BIGSTRN program

The classical treatment for the quantitative determination of the steric effects operative in molecules was developed by Westheimer. Steric effects were considered as the sum of various independent strain producing mechanisms (bond strain, angle strain, torsional strain, non-bonded interaction strain). Westheimer s assumptions proved to be the fundamental basis for the BIGSTRN program as well as for all subsequent molecular mechanics treatments of neutral hydrocarbons and carbocations. Reactivities ranging over 10 ° could be correlated by the strain differences between cation and the neutral precursor. Gleicher and Schleyer s work was a historical breakthrough in the development of molecular mechanics and provided the basis for the predictions of rate constants of solvolysis reactions. For the first time chemical reactions could reliably be predicted by the means of computational chemistry. 

Although satisfying correlations of solvolysis rate constants with strain energy differences of cations and neufi al precursors had already been obtained with Bingham and Schleyer s BIGSTRN program, Muller s UNICAT 4 parameters improved the absolute values of strain energies. 

The Schleyer-Mislow program BIGSTRN (26b) likewise is extensively used. This program incorporates the so-called Engler-Schleyer force field (26a) as well as AUinger s 1971 force field as standard options and is unique in that it uses pattern-search methods of energy minimization. This method is effective at the outset but slows down as the geometry approaches a minimum (Table 2). 

A word of caution on the use of the Schleyer-Mislow program BIGSTRN as obtained from QCPE may be appropriate. The default criteria for termination of the energy minimization loop, namely, 0.01 kcal/mol regardless of the size of molecule, is too loose. One can strengthen the criteria 100-fold with one of the options, with the inevitable consequence of prolonged computation time (29a). 

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