Range-separated density-functional theory applied to the beryllium dimer and trimer

The beryllium dimer and trimer are, despite their small number of electrons, excellent systems for assessing electronic-structure computational methods. With reference data provided by multi-reference averaged coupled-pair functional calculations, we assess several variants of range-separated density-functional theory, combining long-range second-order perturbation theory or coupled-cluster theory with a short-range density functional. The results show that (i) long-range second-order perturbation theory is not sufficient, (ii) long-range coupled-cluster theory gives reasonably accurate potential energy curves, but (iii) provided a relatively large value of bohr(-1) for the range-separation parameter is used.