Diamagnetism molecular orbital approach

Mn2(CO)io provides an example of the simplest sort of metal-metal bond. The 18-electron rule is obeyed the electron use rule is satisfied (by using one electron from each metal atom in u-bond formation, and allowing the others, as in systems M(CO)sX, to ji-bond to the carbonyls), and a molecular orbital approach would simply regard the a-bond as, in effect, a ligand and treat the metal as distorted octahedral diamagnetic Mn+ tig). 

Since the number of integrals to be evaluated increases rapidly with molecular size, semi-empirical calculations are more practical at present for larger molecules. The most widely used semi-empirical approach is based upon Pople s theory (22, 23) of molecular diamagnetism within the independent electron framework. All explicit two electron terms become zero in this method. Thus the necessity of evaluating many integrals is avoided and the remainder can be approximated by the methods implicit in all valence electron molecular orbital calculations. (24)... 

If the Hartree-Fock equations associated with the valence pseudo-Hamiltonian (167) are solved with extended basis sets, then all the above F are almost basis-set-independent. At the present time, and for practical reasons, most of the ab initio valence-only molecular calculations use coreless pseudo-orbitals. The reliability of this approach is still a matter of discussion. Obviously the nodal structure is important for computing observable quantities such as the diamagnetic susceptibility which implies an operator proportional to 1/r. From the computational point of view, it is always easy to recover the nodal structure of coreless valence pseudo-orbitals by orthogonalizing the valence molecular orbitals to the core orbitals. This procedure has led to very accurate results for several internal observables in comparison with all-electron results. The problem of the shape of the pseudoorbitals in the core region is also important in relativity. For heavy atoms, the valence electrons possess high instantaneous velocities near the nuclei. Schwarz has recently investigated the compatibility between the internal structure of valence orbitals and the representations of operators such as the spin-orbit which vary as 1/r near the nucleus. ... 

The contribution of the Fermi contact term to J(NN) in both trans- and C/S-N2H2 was computed using the scanning molecular orbital method, a special approach within the Har-tree-Fock framework [4]. The diamagnetic spin-orbital (DSO) contribution to J(HH) in N2H2 was calculated at the ab initio SCF level of theory [5]. 

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