On the ordering of modified virtual orbitals in doublet state radicals

The orbitally ordered CI method of Cooper and Pounder is applied to open-shell molecules. The modified virtual orbitals (MVO’s) are calculated using an equivalent operator to the closed-shell operator but the ordering process needed to neglect some MVO’s is much more complicated for open-shell systems. It is shown that 80% of the (S+D)CI energy can be recovered using only one-tenth of the configurations.