Excitation wave form, definition

As mentioned earlier, for computational reasons, we have to trimcate the full Cl matrix or equivalently the Cl expansion for the wave function, considering only those configurations which differ from the HF ground-state wave function by no more than a predetermined munber of spin orbitals. The simplest version of this scheme is to deal with single and double excitation out of It d) a SDCI calculation. SDCI and, in fact, aU forms of trimcated Cl deteriorate as the number of electrons increases. The truncation of the Cl expansion leads to problems of size-extensivity. For instance, in a DCI calculation, by definition the wave function of each monomer... 

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