Assessment of the Direct Generalized Bloch Approach B0: Application to the Li and Be Atoms and the Molecules LiH, BeH, and the Phenolate Anion

Besides the necessity of the development of sophisticated methods to calculate correlation energies - be it the coupled-cluster (CC) or the configuration-interaction (CI) methods and their various approaches - one also accentuate the need for efficient and less demanding methods in the area of medium and large molecular systems. Therefore, this article proposes a computational efficient and in our opinion reasonable approach for the calculation of correlation energies for medium and even larger molecules. This approach, named B0, based on the so-called direct generalized Bloch (DGB) equation which has already been successfully applied to small systems. Within those considerations the B0 approach showed promising results so that further investigations are worthwhile. Here, as a further step in the assessment of this method we apply the B0 approach to the Li and Be atoms as well as the LiH and BeH molecules. Molecules which show open and closed shell characteristics in the equilibrium and in the case of dissociation as well. The results are compared with CC and CI and experimental results if available. Since this results are encouraging even when considering small basis sets and with the prospect of larger molecular systems, therefore, we perform also B0 energy calculations for the low-lying states of the phenolate anion which for instance can be used in a simple model of the photoactive yellow protein (PYP) chromophore.