Core-valence correlation effects in calcium hydride

Accurate multi-reference contracted CI calculations have been performed on CaH to determine the bond distance and the dissociation energy. At the Hartree-Fock limit a bond distance 0.10 au longer than in experiments is found. This value is not improved by a valence CI calculation. Including the core-valence (CV) contribution to the correlation energy makes the bond distance too short by 0.06 au, and agreement with experiments is obtained first when the core-core (CC) contribution from the 3p orbitals is added also. It turns out that the only orbital which needs to be treated of the core is the 3p σ orbital, which shows some bonding character at the CI level. Tight d functions to describe a 3d orbital of the type occupied in the ²D state of Ca^?, and f functions to correlate the 3p orbital are necessary for obtaining a good bond distance. For the dissociation energy a different trend is observed with excellent agreement with experiments at the CV level and no improvement over the SCF level at the CC level. The results are discussed and analyzed in detail in the text, where a section on size consistency is also added.