Computational Chemistry

pervades chemistry so thoroughly, and is so useful, that this stark view of a

molecule is unlikely to find many adherents.

References

1. Orville Chapman (1932–2004; professor Iowa State University, UCLA; pioneer in organic
   photochemistry and matrix isolation studies). Remark in a lecture at the University of Toronto,
   ca. 1967


2. Bader RFW (1990) Atoms in molecules. A quantum theory. Clarendon Press, Oxford


3. E.g.: (a) Dobado JA, Martı ́nez-Garcı ́a H, Molina JM, Sundberg MR (1999) J Am Chem Soc
   121:3156. (b) Rozas I, Alkorta I, Elguero J (1977) J Phys Chem A 101:9457


4. Coulson C (1963) “What is a Chemical Bond?” Lecture at Basler Chemische Gesellschaft,
   April 26, 25-page typescript, 3–6, Coulson Papers, Bod. Oxford

Chapter 5, Harder Questions, Answers

Q4

Why is it that (in the usual treatment) the calculation of the internuclear repulsion
energy term is easy, in contrast to the electronic energy term?
It is easy because we know where the nuclei are. In the usual treatment the
nuclei are fixed and the electrons move in their field of attraction; this is the Born–
Oppenheimer approximation. Given the coordinates of the nuclei (which along
with charge and multiplicity define the molecule) the internuclear repulsion
energy is simply obtained as the sum of all pairwise repulsion energies. Of course
the nuclei are actually vibrating aroundaveragepositions,evenat0K.Thezero
point energy (zero point vibrational energy, ZPE or ZPVE) is calculated from the
energies of the normal modes, these energies being obtained from the normal
mode frequencies, which are calculated with the aid of the matrix of second
derivatives of energy with respect to position, the Hessian matrix. The vibra-
tional energy at higher temperatures canbe obtained by the usual thermodynamic
device of calculating the vibrational partition function from the normal mode
frequencies [1].

Reference

1. See e.g. Ochterski JW Thermochemistry in Gaussian, Gaussian White Paper. http://www.
   gaussian.com/g_whitepap/thermo.htm, and references therein. Accessed 2010 August 20

624 Answers