Atomic and molecular applications of open-shell cluster expansion techniques with incomplete model spaces

Pilot numerical applications are reported of two recently formulated cluster expansion formalisms that generate connected effective Hamiltonians even for incomplete model spaces. Results are presented for prototype systems such as (a) the He atom, (b) the H2 molecule and (c) the He22+ ion. For He and H2 we give results for typical examples of incomplete model spaces, and for He22+ we present adiabatic potential energy curves the first four 1Σg+ and the first two 3Σg+states, calculated with an incomplete model space