A simple way of generating starting orbitals for multiconfigurational SCF calculations (particularly of the CAS type) is to use the natural orbitals of the unrestricted Hartree–Fock wave function. Significant fractional occupancy of a UHF natural orbital indicates that the orbital should be included in the active space; this is illustrated for symmetrically stretched water, for NO, N2O4, and for the transition state on the Me3CO→Me2CO+CH3 reaction surface. Average natural orbitals should be used if there are several UHF solutions for the same state; this restores symmetry broken at the the UHF level. Configuration selection based on fractional occupation appears to be the basis for a reliable automated procedure. The UHF natural orbitals provide good starting orbitals for the two-configuration SCF and for the 4×4 CAS wave function in stretched F2, H2O2, and C2H6, methane, water, in twisted ethylene, in ozone, and for various CAS wave functions in the first asynchronous transition state of the HCNO+H2C2 1,3-cycloaddition. Comparison is made with the extended Hartree–Fock method.

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The peroxy isomers of nitrate and nitric acid, peroxynitrite and peroxynitrous acid, are studied with ab initio and density functional methods. The results are compared to the observed Raman and 15N NMR spectra. The harmonic vibrational frequencies, NMR chemical shifts, and energies clearly favor cis ONOO- as the most stable and predominant ONOO- isomer. Peroxynitrite has a large rotational barrier of ∼24 kcal/mol because of partial π-bonding in the central bond. This is confirmed by a bond order of 1.5 for cis and trans ONOO- computed by electron density analysis. Electron correlation is critical in accurately predicting the relative energies for this system, as Hartree−Fock predicts a lower triplet state. The intense, broad band in the solution Raman spectrum centered at 642 cm-1 is predicted to be too low by 100−150 cm-1 if the vibration is the cis torsion. Resolution of this discrepancy is attempted by estimating the effects of solvent and anharmonicity. The results on ONOO- are compared to those for ONOOH, which is adequately described by lower levels of theory. The planar cis−cis isomer of ONOOH is the lowest energy structure by 1−2 kcal/mol.

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