Short-range DFT energy correction to multiconfigurational wave functions for open-shell systems

Abstract

Electronic structure methods emerging from the combination of multiconfigurational wave functions and density functional theory (DFT) aim to take advantage of the strengths of the two nearly antagonistic theories. One of the common strategies employed to merge wave function theory (WFT) with DFT relies on the range separation of the Coulomb operator in which DFT functionals take care of the short-distance part, while long-range inter-electronic interactions are evaluated by using the chosen wave function method (WFT–srDFT). In this work, we uncover the limitations of WFT–srDFT in the characterization of open-shell systems. We show that spin polarization effects have a major impact on the (short-range) DFT exchange energy and are of vital importance in order to provide a balanced description between closed and open-shell configurations. We introduce different strategies to account for spin polarization in the short range based on the definition of a spin polarized electron density and with the use of short-range exact exchange. We test the performance of these approaches in the dissociation of the hydrogen molecule, the calculation of energy gaps in spin-triplet atoms and molecular diradicals, and the characterization of low-lying states of the gallium dimer. Our results indicate that the use of short-range DFT correlation in combination with a (full-range) multiconfigurational wave function might be an excellent approach for the study of open-shell molecules and largely improves the performance of WFT and WFT–srDFT.

Range‐separated multiconfigurational density functional theory methods

“…Recently, yet another non‐SCF method has been applied to nonsinglet excited states from RASCI wavefunctions 176 . The approach absorbs the SR exchange in the wavefunction part by representing the SR exchange functional with the Hartree–Fock expression.…”

Restricted active space configuration interaction methods for strong correlation: Recent developments

“…Very recently, the performance of the RAS‐ sr DFT method, and the WFT‐ sr DFT scheme in general, has been tested in the characterization of open‐shell atoms and molecules 46 . These investigations have revealed a noticeable unbalance in the treatment of open and closed‐shell systems by RAS‐ sr DFT.…”

“…In the last decade, several developments have appeared in order to generalize the RAS‐SF method to other (non‐spin‐flip) excitation operators and optimize its computational implementation, 41–43 improve the accuracy of electronic energies, 44–46 characterize the nature of the wave function in complex systems, 47,48 and to the computation of molecular properties 49 . The present work reviews the theoretical foundations of the general RASCI methodology within the hole and particle truncation of the excitation operator.…”

Molecule‐Specific Uncertainty Quantification in Quantum Chemical Studies

“…However, the neglected dynamic correlation from the huge number of weakly correlated orbitals, which are outside the active space, escapes a treatment on the same level of accuracy; typically only multi-reference perturbation theory to second order represents the highest level of accuracy achievable. [31] Continuous efforts have tried to improve on this situation; examples are tailored coupled cluster, [32][33][34] combinations with short-range DFT [35][36][37][38][39][40][41] or transcorrelation [42][43][44][45][46] to treat the electron-electron cusp due to the singularity of the Coulomb interaction, to mention only a few. Already the amount of suggestions for accurate multi-configurational methods (not reflected in the list of references of this work and beyond its scope) may be taken as an indication that no universally satisfactory and generally accepted best solution has been found so far.…”