  Multi-configurational wave functions are known to describe electronic
structure across a Born-Oppenheimer surface qualitatively correct. However, for
quantitative reaction energies, dynamical correlation originating from the many
configurations involving excitations out of the restricted orbital space, the
active space, must be considered. Standard procedures involve approximations
that eventually limit the ultimate accuracy achievable (most prominently,
multi-reference perturbation theory). At the same time, the computational cost
increase dramatically due to the necessity to obtain higher-order reduced
density matrices. It is this disproportion that leads us here to propose a
MC-srDFT-D hybrid approach of semiclassical dispersion (D) corrections to cover
long-range dynamical correlation in a multi-configurational (MC) wave function
theory which includes short-range (sr) dynamical correlation by density
functional theory (DFT) without double counting. We demonstrate that the
reliability of this approach is very good (at negligible cost), especially when
considering that standard second-order multi-reference perturbation theory
usually overestimates dispersion interactions.
