Interactions with singly occupied orbitals are left uncalculated, and singly occupied orbitals are automatically given the highest possible entropy. Finally, due to the observed biasing of APC entropies towards doubly occupied orbitals a series of virtual orbital removal steps are employed N times in which the highest-entropy virtual orbital is removed from the sums in equations 6 and 7 and the entropies are recalculated; these highest-entropy virtual orbitals are then assigned the highest entropy at the end of the calculation. For small-to-medium sized organic systems we have found good results with N = 2, 12 which we have used here.