energy for RHF/UHF/ROHF references.
The correlation energy is built in the spin-blocked (aa, bb, ab) form on
semicanonicalized orbitals, reusing the validated two-electron driver
(int2_compute) via per-occupied-MO-pair Coulomb builds -- so no full O(N^4)
MO integral tensor is ever stored. A ROHF reference is semicanonicalized first
(occ-occ and vir-vir Fock blocks diagonalized) so the canonical MP2 amplitude
denominators are well defined. Validated to 1e-8 Ha against PySCF UMP2.