Opposite-spin double hybrids
There are several double hybrids that use a different fraction of same-spin MP2 correlation than opposite-spin MP2 correlation.
However; the new API in the master branch currently only supports full MP2 correlation.
So, we have two choices:
- in analogy to what we do for range-separated hybrids, mark the MP2 contribution using full MP2 + additional opposite-spin MP2 correlation
- remove the full MP2 and mark explicitly the same-spin and opposite-spin contributions
Option 1 is easier for both conventional and opposite-spin-only functionals. However, figuring out the fraction of same-spin and opposite-spin correlation requires some arithmetics (% ss = % full MP2, % os = % full MP2 + % os MP2)
Option 2 is simple for opposite-spin only functionals, but requires a bit more work for same-spin functionals. However, it is easier for developers and end users, since both are clear from the outset.