Potential Energy Surfaces from Correlated Wavefunctions

7 Potential Energy Surfaces from Correlated Wavefunctions Hj.—In 1970, Csizmadia et a/.231 carried out an extensive SCF-C3 calculation on the potential energy surface of H+. Four or five s-type GTO s and at least onep-type 

The avoided intersection of the two potential surfaces in the H system was studied in detail in a full Cl calculation by Bauschlicher et a/.338 An uncontracted 5s and 2p GTO basis set was used. The lowest surface was found to be attractive, with a binding energy of 444 kJ mol 1 for H relative to H + and Ha. The second surface which represents H + H+ is reported to be repulsive in nature. 

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Unfortunately there is as yet no known way to obtain the repulsion energy from properties of the separate molecules. An attempt has been made to characterise the repulsive surface of a molecule by performing IMPT calculations between the molecule and a suitable test particle, such as a helium atom. Because the helium atom has only one molecular orbital and is spherically symmetrical, such calculations can be done much more easily than calculations involving two ordinary molecules. From the data for the repulsion between molecule A and the test particle, and between B and the test particle, it may be possible to construct a repulsive potential between A and B. Some limited progress has been made with this idea. An alternative approach has been based on the suggestion that the repulsion energy is closely correlated with the overlap between the molecular wavefunctions, but this seems likely to be more useful as a guide to the form of analytic models than as a direct route to accurate potential functions. 

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