Molecular Orbital and Force-Field Calculations for Structure and Energy Predictions
This article aims to provide a short review of the methods available for the calculation of molecular structures and energies and other properties. The programs employed all use a guessed initial geometry as starting point for the calculation and then optimize the structure in order to find the minimum that can be found by moving down in energy from the starting point. This leads to the first problem with such methods, especially for very large molecules. There are at present very few programs that can investigate a series of possible structures in order to identify as many minima as possible and to be able to find the global (most stable) minimum within any degree of certainty. Even when this is possible (at present only for molecular mechanics calculations) the cost in computer time can be very large. For the other methods, the chemist must have enough imagination to be able to predict all the possibilities open to the molecule in order to find the global minimum.
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