

Atom typing is the process of matching MM atom types to elements. For example, a Carbon atom becomes a C_1, C_2, C_3 or C_R, depending on the number (and type) of bonds it has to neighboring atoms. UFF is capable of finding a matching MM atom type on its own, but might not always succeed in doing so. When doing calculations with UFF, it is important to check the beginning of the output file, as the program will print the detected MM atom types there. You can also take matters into your own hands, and tell UFF what MM atom types you want it to use (see the section on Input and examples).

The atom typing in UFF is mostly controlled by the elements and mmatomtypes parameter files, however, some part of it is hidden in the code itself and is (at the moment) not accessible to users. This is done by UFF to differentiate between:

  • Carbon: double-bonded sp2 carbon (C_2) vs aromatic carbon (C_R), if any of the orders are close to 2, it’s C_2.
  • Oxygen: having a bond to silicon gives O_3_z; otherwise, having bond order of 1.44 or higher gives O_R, provided that the partner is not a hydrogen. Otherwise, it’s just an O_3.
  • Sulfur: having a bond order greater than 1.3 gives aromatic S_R.

If you have source-code access, this behavior can be modified in the MMStructureInfo module.