Accurately solving the electronic Schrِdinger equation of atoms and molecules using explicitly correlated (r12-)MR-CI.: The ground state of beryllium dimer (Be2)

The recently proposed (explicitly correlated) r12-MR-ACPF method is applied to the ground state (X 1Σg+) potential of Be2. A large [19s11p6d4f3g2h] basis set and a valence-CAS reference space (60 CSFs) are used. At long range, the reference space is augmented by {3s, 3p} orbitals (568 CSFs). The calculated dissociation energy (898±8 cm−1) is considerably larger than the experimental estimate (790±30 cm−1). The other molecular constants, however, are in good agreement. Our potential is close to the one calculated by Stärck and Meyer [Chem. Phys. Lett. 258 (1996) 421].