ADF 1999 involves a significant enhancement of functionality to include solvent effects, excitation energies, (hyper) polarizabilities, NMR chemical shifts and the ZORA relativistic formalism, to mention a few major extensions. Technically, the program has been 'upgraded' from the Fortran77 to the Fortran90 language. This implies that, in order to compile the program, you must have a Fortran90 compiler. However, we have also started to distribute complete executables, so the need to compile the program by your self is reduced.
The extended functionality, new language features and other technical matters have made it unavoidable that the format of the TAPE21 output file had to be changed. This implies that the TAPE21 files of ADF2.3 and ADF1999 are technically not compatible, i.e. you cannot hook up a version 2.3 TAPE21 file as a fragment file to a calculation with ADF1999. However, the logical content is largely identical. You may verify this by making an ASCII dump of the files, using the KF utilities. The logical compatibility means that privately developed tools that access TAPE21 files through the KF library procedures should work with the 1999 version as well (using the KF library of the 1999 release, of course), or may need only minor adjustments.
The 1999 release has focused on a significant increase in functionality compared to ADF2.3 and we believe this is a major step forward. However, we appreciate and acknowledge the fact that there is room for improvement in the area of speed, memory, IO, and disk usage and we will give this aspect more attention in the near future.