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 H-miltonian matrices and leads to considerable simplification in the equations to be solved. reducing the inhomogeneous equations to pseudo-eigenvalue form. Typical results are reported for the lowest (2)A(1) state of H2O+ and the lowest (3)A(1) and (1)A(1) excited states of H2O.