where 𝑋 is the cardinal number (called X in the introduction) of the basis set used, 𝐸(𝑋) is the energy calculated with the basis set of cardinal number 𝑋, 𝐸 &-' is the CBS limit, and 𝑓(𝑋) is a function of 𝑋 with a coefficient, 𝐴. The function 𝑓(𝑋) approaches zero as 𝑋 approaches infinity. The coefficient 𝐴 can be eliminated by substituting two consecutive values of 𝑋 and 𝑌 in eq 3 and subtracting the two equations. This also gives the formula for the linear extrapolation to the CBS limit.