A SPIN-ADAPTED COUPLED-CLUSTER BASED LINEAR-RESPONSE THEORY FOR DOUBLE-IONIZATION POTENTIALS

We developed in this article a spin-adapted formulation of the coupled -cluster based linear response theory (CC-LRT) for computing double-io nization potentials (DTPs), which may be experimentally observed by Au ger spectroscopy. CC-LRT is a multireference generalization of the CC theory where the energy differences have no disconnected vacuum (core) diagrams, signifying core-extensivity. For the spin-adaptation of the CC-LRT equations for the singlet and triplet manifolds, we used the Y oung-Yamanouchi orthogonal spin-eigenfunctions. The orbital version of the CC-LRT equations are then automatically generated by the conjugat e projection operators of Young-Yamanouchi spin functions. We illustra ted the working of our spin-adaptation procedure by confining our CC-L RT equations to the space of 2h and 1p-3h ionized determinants. As num erical application of our formalism, we computed the Auger kinetic ene rgies of HP and H2O. We also analyzed the nature of size-extensivity o f the DTPs generated by CC-LRT and showed explicitly that when the mol ecule is composed of two noninteracting fragments the computed DIPs ar e either DIPs of fragment A or B or a composite DIP depending on both A and B, which are just not sum of ionization potentials (IPs) of A an d B. Tnis analysis is done to underscore the fact that DIPs from CC-LR T is only core-extensive and not fully extensive. (C) 1996 John Wiley & Sons, Inc.