Development of such basis sets subsequently led to the proposal of numerous and mostly empirical extrapolation schemes to estimate values in the complete basis set (CBS) limit , some of which are detailed later in Section II C 1. Although the BSSE vanishes in the CBS limit, these extrapolation procedures are not intended to directly correct for it, but rather attempt to eliminate the error due to the finite basis that we call here the basis set incompleteness error (BSIE) . While most extrapolation schemes are of a fully empirical nature, some are based on theoretical motivations about the leading behavior of approximate atomic wave functions, so that their applicability to molecular systems requires that the correlation energy be dominated by the electron-electron (Coulomb) cusp and that assumptions are transferable to polyatomic systems . In addition, it is generally assumed that extrapolations are applicable from one correlated method to another. For example, the usual X -3 scheme introduced by Helgaker 1,26 relies on the finding that the principal expansion of the helium configuration-interaction (CI) energy holds for energies obtained with correlation-consistent bases, because both converge according to the principal quantum number n for this two-electron atom. Transferred to molecules, this one would then assume that lower order terms as well as chemical bonding effects are negligible, and that the expression applies equally to all system sizes and quantum chemical methods .