The direct RPA correlation energy is defined in a Kohn-Sham context withoutinclusion of exchange integrals and therefore the use of self-consistent KS orbitalsobtained from (semi-)local functionals is recommended. HF-orbitals or KS-orbitalsobtained form hybrid functionals lead to inferior results.
Experience has demonstrated that the difference in RPA correlation energies obtainedfrom different (semi-)local functionals is very small (much smaller than the inherenterror of the method).
Like MP2, RIRPA results are known to converge very slowly with increasing basis setsize, in particular slowly with increasing l-quantum number of the basis set. For reliableresults the use of QZVP basis sets (or higher) is recommended. For non-covalentlybound systems larger basis sets (especially, with more diffuse functions) are needed.
It is recommended to exclude all non-valence orbitals from RIRPA calculations,as neither the TURBOMOLE standard basis sets SVP, TZVPP, and QZVPP nor thecc-pVXZ basis set families (with X=D,T,Q,5,6) are designed for correlation treatmentof 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. arefrozen) provides a reasonable guess. If core orbitals are included in the correlationtreatment, it is recommended to use basis sets with additional tight correlationfunctions 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 forrirpa [170] correlation energy calculations.
For systems where ECPs are required as well as within the two-component relativisticimplementation, RIRPA total energies (HF@KS + correlation) must be computed intwo steps. RIRPA correlation energies can be obtained using the nohxxoption, and theHF energy can then be computed separately, e.g., in ridftif the RI-J approximationis used for the Coulomb integrals. To compute the HF@KS energy, compute the KSorbitals first; then disable $dftand set $scfiterlimit 1in the controlfile toperform a single SCF iteration. Finally add the total HF@KS energy from ridfttothe correlation energy from the nohxx-rirpacalculation to obtain the total RIRPAenergy. Note: the molecular orbitals are altered by ridftafter a single-iteration, sothe HF@KS energy must be computed after the RIRPA correlation energy.
Tight SCF ($scfconv 7) and one-electron density matrix ($denconv 1d-7)convergence criteria, large basis sets (QZVP), and large frequency grids which ensure asensitivity measure of no more than 1d-4should be used in combination with rpagradfor accurate results.