Potential energy surface computations

S. P. A. Sauer and M. J. Packer, The Ab Initio Calculation of Molecular Properties other than the Potential Energy Surface, Computational Molecular Spectroscopy, P. R. Bunker and P. Jensen, eds., Wiley, London, 2000, Chapter 7, pp. 221. 

The only type of chemical reaction we are likely to ever be able to solve rigorously in a quantum mechanical way is a three-body reaction of the type A+BC - AB+C. (See Fig. 5.) The input information to the dynamicist is the potential energy surface computed by the quantum structure chemist. Given this potential surface, we treat the nuclear collision dynamics using Schrodinger s equation to model the chemical reaction process. 

Calculations have underscored the dominating importance of electron correlation in understanding the behavior of these systems. Whereas potential energy surfaces computed at the Hartree-Fock level typically indicate the presence of two distinct minima, one corre-... 

Potential Energy Surfaces, Computer Simulations, and the Structure of Water... 

The various approaches discussed in this chapter all stem from elementary Hiickel Molecular Orbital theory. Why do qualitative arguments based on so approximate a set of assumptions work as well as they do Can such naive considerations still serve a useful purpose in this day of sophisticated semi-empirical and ab initio multiconfigurational potential energy surface computations, and -audacious presumption - perhaps even suggest ways of improving the efficiency and reliability of these very computations ... 

Figure 1 The core contribution to the potential energy surface computed with a valence and core augmented basis set where the bond lengths are fixed at r = 0.8 A and ri 0.9 A...

In summary, spin contamination can cause problems for potential energy surfaces computed by methods based on UHF determinants. Spin projection can improve the behavior of UMPn calculations, but normally should not be used for UHF and DFT calculations. Methods such as QCISD, CCSD, and BD are much less susceptible to the effects of spin contamination. 

Sauer, J. and Dobler, J. (2005) Gas-phase infrared spectmm of the protonated water dimer molecular dynamics simulation and accuracy of the potential energy surface. Comput. Phys. Commun., 6, 1706-1710. 

M. L. McKee. Ab initio study of the N204 potential energy surface. Computational evidence for a new N204 isomer. J. Am. Chem. Soc. 117, 1995,1629-1637. 

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