Multiconfigurational Hartree–Fock studies of avoided curve crossing using the Newton–Raphson technique

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Abstract
A mode controlling scheme for use in second order Newton–Raphson multiconfigurational Hartree–Fock calculations has been developed and applied on the avoided curve crossing for the lowest three states of 4Π symmetry of BO and on the valence-Rydberg mixing in the lowest two states of 3Σ−u symmetry in O2. Convergence of the multiconfigurational Hartree–Fock calculations was easily obtained in all calculations, even where root-flipping occurred. We further demonstrate how an MCSCF calculation may occasionally be reparametrized, that is, the same MCSCF total energy may be obtained using two different sets of configuration lists. One parameter list may give root-flipping, whereas another parameterization may not. This is exemplified in a calculation on the E 3Σ−u state of O2 at 2.13 a.u..