Developing analytical tools 0, approximately

A further model Hamiltonian that is tailored for the treatment of non-adiabatic systems is the vibronic coupling (VC) model of Koppel et al. [65]. This provides an analytic expression for PES coupled by non-adiabatic effects, which can be fitted to ab initio calculations using only a few data points. As a result, it is a useful tool in the description of photochemical systems. It is also very useful in the development of dynamics methods, as it provides realistic global surfaces that can be used both for exact quantum wavepacket dynamics and more approximate methods. 

In applying quantum mechanics to real chemical problems, one is usually faced with a Schrodinger differential equation for which, to date, no one has found an analytical solution. This is equally true for electronic and nuclear-motion problems. It has therefore proven essential to develop and efficiently implement mathematical methods which can provide approximate solutions to such eigenvalue equations. Two methods are widely used in this context- the variational method and perturbation theory. These tools, whose use permeates virtually all areas of theoretical chemistry, are briefly outlined here, and the details of perturbation theory are amplified in Appendix D. 

This Chapter is concerned with some of the mathematical tools required to describe special properties of curved surfaces. The tools are to be found in differential geometry, analytical function theory, and topology. General references can be foimd at the end of the Chapter. The reader xminterested in the mathematics can skip the equations and their development. The ideas we want to focus on will be clear enough in the text. A particular class of saddle-shaped (hyperbolic) surfaces called minimal surfaces will be treated with special attention since they are relatively straightforward to treat mathematically and do form good approximate representations of actual physical and chemical structures. 

After only approximately a decade since it was developed, Crystaf has become one of the most important analytical techniques in polyolefin characterization laboratories. It can provide fast and crucial information required for the proper imderstanding of polymerization mechanisms and structure-property relationships. In industry, it has been established as an indispensable tool, together with Tref, for product development and product quality monitoring. 

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