(a) As noted in the text, for the cc-pVnZ basis sets, it is best not to use the n = 2 energy value in extrapolations. Also, the n = 6 energy value is often not available. Use (15.23) and the H2O HF/cc-pVnZ energies for n = 3, 4, and 5 given just before (15.23) to find the values of the three parameters in (15.23). There are three equations in three unknowns. Subtract the n = 3 equation from the n = 4 equation and from the n = 5 equation to obtain two equations in two unknowns. Then combine these equations to eliminate A. To solve the remaining equation for B, put all terms on one side of the equation and use the Excel Solver to find B. Then find the other two unknowns.
(b) Now use the n = 3, 4, 5, and 6 energies and the Excel Solver to find the values of the three parameters in (15.23) that minimize Σ6n = 3(E(15.23) - Ecc.pVAZ)2. Take the initial guesses for the parameters as the results from part (a). In Excel 2010, be sure and uncheck the Make Unconstrained Variables Non-Negative box in the Solver Parameters box.