Homonuclear Diatomic Molecules of First-row Elements

5 Homonuclear Diatomic Molecules of First-row Elements We have referred above to dementi and Lie s work on the correlation energy of the first-row hydrides.125 The authors218 have also presented results for the homonuclear diatomics of the first row, namely the molecules HaC-Y1 ), Be2(X1SJ), 

Kutzelnigg, V. Staemmler, and M. G61us, Chem. Phys. Letters, 1972, 13, 496. 

Attempts to improve molecular wavefunctions so as to be able to calculate properties more accurately continue to be made, particularly via the constrained variational procedure. Two-particle hypervirial constraints were considered by Bjoma within the SCF formation,282 and he presented a perturbational approach to their solution.233 Using Scherr s wavefunction, and constraining p to satisfy the molecular virial theorem, a calculation on N2 led to rapid convergence.234-235 The constrained SCF orbitals are believed to be a closer approximation to the true tfi nearer the nucleus than further out. A later paper discussed the electron-density maps in comparison to the SCF derived maps, which confirm the conclusion that the wavefunction near the nucleus is improved.236 

Electron scattering from molecules is receiving increasing attention, and theoretically it can be treated by calculation of the static potential (the interaction potential of an electron with the unperturbed charge distribution). Ab initio calculations for N2 using wavefunctions varying between minimal-basis and near-HF quality have been reported by Truhlar et a/.,237 and compared with semi-empirical INDO calculations. The anisotropy of the potential is only correctly described if tf-functions are included in the basis set. 

Probably the most extensive calculation yet carried out on N2 is that of Langhoff 

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