Density fitting can be used to approximate the integrals in spin restricted Hartree-Fock (HF), density functional theory (KS), second-order Møller-Plesset perturbation theory (MP2 and RMP2), explicitly correlated MP2 (MP2-F12), and all levels of closed-shell local correlation methods (LCC2, LMP2-LMP4, LQCISD(T), LCCSD(T)). In the literature the corresponding methods are often also denoted as RI-HF, RI-DFT, or RI-MP2 etc. We prefer the acronym DF since RI is used for another purpose in explicitly correlated methods (cf. section 15).
These methods are either direct or semi-direct, i.e. only relatively small sets of transformed four-index integrals are stored. They are very much faster than standard direct calculations and highly recommended, in particular for calculations on larger molecules. The errors caused by density fitting approximations are generally negligible.
Density fitting is invoked by adding the prefix DF- to the command name, e.g. DF-HF, DF-KS, DF-MP2 and so on. Gradients are available for DF-HF, DF-KS, and DF-LMP2. Symmetry is not implemented for density fitting programs. Therefore, symmetry is turned off automatically if DF- is found in the input.
By default, a fitting basis set will be chosen automatically that corresponds to the current orbital basis set and is appropriate for the method. For instance, if the orbital basis set is VTZ, the default fitting basis is VTZ/JKFIT for DF-HF or DF-KS, and VTZ/MP2FIT for DF-MP2. Other fitting basis sets from the library can be chosen using the DF_BASIS option, e.g.
BASIS=VTZ !use VTZ orbital basis DF-HF,DF_BASIS=VQZ !use VQZ/JKFIT fitting basis DF-MP2,DF_BASIS=VQZ !use VQZ/MP2FIT fitting basis
The program then chooses automatically the set which is appropriate for the method. Optionally, the basis type can appended to the basis name and then this supercedes the default, e.g. DF-HF,DF_BASIS=VQZ/JKFIT !use VQZ/JKFIT fitting basis
There are many options affecting, e.g. screening and other details, but the default values should normally be appropriate. A full description can be found in the MOLPRO users manual.
molpro@molpro.net 2012-02-11