SCF Stability Calculations

The stability of SCF solutions for unknown systems should always be tested. Stability considerations apply to and may be tested for in calculations using Density Functional Theory methods as well. 

Stable Tests the stability of the SCF solution computed for the molecule. This 

In order to illustrate how stability calculations work, we ll run the following RHF calculation on molecular oxygen  

Exploring Chemistry with Electronic Structure Methods 

We can be sure that the RHF wavefunction for molecular oxygen is unstable, since we know the ground state of the molecule is a triplet. The output from the stability calculation confirms this  

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SCF stability calculations were performed for every molecule ai every model chemistry to ensure thui the lowest energy electronic vtale of the proper type was used. 

Recent SCF—XaSW calculations indicate that the above geometry is stabilized by attractive ligand-ligand nonbonded interactions231, 232 ... 

Insights into the acidolysis of ozonides have been gained using ab initio SCF-MO calculations at the split valence 4-31G level (Section 4.16.5.2.1). Comparisons were made between experimental observations of reaction products and calculated stabilities and charge distributions for the species involved <83JOC2366). 

Against this, non-empirical SCF—MO calculations on the C2H8S+ ion show (Denes et aX., 1971) that the bridged structure (type 162) is about 66 kcal mole-1 more stable than the linear structure. These results, although preliminary, accord very well with the chemical evidence (see infra) and point to a great influence of the nature of the electrophile on the relative stability of open and bridged structures. 

An experimental study of diacetylene and HF in solid argon suggested both sorts of complexes (a and b in Fig. 6.6) were present and that they are of comparable stability. Correlated (MP2) calculations with a 6-31 -I- -l-G(d,p) basis set found the perpendicular complex (a in Fig. 6.6), wherein FH approaches one of the two triple bonds of diacetylene, is more stable than is complex b wherein C—H acts as proton donor. The electronic contributions to the binding energies of complexes a and b are calculated to be —3.8 and —2.6 kcal/rnol, respectively. However, these values are surely inflated by the failure to correct them for BSSE. One can conclude that the triple bond is a better proton acceptor than the alkynic C—H is a donor, at least when paired with the rather strong acid HF. The preference for this sort of geometry is confirmed by gas-phase measurements, and are valid also when HF is replaced by HCl . The importance of using a satisfactory level of theory for such complexes is reinforced by comparison with earlier SCF-level calculations which predicted a structure like b to be most stable. 

The CNDO/2 calculations correctly predict tautomers 32 of uracil and thymine to be more stable than tautomers 28. It is interesting to observe that 5-fluorouracil is predicted to be more easily converted into lactim form 28 than uracil and thymine, considering that 5-fluorouracil is mutagenic. As to the relative stability of uracil tautomers, the different approaches give different results. For instance, the tt-SCF MO calculations including a-polarization effectpredict the relative stability of uracil tautomers to be 32 > 31 > 29 > 28 > 27, while the CNDO/2 approach gives the order 32 > 27 > 31 > 29 > 30 > 28. 

Electron delocalization of 1 has been estimated previously by SCF MO calculations based on crystallographic data <1983CPB3762>. A novel procedure for constructing a localized fragment MO basis set has been developed to allow new insights into aromaticity and conjugation. The effects that 7t-delocalization have on the tr-framework need to be taken into account <2000PCA1736>. For 1 as an example of a benzene-like compound, it is demonstrated that both the 7t- and the a- system are stabilized by rt-delocalization. 

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