AB-INITIO CALCULATIONS OF HARDNESS AND CHEMICAL-POTENTIAL OF OPEN-SHELL SYSTEMS USING SCF, MP2 AND MP4 METHODS

The hardness (eta) and chemical potential (mu) of a number of open she ll systems have been calculated by ab initio SCF, MP2 and MP4 methods. Both vertical and adiabatic values of ionisation potential (I) and el ectron affinity (A) have been used to evaluate eta and mu. The effect of spin contamination on various calculated quantities has been examin ed by performing UHF as well as ROHF calculations in appropriate cases . It has been observed that in the case of closely related species the hardness values obtained by almost all the methods of calculation emp loyed here are capable of predicting the empirical hardness sequence o f the corresponding anions. However, overall best agreement between th eory and experiment is achieved only at the MP4 level. Barring minor e xceptions the corresponding calculated values follow the experimental order for both eta and mu. (C) 1998 Elsevier Science B.V.