Examples of Coordination Sphere Building Procedures

The manner in which MOLPAK derives coordination spheres is illustrated by three examples, one for P-1 and two for P2i/c, the most important of the space groups for light atom containing organic substances. The procedure for each type is unique, but these examples show some of the techniques developed. 

Among the most important computational aspects of MOLPAK are replacement of the standard interatomic potential by a repulsion-only potential 

Numerous other tricks were used to speed the calculations, such as the initial calculation of an Ey vs. ry2 table for atom ij pairs and a table look-up based on the ry2 between atoms. This avoided the repeated calculation of Ey and the need to take square roots to obtain ry. 

Ideally, a single A (or B) coefficient would be appropriate for each element type such as one AH for all hydrogens. It has been our experience, however, that atoms of the same element often require different coefficients in different functional groups. There would be, for example, different AH s for H in H-O, H-NR2, H-C(sp2), H-C(sp3), etc. Other complications may involve separate cross terms for each of the atom i - atom j interactions. For a simple molecule such as formaldehyde (H2CO), the Ay cross terms are handled in WMIN with three AH, Ac and Ao coefficients that are used to form the six possible cross terms. But other programs, (e.g. DMAREL) may require cross terms that encompass the Ahh, AHo AHc, ACc, ACo and Aoo possibilities. This is undoubtedly more flexible and possibly more accurate but does have the disadvantage of requiring the evaluation of many cross term coefficients. Of course, in cases with separate cross terms, one can always use the WMIN simplification of AHo = (AH A0) /2, etc. 

Several examples of the MOLPAK + WMIN structure prediction procedures are given in the next sections. The problem of identifying the correct crystal structure from literally thousands of possible structures remains. The which is the best/correct solution was an important topic during the 1999 and 2001 CCDC sponsored blind tests [12]. Calculated lattice energies are adequate in many cases, but in others small lattice energy differences make it difficult to separate the wheat from the chaff. Other criteria, such as good or bad patterns of intermolecular contacts in comparison with known crystal structures could be helpful. 

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