# Extended Hybrid Methods

Hybrid DFT methods, where the functional is supplemented with Hartree-Fock exchange, have become increasingly popular due to their low cost and decent accuracy. Double hybrids, which mix in an MP2 contribution, have also been developed as a way to improve the accuracy.

Has this idea been extended to other Post-SCF methods? For example, one could consider doing coupled cluster or configuration interaction calculations where a DFT wavefunction is used as the primary configuration. If not, is there a major hurdle, technical or theoretical, that prevents this from being done?

• @NikeDattani I had initially thought of asking about a "reverse hybrid", but after reading a bit more on double hybrids, apparently there isn't much of a distinction. Double hybrid DFT methods actually perform an MP2 calculation with the DFT wavefunction, so its not necessarily a clear distinction whether you are improving DFT with MP2/Post-SCF or the other way around. – Tyberius May 16 '20 at 17:19
• I once asked about mixing in coupled cluster. The main reluctance to mix in CC is probably that the cost would become $N^6$ for CCSD, which might defeat the purpose of DFT (some people might say that the purpose of DFT is to provide a low-cost method). The nice thing about DFT+MP2 is that the accuracy tends to be better than standard MP2, and gets quite close to CCSD(T) accuracy with roughly MP2 cost. But let me ask Stefan Grimme for a better answer! – Nike Dattani May 30 '20 at 4:45