HIGH LEVEL <i>AB INITIO </i> ANALYSIS OF MAGNETIC INTERACTIONS IN DIRADICALS AND MOLECULAR TRANSITION-METAL COMPLEXES
In this paper, we have tested two multiconfiguration approaches to defining isotropic exchange. In both of them, the complete active space self-consistent field method is used first to build the multiconfiguration reference wave function. Then this multiconfiguration reference wave function is used to calculate the corrections for the energies of electronic states - either within the framework of the multiconfigurational second order perturbation theory (CASSCF(n,m)/NEVPT2) or variationally (I-DDCI3). The calculations were performed for two-center exchange clusters with one unpaired electron in each center. The examples of such exchange clusters are organic diradicals and dinuclear copper (II) complexes. The findings show that the multiconfiguration computational models allow evaluating the parameter of isotrope eachange JAB more accurately, as compatrd to the computational models using unideterminant methods, such as the density functional methods. In case of nitronyl-nitroxyl diradical, a good agreement with the experimental value of JAB is achieved within the first approach, using an extended CAS(20,15)-space comprising all valent π-orbitals. In case of dimer copper(II) acetate, the consistent extension of the active space leads to improving, but it does not allow us to sufficiently approach to the experimental value of JAB; only using the second (variational) approach makes it possible to define the isotrope exchange highly accurately. Unfortunately, highly consuming computer resources and valuable machine time confine the application of multiconfiguration approaches. In case of multinuclear exchange clusters, only unideterminant calculations remain.