Why doubly excited determinants govern configuration interaction   calculations of electron correlations
  Computational evidence shows that, when using natural orbitals to study (dynamical and non-dynamical) electron correlation, determinants with an odd number of excitations play a negligible role. Instead, doubly excited determinants rule the rostrum in this kind of configuration interaction calculations. We explain mathematically why it must be so. 
Carlos L. Benavides-Riveros, Jos\'e M. Gracia-Bond\'ia and Michael   Springborg
quant-ph cond-mat.str-el physics.atm-clus physics.chem-ph