Electronic couplings for molecular charge transfer: benchmarking CDFT, FODFT and FODFTB against high-level ab initio calculations. II

Abstract

A new database (HAB7-) of electronic coupling matrix elements (Hab) for electron transfer in seven medium-sized negatively charged π-conjugated organic dimers is introduced. Reference data are obtained with spin-component scaled approximate coupled cluster method (SCS-CC2) and large basis sets. Assessed DFT-based approaches include constrained density functional theory (CDFT), fragment-orbital DFT (FODFT), self-consistent charge density functional tight-binding (FODFTB) and the recently described analytic overlap method (AOM). This complements the previously reported HAB11 database where only cationic dimers were considered. The CDFT method in combination with a functional based on PBE and including 50% of exact exchange (HFX) was found to provide best estimates, with a mean relative unsigned error (MRUE) of 8.2%. CDFT couplings systematically increase with decreasing fraction of HFX as a consequence of increasing delocalisation of the SOMO orbital. The FODFT method is found to be very robust underestimating electronic couplings by 28%. The FODFTB and AOM methods, although orders of magnitude more efficient in terms of computational effort than the DFT approaches, perform well with reasonably small errors of 54% and 29%, respectively, translating in errors in the non-adiabatic electron transfer rate of a factor of 2.4 and 1.7, respectively. We discuss carefully various sources of errors and the scope and limitations of all assessed methods taking into account the results obtained for both HAB7- and HAB11 databases.

Structure and Charge Transport Properties of Cycloparaphenylene Monolayers on Graphite

“…ℏ and k B are fundamental constants, and T is the temperature (fixed here at 300 K). Therefore, the explicit expression for t ab , [44,45] also referred here as charge transfer integral (CTI), is defined in terms of the nonorthonormal Hamiltonian as [39], |t ab | depends of the relative position, orientation, and internal degrees of freedom of the monomers in a dimer configuration extracted from the supramolecular assembly.…”

“…[41,42] However, the Hamiltonian in the new orbital basis lacks the orthonormalization condition and thus a Löwdin's orthonormal transformation [43] is needed. Therefore, the explicit expression for t ab , [44,45] also referred here as charge transfer integral (CTI), is defined in terms of the nonorthonormal Hamiltonian as…”

Ecoupling server: A tool to compute and analyze electronic couplings

“…Thus, additional and user-friendly tools to compute and analyze EC from external wave functions will be of high value. It would be also very convenient that these tools allow to handle with several calculations in a timely manner, to explore many conformations required for a realistic coupling values 26,27 . Therefore, we introduce in this communication the ecoupling server to perform EC calculations from the output of common QM and QM/MM software, and analyze the results in a handy manner.…”

First principles modeling of electron tunneling between defects in m-HfO2

“…The calculation of H ab is corrected for finite size effects as described previously for MgO [12] but here we find corrections are small (<10 meV) and do not effect the main conclusions. The accuracy of CDFT electronic coupling matrix element calculations was benchmarked recently for molecular charge transfer and it was found that the results compare very favorably with correlated wavefunction methods [19,20].…”