Theory and Computational Approaches

The choice of cell parameters is not unique because the same lattice can be described by different sets of parameters. Rules exist for obtaining a set of standard cell parameters for a given lattice, called the conventional cell. The reduced cell is the standard primitive (noncentered) cell to describe a given lattice. Andrews and Bernstein describe a method to determine the reduced cell from a given set of cell parameters, and in that report an overview of earlier methods is presented. More recently, a new algorithm for this purpose was developed by Zuo et al.  

The relative stability of polymorphs at a given temperature and pressure is determined by their differences in Gibbs energy, AG  

A number of computational methods are frequently used in crystal structure prediction programs. To assess the differences between the programs, we list the most important techniques commonly used and mention their strengths and weaknesses. 

A separate energy term to account for hydrogen bonds is used in some force fields, and cross-terms (such as stretch-bend) are often used. 

Further simplifications to the MM energy function can be introduced such as by ignoring the electrostatic contribution for nonpolar compounds, and/or using mainly the repulsive part of the van der Waals function by ignoring the van der Waals energy for atom pairs with an interatomic distance greater than a certain threshold. Obviously, these simplifications will generally improve the speed of calculations, but they do so at some cost in accuracy. 

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I. Shavitt, Advanced Theories and Computational Approaches to the Electronic Structure... 

I. Shavitt, Advanced Theories and Computational Approaches to the Electronic Structure of Molecules C. E. Dykstra, Ed., 185, Reidel, Dordrecht (1984). 

I. Shavitt, The Method of Configuration Interaction. In H. F. Schaefer III (Ed.) Methods of Electronic Structure Theory Modem Theoretical Chemistry, Vol. 3. (Plenum, New York, 1977), pp. 189-275 The Treatment of Electron Correlation Where Do We Go from Here In C. E. Dykstra (Ed.) Advanced Theories and Computational Approaches to the Electronic Structure of Molecules (Reidel, Dordrecht, 1984), pp. 185-196. 

Shavitt I (1984) In Dykstra CE (ed) Advanced theories and computational approaches to the electronic structure of molecules. Reidel, Dordrecht p 192... 

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