An extension of Huzinagarsquo;s model potential method is applied to the study of the equilibrium geometry of the ground electronic state of I2and HI. In the extended model potential method (EMP) the exchange interactions between the core and valence shells are handled more efficiently than in the standard MP method; furthermore, the long range behavior of the model potential does not depend on the reduction of the valence basis set. Consequently, the results of the EMP molecular calculations are very close to the reference allhyphen;electron calculations, with errors of 0.04ndash;0.02 Aring; (equilibrium distance), 3percnt;ndash;2percnt; (vibrational frequency), 0.4 eV (binding energy), and 10minus;3a.u. (molecular orbital energies). The present results show that in molecules with large corendash;valence interactions the EMP method, which explicitly involves only the valence electrons and uses the theoretical core projection parameters, offers the same accuracy as the standard MP method enhanced by the promotion of the outermost core orbital to the valence space and by using adjustable (soft) core projection parameters.
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