Here, 𝑋 is the cardinal number of the basis set and 𝐸 𝐷𝐹𝑇,𝑋→∞ corresponds to the total energy in the CBS limit. This energy is obtained as intercept of a linear fit with the slope 𝐴. The cardinal number 𝑋 corresponds to 3 for TVZP-/TZV2P-GTH basis sets and 4 for QZV2P-/QZV3P-GTH basis sets. The cubic extrapolation formula was originally proposed for an extrapolation of the correlation energy part in post-Hartree Fock calculations, but was subsequently also applied to total energies. Although tailored variants of the extrapolation formula have been proposed, e.g., for use with specific types of basis sets, they tend to give similar results as equation 2 if the larger basis set is of quadruple-zeta or higher quality.