Linked-graph expansions in mixed spaces

Several authors have discussed the problems one confronts in attempting to obtain linked-graph energy expansions in a finite-dimensional model space including 2 hω excitations (in an oscillator basis), and have indicated that unlinked graphs arise in energy diagonalisations in such spaces. A situation somewhat more complicated in form is considered, namely the case when two excitation spaces are included in addition to the unperturbed model state. One conclusion from this work is that unlinked graphs become more important in enlarged mixed-space diagonalisations selective in the type of high-lying excitations one includes. (author)