Two of the files that are produced by ADF deserve special attention. The first is the general result file TAPE21 (.t21 files). It is a binary file that contains a lot of information about the calculation, such as the one-electron orbitals expressed in the basis functions. It can be used as a fragment file for subsequent calculations (although only TAPE21 files from spin-restricted calculations can be used as fragment files). Like all files produced by the program, it is generated in the directory where the job runs. Having done a calculation, you will usually store TAPE21 somewhere under a suitable name so that you can later reuse it, as a fragment file, for a restart, to feed it to an analysis program, and so on.
The second is an ASCII log file, called logfile. It accumulates messages from ADF into a (brief) summary of the run. You can inspect it during the calculation to check how far the calculation has proceeded, whether there are important warnings and so on. At the end of the run this file is copied to the tail of the normal standard output file.
