Question

A student wishes to calculate the gas phase Gibbs free energy change for the following reaction (at 298 K and 1 atm): CH3CH2CH2COOH(g) --> CH3CH2CH2COO-(g) + H*(g) To do so, the student performed a HF/6-31G(d) geometry optimisation and frequency calculation for each species but the result differs significantly from the experimental value. There are several options available to the student to improve the accuracy of this prediction. Which of the following is least likely to remedy the error in the calculation? Select one: O a. Perform a CCSD(T)/aug-CC-PVTZ calculation for the proton. b. Perform a systematic conformer search to identify the lowest energy conformer of each species c. Repeat the above calculation at the HF/6-31+G(d) level of theory d. Apply scaling factors to the vibrational frequencies to account for anharmonic effects.

Please explain your answer

Answer 1

The correct option will be D.

Scaling factor applied for anharmonic effects are only provide frequency correction but it does not affect the optimization.

All the other options will give better and more precise optimization and therefore give approximate experimental results because either they have high basis sets or advanced method for the calculation.