Study of Linear Stretch in Polyatomic Molecules: Accurate SCF MO Wavefunctions for CO2 and BeF2

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Abstract
Accurate SCF MO calculations are carried out for CO2, BeF2, and various AH2 and AH3 molecules as a function of internuclear distance. It is noted that the resulting sum of orbital-energy curves show minima at much smaller internuclear distances than the corresponding total energy curves, even though it has previously been shown that derivatives of ET and ∑ εi with respect to internuclear angle quite generally vanish at approximately equal angles. This distinction emphasizes the point that the close correlation between angular derivatives of ET and ∑ εi is in large part caused by the lack of changes in internuclear distances between closely interacting atoms in the process of angular variations. It is further shown that an elementary application of Koopman's theorem enables one to make qualitative determinations of both relative bond angles and relative bond lengths by adding or subtracting various differentiating orbital-energy curves to an appropriate total-energy curve.