Calculation of local excitations in large systems by embedding wave-function theory in density-functional theory

Abstract

We present a simple and efficient embedding scheme for the wave-function based calculation of the energies of local excitations in large systems. By introducing an embedding potential obtained from density-functional theory (DFT) it is possible to describe the effect of an environment on local excitations of an embedded system in wave-function theory (WFT) calculations of the excitation energies. We outline the implementation of such a WFT-in-DFT embedding procedure employing the ADF, Dalton and DIRAC codes, where the embedded subsystem is treated with coupled cluster methods. We then evaluate this procedure in the calculation of the solvatochromic shift of acetone in water and of the f-f spectrum of NpO22+ embedded in a Cs2UO2Cl4 crystal and find that our scheme does effectively incorporate the environment effect in both cases. A particularly interesting finding is that with our embedding scheme we can model the equatorial Cl- ligands in NpO2Cl42- quite accurately, compared to a fully wavefunction-based calculation, and this opens up the possibility of modeling the interaction of different ligands to actinyl species with relatively high accuracy but at a much reduced computational cost.

PyADF — A scripting framework for multiscale quantum chemistry

“…65,66 In ref. 33, three of us proposed a simplified scheme for the calculation of local excitation energies with WFT-in-DFT embedding. In many cases in which TD-DFT fails (such as in the case of chargetransfer excitations), the density obtained for the ground-state is still rather accurate.…”

“…Such a scheme was applied in ref. 33 to investigate local excitations of a neptunyl ion (NpO 2+ 2 ) embedded as a defect in a Cs 2 UO 2 Cl 4 crystal.…”

“…For WFT-in-DFT embedding calculations (see, e.g., ref. 33), it is necessary to combine different quantum-chemical program packages and to pass the embedding potential and/or the frozen environment electron density between these programs.…”

Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2

“…23,24,26 The electronic spectra of the solutions were first assigned using semi-empirical ligand-field theory (NpO 2 +27 and NpO 2 2+ 28 ) and were revised later by quantum chemical calculations. 19,25,[29][30][31][32][33][34][35] Matsika et al 19,25,29 used configuration interaction methods in conjunction with relativistic effective core potentials that incorporate also spin-orbit coupling 36 (SO-MRCI). In one of these studies, 19 coordination models with five explicit coordinating water molecules and chloride ions, respectively, were tested.…”

“…32 The ground and the four lowest-energy excited states of NpO 2 2+ and of the NpO 2 Cl 4 2− complex ion (model for the Cs 2 NpO 2 Cl 4 solidphase spectra) were investigated by Gomes et al using IHFSCC. 31 They considered further environmental effects on NpO 2 Cl 2− 4 by means of density functional theory (DFT) embedding. The hitherto most complete assessment of the electronic spectrum of Cs 2 NpO 2 Cl 4 at an adequate level of theory was performed by Su et al 33 who calculated both the vertical and adiabatic transitions of NpO 2 2+ and NpO 2 Cl 4 2− up to 21 000 cm −1 using the SO-RASPT2 method.…”