Search Results (5)

The ionization of ferrocene (Fc) remains an active topic of interest due to its complex, ulti-electron character. Accurate prediction of its first ionization potential (IP) requires methods that go beyond single-particle approximations, as Koopmans’ theorem, Janak’s theorem, and the outer valence Green function (OVGF) approach prove inadequate. Using the ΔSCF method, the first IP of Fc was calculated to be ~6.9 ± 0.1 eV, which is in close agreement with experimental values (6.72–6.99 eV). To benchmark computational accuracy, 42 models were evaluated using the CCSD, CCSD(T), and B3LYP methods with Pople and Dunning basis sets, including Fe-specific modifications to better capture 3d electron behavior. The results underscore the importance of proper treatment of Fe 3d orbitals, with B3LYP/m6-31G(d) offering the best compromise between accuracy and computational efficiency. Notably, the singly occupied molecular orbital (SOMO) in Fc⁺ is identified as the 8a₁’ orbital, which is dominated by its Fe 3d character. This orbital, although not the α-HOMO in Fc⁺, becomes the LUMO upon ionization. Analysis of the excess orbital energy spectrum (EOES) reveals substantial energy shifts upon ionization, particularly in Fe-centered orbitals spanning both the core and valence regions. Theoretical momentum distribution (TMD) analysis of the 8a₁’ orbital further quantifies orbital differences before and after ionization, providing complementary insights in momentum space. Finally, energy decomposition analysis (EDA) shows that while most interaction energy components become less stabilizing upon ionization, steric and Pauli terms contribute a small stabilizing effect. Full article

Accurate determination of carbon core-electron binding energies (C1s CEBEs) is crucial for X-ray photoelectron spectroscopy (XPS) assignments and predictive computational modeling. This study evaluates density functional theory (DFT)-based methods for calculating C1s core-electron binding energies (CEBEs), comparing three functionals—PW86x-PW91c (DFTpw), mPW1PW, and PBE50—across 68 C1s cases in small hydrocarbons and halogenated molecules (alkyl halides), using the delta self-consistent field ΔSCF (or ΔDFT) method developed by one of the authors over the past decade. The PW86x-PW91c functional achieves a root mean square deviation (RMSD) of 0.1735 eV, with improved accuracy for polar C-X bonds (X=O, F) using mPW1PW and PBE50, reducing the average absolute deviation (AAD) to ~0.132 eV. The study emphasizes the role of Hartree–Fock (HF) exchange in refining CEBE predictions and highlights the synergy between theoretical and experimental approaches. These insights lay the groundwork for machine learning (ML)-driven spectral analysis, advancing materials characterization, and catalysis through more reliable automated XPS assignments. Full article

In this study, we conducted an extensive investigation into broadband near-infrared luminescence of Cr³⁺-doped Ca₃Y₂Ge₃O₁₂ garnet, employing first-principles calculations within the density functional theory framework. Our initial focus involved determining the site occupancy of Cr³⁺ activator ions, which revealed a pronounced preference for the Y³⁺ sites over the Ca²⁺ and Ge⁴⁺ sites, as evidenced by the formation energy calculations. Subsequently, the geometric structures of the excited states ²E and ⁴T₂, along with their optical transition energies relative to the ground state ⁴A₂ in Ca₃Y₂Ge₃O₁₂:Cr³⁺, were successfully modeled using the ΔSCF method. Calculation convergence challenges were effectively addressed through the proposed fractional particle occupancy schemes. The constructed host-referred binding energy diagram provided a clear description of the luminescence kinetics process in the garnet, which explained the high quantum efficiency of emission. Furthermore, the accurate prediction of thermal excitation energy yielded insights into the thermal stability of the compound, as illustrated in the calculated configuration coordinate diagram. More importantly, all calculated data were consistently aligned with the experimental results. This research not only advances our understanding of the intricate interplay between geometric and electronic structures, optical properties, and thermal behavior in Cr³⁺-doped garnets but also lays the groundwork for future breakthroughs in the high-throughput design and optimization of luminescent performance and thermal stability in Cr³⁺-doped phosphors. Full article

Despite great advances in X-ray absorption spectroscopy for the investigation of small molecule electronic structure, the application to biosystems of experimental techniques developed within this research field remains a challenge. To partially circumvent the problem, users resort to theoretical methods to interpret or predict the X-ray absorption spectra of large molecules. To accomplish this task, only low-cost computational strategies can be exploited. For this reason, some of them are single Slater determinant wavefunction approaches coupled with multiscale embedding techniques designed to treat large systems of biological interest. Therefore, in this work, we propose to apply the recently developed IMOM/ELMO embedding method to the determination of core-ionized states. The IMOM/ELMO technique resulted from the combination of the single Slater determinant Δself-consistent-field-initial maximum overlap approach (ΔSCF-IMOM) with the QM/ELMO (quantum mechanics/extremely localized molecular orbital) embedding strategy, a method where only the chemically relevant region of the examined system is treated at fully quantum chemical level, while the rest is described through transferred and frozen extremely localized molecular orbitals (ELMOs). The IMOM/ELMO technique was initially validated by computing core-ionization energies for small molecules, and it was afterwards exploited to study larger biosystems. The obtained results are in line with those reported in previous studies that applied alternative ΔSCF approaches. This makes us envisage a possible future application of the proposed method to the interpretation of X-ray absorption spectra of large molecules. Full article

Ionization potentials (IPs) and electron affinities (EAs) are important quantities input into most models for calculating the open-circuit voltage (V_oc) of organic solar cells. We assess the semi-empirical density-functional tight-binding (DFTB) method with the third-order self-consistent charge (SCC) correction and the 3ob parameter set (the third-order DFTB (DFTB3) organic and biochemistry parameter set) against experiments (for smaller molecules) and against first-principles GW (Green’s function, G, times the screened potential, W) calculations (for larger molecules of interest in organic electronics) for the calculation of IPs and EAs. Since GW calculations are relatively new for molecules of this size, we have also taken care to validate these calculations against experiments. As expected, DFTB is found to behave very much like density-functional theory (DFT), but with some loss of accuracy in predicting IPs and EAs. For small molecules, the best results were found with ΔSCF (Δ self-consistent field) SCC-DFTB calculations for first IPs (good to ± 0.649 eV). When considering several IPs of the same molecule, it is convenient to use the negative of the orbital energies (which we refer to as Koopmans’ theorem (KT) IPs) as an indication of trends. Linear regression analysis shows that KT SCC-DFTB IPs are nearly as accurate as ΔSCF SCC-DFTB eigenvalues (± 0.852 eV for first IPs, but ± 0.706 eV for all of the IPs considered here) for small molecules. For larger molecules, SCC-DFTB was also the ideal choice with IP/EA errors of ± 0.489/0.740 eV from ΔSCF calculations and of ± 0.326/0.458 eV from (KT) orbital energies. Interestingly, the linear least squares fit for the KT IPs of the larger molecules also proves to have good predictive value for the lower energy KT IPs of smaller molecules, with significant deviations appearing only for IPs of 15–20 eV or larger. We believe that this quantitative analysis of errors in SCC-DFTB IPs and EAs may be of interest to other researchers interested in DFTB investigation of large and complex problems, such as those encountered in organic electronics. Full article