Semiempirical quantum-chemical methods for examining ?-electron systems and calculation of ?-electron radicals

Abstract

The field of semiempirical quantum-chemical calculations of σ-electron systems has undergone vigorous development in the investigations of the last ten years. One of the first formulated was the EHT method in which overlap was taken into account in explicit form. This was no accident since for a long time the opinion prevailed that disregarding overlap in calculations of σ-electron systems is an unjustifiably daring approximation. However, the general opinion with respect to this problem began to change after practical successes of a number of semiempirical computational schemes which disregard overlap. The results of such calculations with appropriate selection of parameters agree with experiment at least not worse than the results of calculations in which overlap is taken into account in explicit form. In the second section of this review, this was illustrated in the case of the HFI constants in σ-electron radicals. This is also valid with respect to many other molecular properties: energies, dipole moments, ionization potentials, etc. In our opinion, one of the reasons for the success of the methods which “disregard” overlap consists in the fact that overlap is partially taken into account by the effective selection of parameters (see the discussion of the hybrid-basis method). In this connection, the path for constructing huge computational schemes which explicitly take overlap into account is less valid. It should also be emphasized that familiarization with rather complex and laborious methods is not necessary in semiempirical calculations which disregard overlap since simpler and more obvious schemes often turn out to be just as effective and lead to results which are no worse, and sometimes better, than the results of more complex methods.