Very accurate correlated calculations on diatomic molecules with numerical orbitals: The hydrogen fluoride molecule

Numerical orbital coupled-cluster calculations on the FH molecule are reported, which obtain 99% of the possible correlation energy. Instead of requiring explicit $r_{12}$ functions, it is demonstrated that a one-particle but very well-chosen numerical orbital basis set is adequate to recover essentially all effects of electron correlation. Further reduction of the dimension of the numerical orbital basis set is accomplished via our recently proposed optimized virtual-orbital-space technique. Augmented by the exact second-order energy, the latter allows a reduction to as small as 25% of the original space with no appreciable errors in the correlation energy.