Theoretical ab initio studies of neutral, cationic and anionic Cr 2 , Mn 2 , and CrMn dimers have been carried out to explore the progression of magnetic coupling with the number of electrons. It is shown that while Cr 2 and Cr 2 Ϫ have antiferromagnetically coupled atomic spins, Cr 2 ϩ has a ferromagnetic ground state closely followed by an antiferromagnetic state. On the other hand, all Mn 2 dimers are ferromagnetic, irrespective of the charge. The neutral CrMn is ferrimagnetic while the charged CrMn are antiferromagnetic. In all cases, the charged dimers are found to be more stable than the neutral ones. The results are compared with available calculations and experiments and the difficulties associated with theoretical description and the experimental interpretations are discussed.

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N. DESMARAIS, G. DANCAUSSE, and S. FLISZAR. Can. J. Chem. 71, 175 (1993).A quality test is proposed for SCF atomic orb~tals, 4, = CI c~,~: " ' , approximated as finite linear cpmbinations of suitable basis functions x:' " The key is in a function, readily derived from the Hartree-Fock equation F+, = E, +,, which is identically zero for true Hartree-Fock spin orbitals and not so for approximate orbitals. In this way, our test measures how closely approximate orbital descriptions approach the true Hartree-Fock limit and thus provides a quality ordering of orbital bases with respect to one another and with respect to that limit, in a scale uniquely defined by the latter. Moreover, this analysis also holds for atomic subspaces of our choice, e.g., the valence region. Examples are offered for representative collections of Slater-and Gaussian-type orbital expansions. N. DESMARAIS, G. DANCAUSSE et S. FLISZAR. Can. J. Chem. 71, 175 (1993).On propose un critkre pour Cvaluer la qualit6 des orbitales atomiques, 4; = 2, ckiXP, dCcrites par des combina!sons 1inCraires finies de fonctions de base XI(" La cle est dans une fonction decoulant de l'equation de Hartree-Fock F+, = E; bir qui est identiquement nulle pour les vraie orbitales de spin Hartree-Fock, mais non nulle autrement. Ceci permet de mesurer comment la description des orbitales leur permet de se rapprocher de la lirnite exacte de Hartree-Fock et fournit une apprkciation de la qualit6 des bases dans une echelle dkfinie par cette limite. Cette analyse est Cgalement possible pour des sous-espaces atomiques, par exemple la rCgion de valence. Les exemples portent sur une sCrie de fonctions gaussiennes et de Slater.