Alternative Techniques in Open-Shell SCF Theory

摘要

An alternative to the Roothaan's open-shell method that does not involve off-diagonal Lagrangian multipliers has been proposed.The orthogonality of the closed-and open-shell orbitals is discussed.Two possible schemes of designing the restricted open-shell Hartree-Fock(HF)wave functions are discussed:In the first scheme the self-consistent field(SCF)functions are linear combination of Slater determinants with coefficients fixed by symmetry whereas the second one uses the unrestricted HF formalism to build a single open-shell Slater determinant from which a well-defined open-shell M011er-Plesset-like perturbation theory can be performed.It has been shown that the Euler SCF equations based on the proposed technique lead to an optimum set of orbitals satisfying the correct variational conditions.A structure and correspondence of the proposed equations to those of the conventional approach are discussed.Practical calculations carried out at SCF and perturbation theory levels demonstrate some features of the proposed technique.