The orthogonality constrained basis set expansion method for treating off-diagonal lagrange multipliers in calculations of electronic wave functions

We propose an alternative to the coupling operator approach for the solution of the self-consistent field equations containing off-diagonal Lagrange multipliers. This method involves a transformation of the basis set after formation of the Hamiltonian matrices and leads to considerable simplification in the equations to be solved, reducing th ehomogeneous equations to pseudo-eigenvalue form. Typical results are reported for the lowest ²A₁ state of H₂O⁺ and the lowest ³A₁ excited states of H₂O.