Multireference configuration interaction calculations on Cr₂: Passing the one billion limit in MRCI/MRACPF calculations

The region around the minimum of the potential-energy curve of Cr₂has been calculated at the multireference configuration interaction (CI) level including almost 1.3 billion configurations in the CI calculation. The computational techniques as implemented on massively parallel computers which enabled this calculation are described. The calculated results are R_e = 1.72 Å, D_e = 1.09 eV, and ω_e = 338.7 cm^-1 as compared to experimental values of R_e = 1.679 Å, D_e = 1.50 ± 0.05 eV, and ω_e = 452.34(ΔG_1/2) cm^-1. The error of 0.4 eV in the dissociation energy can be attributed to relativistic effects following other authors (0.2 eV) and the need for higher angular momentum basis functions in the one-particle set (0.2 eV).