Self-Adapting Short-Range\nCorrelation Functional for\nComplete Active Space-Based Approximations

We propose a short-range correlation energy correction\ntailored\nfor active space wave function models. The correction relies on a\nshort-range multideterminant correlation functional computed with\na local range-separation parameter that self-adapts to the underlying\nwave function. This approach is analogous to that of Giner et al.\n[<i>J. Chem. Phys.</i> <b>2018</b>, 149, 194301] which\naddresses the basis set incompleteness error, with the vital distinction\nthat in our protocol the range-separation parameter remains finite\nin the complete basis set limit, ensuring nonzero short-range correlation.\nThe proposed correlation functional compensates for the missing short-range\ncorrelation via two mechanisms: (i) an automatically adapting short-range\nparameter, which gauges the missing correlation in the electron vicinity,\nand (ii) the functional’s explicit dependence on the on-top\npair density, which reduces short-range correlation in regions where\nelectron correlation is mainly static. We integrate our method into\nthe multireference adiabatic connection theory for CASSCF wave functions.\nThe performance of the introduced CAS-AC0-(c,md) model is verified\nby calculating potential energy curves for alkaline-earth metal dimers\n(Be<sub>2</sub>, Mg<sub>2</sub>, Ca<sub>2</sub>) and for the chromium\ndimer, in all cases obtaining promising results.