I’ve come across an issue with SOS-OMP2 (scaled opposite spin orbital optimized MP2) as implemented in PSI4.
In a single point calculation, I get identical energies (OMP2 total energy, scaled_SS energy, scaled_OS energy, SOS-OMP2 total energy, etc) using OMP2 as when I use SOS-OMP2. This is problematic, as the different scaling of the energy terms should be lead to PSI4 rotating the orbitals differently.
I couldn’t locate the source code for SOS-OMP2 to check whether it was a complete implementation (affecting density) or just a final scaling factor of the energy after performing OMP2 (certainly looks to be the case, even though do_sos is set to true).
I know it’s a rather niche method, but has anyone come across this, or any devs know how it’s implemented?