Geometry Specification for Band Structure Calculations

Before electronic band structure calculations can be done, the sequence of atoms in the polymer must be supplied in a highly specific order. For a simple polymer, the coordinates of all the atoms in the first fundamental unit cell are given. These atoms can be in any order. The next set of atoms defined are those for the next unit cell. These atoms must be in the same order as the atoms in the first unit cell. For band structures at least two unit cells must be defined. If more than two unit cells are defined, the atoms in the other unit cells must be defined in the same order as those in the first unit cell.

For all polymer calculations except band structures, the order of atoms is not important. An example of such a data set is shown for polytetrahydrofuran. When band structures are to be calculated, the order of atoms is important.

Because of the difficulty in generating data sets for band-structure work, program BZ was written. Given a suitable data-set, BZ will generate a MOPAC data set which can then be used for the calculation of band structures.