Size inconsistency of the IOPT method

Several recently proposed zeroth-order Hamiltonians for open-shell perturbation theory include terms non-linear in the number operators. These Hamiltonians are, in general, described as a correction, which is to be added to a conventional Fock operator. It has previously been pointed out that one of these corrections, OPT2, gives energies for twisted ethylene that vary with rotation among the singly occupied orbitals, and in a recent article a new operator, called the IOPT operator, was proposed as a remedy. However, this proposed cure is worse than the disease, since the resulting theory has an unacceptably large size consistency error. The properties of a molecule will depend upon the properties of infinitely removed molecules, and this unphysical action at distance is of the same or larger order of magnitude as the proposed correction. The problem is demonstrated by the singlet-triplet splitting of methylene, which varies by 5.5 kcal/mol when other atoms are placed at 1000.0 α0 distance.