Ab Initio LCAO-MO Methods

Ab initio LCAO-MO Methods.—The so called ab initio MO method is defined by the discussion so far. The two approximations made are the MO form of the wave-function and the necessary truncation of the basis set. It is important to realize that all the ab initio calculations on large molecules are far from the Hartree-Fock limit and that even this limit may be inadequate to describe a particular feature of the molecule. An excellent introduction to ab initio calculations is the recent book by 

9c A critical discussion of calculations mainly on small molecules is given in Schaeffer s book.9 The inadequacies of Hartree-Fock theory and small basis sets are clearly discussed here, and several calculations on large molecules are discussed in detail. 

Another widely used choice of basis set is a small set of spherical GTOs centred in bond and lone-pair regions as well as in atoms. Such a basis set can give remarkably 

11 (a) J. Hinze, Adv. Chem. Phys., 1974, 26, 213 (A) W. G. Richards, T. E. H. Walker, L. Farnell, and P. R. Scott, Bibliography of Ab initio Molecular Wave Functions , supplement for 1970-1973, Clarendon Press, Oxford, 1974. 

19 Quantum Chemistry - the State of the Art , Volume 3, Hartree-Fock Theory , Proceedings of S.R.C. Atlas Symposium No. 4, ed. V. R. Saunders and J. Brown, Atlas Computer Laboratory, Chilton, near Didcot, Oxford, 1974. 

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The four most important tautomers of pyrazolo[4,3-<7]pyrimidine-5,7(477,677)-dione (52a-d) were studied using ab initio LCAO-MO methods <91CPL(181)123>. Calculation indicates that structure (52a) is the predominant form both in nonpolar and polar environments. The second most stable species, the oxo hydroxy tautomers (52b) and (52c), have relative energies of 38.7 kJ mol . For the dioxo to oxo-hydroxy transition, the calculated energy gap corresponds to an equilibrium constant... 

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