Chemistry without potential energy surfaces

Chemistry without potential energy surfaces Highly quasi-degenerate electronic states... 

Computational methods typically employ the Born-Oppenheimer approximation in most electronic structure programs to separate the nuclear and electronic parts of the Schrodinger equation that is still hard enough to solve approximately. There would be no potential energy (hyper)surface (PES) without the Born-Oppenheimer approximation -how difficult mechanistic organic chemistry would be without it ... 

One of the most difficult problems for ab initio quantum chemistry is to determine the potential energy function for a chemical reaction on a metal surface. Why is this so First of all, the metal substrate is strongly delocalized. This means that the system cannot be modeled [1] by considering just a small or medium-sized cluster of metal atoms. On the other hand, the band structure techniques that would simplify calculations for a bare metal surface cannot be directly applied because the translational symmetry is broken by the presence of the reactants. As a result one has the difficulty of dealing with extended interactions without the benefit of simplifications due to symmetry. Many problems involving surfaces, interfaces, impurities, or defects in solid state materials fall under this broad rubric along with various solution phenomena as well. 

Clementi (1985) described ab initio computational chemistry as a global approach to simulations of complex chemical systems, derived directly from theory without recourse to empirical parametrizations. The intent is to break the computation into steps quantum mechanical computations for the elements of the system, construction of two-body potentials for the interactions between them, statistical mechanical simulations using the above potentials, and, finally, the treatment of higher levels of chemical complexity (e.g., dissipative behavior). This program has been followed for analysis of the hydration of DNA. Early work by Clementi et al. (1977) established intermolecular potentials for the interaction of lysozyme with water, given as maps of the energy of interaction of solvent water with the lysozyme surface. 

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