DIRECT CALCULATION OF ENERGY DIFFERENCES BY A COMMON UNITARY TRANSFORMATION OF 2 MODEL STATES, WITH APPLICATION TO IONIZATION-POTENTIALS

The approximate wavefunctions φ0 and φ′ for two states can be transformed by a common unitary operator U = exp(σ), σ+ = -σ, to the corresponding exact states ψ0 and ψ′. The energy correction to second order in σ is evaluated with σ chosen such that it makes the energy difference stationary. The contributions of various types of excitation operators contained in σ are analyzed. Contributions that are identical for the two states cancel automatically. Numerical results for H2O and Ne indicate that the Koopmans correction is to a large extent recovered and given in terms of physically interpretable quantities. © 1979.