PES: Exercise 3

Relative energies of H₂O reactions

Some important relative energy quantities in chemistry are shown in the slide above.

Calculate all of them for the water molecule using the HF-3c level of theory:

1. Bond dissocation energy: homolytic and heterolytic

2. Atomization energy

3. The vertical 1st ionization energy of H₂O and the adibatic ionization energy (this means you relax the [H₂O]⁺ molecule by a geometry optimization)

4. The vertical electron affinity and the adiabatic electron affinity (this means you relax the [H₂O]^- molecule by a geometry optimization)

Bug: It turns out that the minimal basis set in the HF-3c method is too small to describe the [H₂O]^- correctly. To describe the molecule correctly requires you to use a larger basis set and thus the following input instead: !HF def2-TZVP (instead of ! HF-3c ). You are also allowed to skip this part.

All relative energies should be given as either kcal/mol or eV.

5. Look up reference data for these energetic quantities in either textbooks or online databases. Note that experimental electron affinities for water are not to be found.

How do your calculated values compare? Note that the simple HF-3c method will give you pretty bad accuracy (actually it's crap :) ) but your numbers should be of roughly the right magnitude. We are also ignoring thermal contributions. Let me know if you don't find valid reference data. Is there a big difference between your vertical and adiabatic values?

In a later exercise we will recalculate these quantities with a high-accuracy method and the agreement with experiment will improve.

Notes:

1. When calculating atoms, you never use the Opt keyword. There is nothing to optimize as there is no internal degree of freedom.

2. Make sure to choose the correct charge and spin for the fragments. Note that spin multiplicity is : 2S+1 where S is the spin of the atom/molecule.

3. Vertical ionization energy/electron affinity means that you use the optimized geometry of the normal water molecule and do a single-point energy calculation on the cation or anion (simply don't include the Opt keyword). Adiabatic means that you relax the structures (optimize).

4. Experimental data might be found in NIST databases, e.g. Computational Chemistry Comparison and Benchmark database and various other places. If you don't find experimental data, then theoretical data is also fine to compare to.