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 the homogeneous equations to pseudo-eigenvalue form. Typical results are reported for the lowest ^2A_1 state of H_2O^+ and the lowest ^3A_1 excited states of H_2O.