- The direct RPA correlation energy is defined in a Kohn-Sham context without inclusion of exchange integrals and therefore the use of self-consistent KS orbitals obtained from (semi-)local functionals is recommended. HF-orbitals or KS-orbitals obtained form hybrid functionals lead to inferior results.
- Experience has demonstrated that the difference in RPA correlation energies obtained from different (semi-)local functionals is very small (much smaller than the inherent error of the method).
- Like MP2, RIRPA results are known to converge very slowly with increasing
basis set size, in particular slowly with increasing
-quantum number of the basis set. For reliable results the use of QZVP basis sets (or higher) is recommended. For non-covalently bound systems larger basis sets (especially, with more diffuse functions) are needed.*l* - It is recommended to exclude all non-valence orbitals from RIRPA
calculations, as neither the
`TURBOMOLE`standard basis sets SVP, TZVPP, and QZVPP nor the cc-pVXZ basis set families (with X=D,T,Q,5,6) are designed for correlation treatment of inner shells (for this purpose polarisation functions for the inner shells are needed). The default selection for frozen core orbitals in`Define`(orbitals below -3 a.u. are frozen) provides a reasonable guess. If core orbitals are included in the correlation treatment, it is recommended to use basis sets with additional tight correlation functions as e.g. the cc-pwCVXZ and cc-pCVXZ basis set families. - We recommend the use of auxiliary basis sets
optimized for the corresponding (MO) basis sets. The auxiliary basis sets optimized for
RI-MP2 and RI-CC2 are suitable for
`rirpa`[133] correlation energy calculations. - For systems with heavy atoms where ECPs are required,
HXX+RIRPA total energies can be computed in two steps.
RIRPA correlation energies can be obtained using the
`nohxx`option, and the HXX energy can then be computed separately, for example, in`ridft`if the RI-J approximation is used for the HXX Coulomb integrals. To compute the HXX energy, disable`$dft`in the`control`file and perform a single SCF iteration by setting`$scfiterlimit 1`. Adding the total energy from`ridft`and the correlation energy from`rirpa`together gives HXX+RIRPA total energies.*Note:*the molecular orbitals are altered by`ridft`after a single-iteration, so the HXX energy must be computed*after*the RIRPA correlation energy.