MOLPRO Basis Query (current), basis=def2-TZVPP_OPT

   - The "OPTRI" basis sets must be considered as experimental. They
     were automatically generated to extend the valence space and
     valence-shell-minus-1 core space of the orbital basis set. The
     concrete procedure is described in the thesis of Gerald Knizia
     (can be obtained online). These were not actually optimized to
     minimize RI errors; rather, they just fill "holes" in the exponent
     coverage of the orbital sets, and thereby more or less saturate
     the region of the valence space covered by the orbital basis sets
     The sets are mainy provided as an intermediate solution until
     better basis sets are developed. Nevertheless, they allow F12
     calculations for nearly all elements in the periodic table, and for
     the cases in which this approach was actually tested (main group
     elements up to 2nd row), it appeared to work decently. If using
     them, some validation of the calculations with conventional
     methods or larger basis sets is strongly recommended.

   - These OPTRI sets are complementary auxiliary sets: An RI set here
     can only be used in conjunction with the def2-nZVPP orbital set for
     which it was generated and its singly augmented modification.

 - Gerald Knizia, 2009-11-20