Database

The ADF package is equipped with a database to help you generate basic atoms. Each data file in the database contains a standard basis set (and related information) for the creation of one basic atom. The data files are relatively small ASCII files. You can easily inspect them. In Appendix 1 a definition is given of such a file. This enables you to create variations and construct your own adapted basis sets. Important: names of the standard basis sets have changed starting from the ADF 2002.01 version to more intuitive names: I→SZ, II→DZ, III→DZP, IV→TZP, and V→TZ2P. Note that in some places the old names are still not replaced by the new names.

The basis functions used in ADF are commonly known as Slater Type Orbitals (STOs). A basis set can roughly be characterized by its size (single-, double-, triple-zeta; with or without polarization) and by the level of frozen core approximation. Initially, the only basis sets provided with ADF were those in the directories I, II, III, IV, V, which now have the more intuitive names SZ, DZ, DZP, TZP, and TZ2P, respectively. The increasing numbers point to an increase in size and quality. It is not possible to give a formally correct short general classification for each basis set directory. However, generally speaking we can say that SZ is a single-zeta basis set, DZ is a double zeta basis set, DZP is a double zeta polarized basis, TZP is a core double zeta, valence triple zeta, polarized basis set, and finally TZ2P is a core double zeta, valence triple zeta, doubly polarized basis. This explains the more intuitive names that are given for the basis sets. The names have also been changed since some of the basis sets have been modified substantially.

In addition, the database contains directories with special basis sets:

ZORA contains basis sets that should be used (exclusively) for relativistic calculations with the ZORA approach. Using 'normal' basis sets in a ZORA calculation may give highly inaccurate results, in particular for heavy elements. The same is classification is used for the directories ZORA/SZ-TZ2P as in the non-relativistic directories. The ZORA basis sets were added later because of the special requirements on basis sets for ZORA relativistic calculations, especially in the core region. The ZORA/QZ4P basis set can be loosely described as core triple zeta, valence quadruple zeta, with four sets of polarization functions.

ET contains several even tempered basis sets which enables one to go to the basis set limit. The accuracy of the smallest basis set in this directory can loosely be described as quadruple zeta in the valence with three polarization functions added. This directory also contains basis sets with extra diffuse functions. In Response calculations one should use such large basis sets. Very diffuse functions are absolutely necessary to get good results for excitation energies corresponding to high lying orbitals.

AUG contains several augmented standard basis sets which enables one to get reasonable results for excitation energies with relatively small basis sets.

OLD contains basis sets that were contained in the 2.3 release, but that we feel should not be used anymore unless with great care, primarily because the involved frozen cores are too large to justify the frozen core approximation. In some cases we found uncomfortably large errors in equilibrium geometries resulting from such too-large cores.

Furthermore, you will find in the database:

Special/AE contains non-relativistic basis sets for all-electron calculations. However, these files cannot be used as such, because they don't contain any fit sets. Using basis sets without fit sets is pointless and is in fact not possible at all. (The usage and relevance of fit functions is explained later). Therefore, they serve as starting point for the development of (new) basis sets. For some of the all-electron sets appropriate fit sets have already been generated. The corresponding data base files can be found in the appropriate subdirectories SZ, DZ, DZP, et cetera.

Special/Vdiff contains non-relativistic basis sets that include very diffuse functions. These were recommended to be used for Response calculations. Very diffuse functions are absolutely necessary to get good results for excitation energies corresponding to high lying orbitals. Recommendation: use the even tempered basis sets in the ET directory, since these basis sets are better.

Special/MDC contains non-relativistic basis sets with optimized fit functions especially useful for accurate Multipole Derived Charges. These are available only for a limited number of basis sets.

Cerius contains data files that are used in the Cerius2-ADF graphical user interface.

Dirac contains the input files for the DIRAC auxiliary program (see the UTILITIES document)

Band contains input files for the BAND program (see the BAND User's Guide)

ForceFields contains force field files to be used in the QM/MM functionality. Their structure and contents are described in the QM/MM manual. See also the pdb2adf utility (Utilities document), which transforms a PDB file into an ADF input file, for use with QM/MM.

The files in SZ/ (old name I/) (and ZORA/SZ/) have minimal basis sets: single-zeta without polarization. The exponents of the functions correspond to the standard STO-3g basis sets used in programs that employ Gaussian type basis functions. The frozen core approximation is applied, however, for the inner atomic shells. Type-SZ database files are provided only for the lighter elements, up to Kr.

The files in DZ (old name II) can be characterized as double-zeta basis sets without polarization functions. A triple-zeta set is used for the 3d shells of the first row transition metals, the 4f shells of the Lanthanides, and the 5f shells of the Actinides. In all these cases a double-zeta set provides a rather poor expansion basis for the true (numerically computed) atomic orbital.

The basis sets in DZP (old name III) are derived from DZ (old name II), extended with a polarization function. This type of basis sets is thus far provided only for the elements up to Ar, and for the 4p series Ga through Kr.

TZP (old name IV) contains triple-zeta basis sets. A polarization function is added for H through Ar and for Ga through Kr (from DZP).

TZ2P (old name V) finally gives extended basis sets: triple-zeta with two polarization functions, for H through Ar and Ga through Kr (from DZP). Note that the TZ2P database files are provided only for the lighter elements, up to Kr. The ZORA/TZ2P database files are provided for all elements. Typically for all elements one polarization function is added compared to the corresponding TZP basis set. Note, however, that TZ2P will not always give you extra basis functions for most lanthanide and actinide frozen core basis sets.

Multiple occurrences of one chemical element in the same basis set subdirectory correspond to different levels of the frozen core approximation. Manganese for instance may have a basis set for an atom with a frozen 2p shell and another one with a frozen 3p shell. The file names are self-explanatory: Mn.2p stands for a data file for Manganese with frozen core shells up to the 2p level. An all-electron basis set would correspond to a file that has no frozen-core suffix in its name.

Another type of multiple occurrence of one element in one database directory may be found when basis sets have been developed for different electronic configurations: the Slater-type basis sets are fitted then to numerical orbitals from runs with different occupation numbers. Currently this applies only for Ni (in database directories DZ, TZP and TZ2P), where basis sets are supplied for the d8s2 and the d9s1 configurations respectively. Since in earlier releases only the d8s2 variety was available, the names of the database files are Ni.2p (for d8s2) and Ni_d9.2p, and likewise Ni.3p and Ni_d9.3p.

As mentioned above, some all-electron basis sets are present in the basis set directories SZ through TZ2P (old names I-V), but not for all elements of the periodic table. The heavier elements, from Rb on, the non-relativistic all electron basis sets are missing. In the ZORA basis sets directory you will find all-electron basis sets for all elements (Z = 1-118), which also could be used in non-relativistic calculations. Note, however, that these basis sets were optimized for ZORA calculations, which means that non-relativistic calculations will not always give you the expected accuracy. Warning: the frozen core basis sets in the ZORA directory should never be used in non-relativistic calculations. Non-relativistically optimized basis sets for the heavier elements are provided in a separate directory AE, which contains basis sets of single-, double- and triple-zeta quality indicated respectively by suffixes 'sz', 'dz', and 'tz'. The files in Special/AE/ are not complete database files, because they don't contain fit sets (the usage and relevance of fit functions is explained later).

The development of fit sets and their testing is not a triviality. It is absolutely a bad idea to take a fit set from another database file, corresponding to some frozen core level, and use that in an all-electron basis set: this will give significant errors and make results worthless. In the ZORA directory one can find all-electron basis sets with good fits sets for the heavier elements.