Perhaps surprisingly, pair natural orbitals (PNOs) have a long history in quantum chemistry, with the work of Edmiston and Krauss on "pseudonatural orbitals," published in 1965, eighteen years before Pulay's seminal publication on projected atomic orbitals. PNOs were used in the early PNO-CI and IEPA-PNO 99 methods. However, the advent of more efficient direct CI approaches