Small-occupation density functional correlation energy correction to wavefunction approximations

In this work, we introduce a novel hybrid approach, termed WFT-soDFT, designed to seamlessly incorporate DFT correlation into wavefunction ansatzes. This is achieved through a partitioning of the orbital space, distinguishing between large and small natural occupation numbers (NOONs) associated with wavefunction theory (WFT) and DFT correlation, respectively. The method uses a novel criteria for partitioning the orbital space and mapping the electron density in natural orbitals with small occupation with the correlation energy of fast electrons within the homogeneous electron gas. Central to our approach is the introduction of a separation parameter $\nu$, the choice of the WFT approach, and the correlation functional. Here, we combine the RASCI wavefunction with hole and particle truncation with a local density correlation functional to only account for small-occupation correlation energy. We investigate the performance of the method in the study of small but challenging chemical systems, for which WFT-soDFT demonstrates notable improvements over pristine wavefunction calculations. These findings collectively highlight the potential of the WFT-soDFT approach as a computationally affordable strategy to improve the accuracy of WFT electronic structure calculations.