RESULTS OF DIAGONAL ARPA CALCULATIONS FOR LIH, HF, H2O AND NH3

The augmented random phase approximation (ARPA) in its diagonal form has been employed to compute the linear response function for the molecules LiH, HF, H2O and NH3. The calculations have been carried out including: (a) particle--particle plus hole--hole (pp + hh) terms; (b) particle--hole plus hole--particle (ph + hp) terms and (c) pp + hh + ph + hp terms in order to have a numerical test on the capability of the ARPA in comparison with the standard random phase approximation (RPA) technique. It was found that by using large basis sets, the differences between the two methods are neglible from a numerical viewpoint.