Abstract
Truncated configuration interaction (CI) calculations are known1 to yield total electronic energies which do not scale properly with the number of electrons in the system. This so called “size extensivity” (SE) problem causes truncated CI energies (e.g., those which include all excitations up through some specified level) energies to be of questionable value, for example, in computing thermodynamic energy differences such as chemical reaction energies. Specifically, for the reaction $$ A+B\rightarrow C+D $$ (1) truncated-CI energies of the individual species (C, D, A, and B) can not be used to evaluate ΔE; one must evaluate, within the truncated-CI method, the energy of the C + D and A + B “supermolecules” at large C — D and A — B separations, and then subtract these two energies to evaluate ΔE.
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