Amount Of Hartree-Fock Exchange Research Articles

Density functional theory‐based rapid and accurate estimation of reduction potentials of acridinium derivatives in ground and excited state

AbstractWe devised a quantum chemical simulation protocol that can rapidly and accurately predict ground and excited state reduction potentials (E0 and E*, respectively) of acridinium photoredox catalysts (PCs). To bypass time‐consuming excited state geometry optimizations, we used ground state equilibrium geometries to compute the electronic energy of excited states, which provides reasonable E0 and E* estimates. The contribution of Hartree‐Fock exchange (HFX) in density functionals was systematically varied to estimate E0 and E*. In addition to reproducing experimental results, physically sensible models, such as correct descriptions of exciton behavior, are highly necessary. Based on the exciton correlation values, the appropriate amount of HFX in the B3LYP functional was determined to be 30 % to yield physically sensible bound electron‐hole pairs for small organic molecules. We also investigated the impact of basis sets on the predictability of E0 and E*. Geometry optimizations with B3LYP‐D2(HFX 20 %)/6‐31G and single point energy refinement with (TD‐)B3LYP‐D2(HFX 30%)/6‐311++G(d,p) yielded the best results. The transferability of the suggested protocol was confirmed with a test set consisting of eight acridinium derivatives. This study can provide reasonably accurate results with relatively small amounts of computational resources, and it is therefore expected to greatly contribute to the development of new acridinium PCs.

Calibrating TDDFT Calculations of the X-ray Emission Spectrum of Liquid Water: The Effects of Hartree-Fock Exchange.

The structure and dynamics of liquid water continue to be debated, with insight provided by, among others, X-ray emission spectroscopy (XES), which shows a split in the high-energy 1b1 feature. This split is yet to be reproduced by theory, and it remains unclear if these difficulties are related to inaccuracies in dynamics simulations, spectrum calculations, or both. We investigate the performance of different methods for calculating XES of liquid water, focusing on the ability of time-dependent density functional theory (TDDFT) to reproduce reference spectra obtained by high-level coupled cluster and algebraic-diagrammatic construction scheme calculations. A metric for evaluating the agreement between theoretical spectra termed the integrated absolute difference (IAD), which considers the integral of shifted difference spectra, is introduced and used to investigate the performance of different exchange-correlation functionals. We find that computed spectra of symmetric and asymmetric model water structures are strongly and differently influenced by the amount of Hartree-Fock exchange, with best agreement to reference spectra for ∼40-50%. Lower percentages tend to yield high density of contributing states, resulting in too broad features. The method introduced here is useful also for other spectrum calculations, in particular where the performance for ensembles of structures are evaluated.

Constrained density functional theory calculations for estimation of forward and backward intermolecular charge transfer energy

AbstractCharge transfer (CT) in donor–acceptor complexes can occur in two directions: from donor to acceptor (forward CT, CTF) and from acceptor to donor (backward CT, CTB). For an ethylene and tetrafluoroethylene model system, coupled cluster calculations predict that CTF is more stable than CTB. Meanwhile, density functional theory (DFT) and time‐dependent DFT (TDDFT) employing B3LYP functional show a different CT state order depending on the Hartree–Fock exchange (HFX) contribution. The impact of basis sets as a possible cause to induce such unreasonable CT state reverse along with the increase of HFX was investigated. The physical origin of CT state reversal is examined in terms of molecular orbital energy. We analyzed two additional molecular pairs that feature CT state reversal along with the amount of HFX in B3LYP functional. Despite the reduced gap, we suggest constrained DFT using M06‐HF functional as an effective approach to estimate two CT state energies on equal footing.

Effect of Exchange–correlation Functionals on Ground State Geometries, Optoelectronic and Charge Transfer of Triphenylamine-based Dyes

The importance of the Density Functional Theory (DFT) calculation approach lies in their ability to provide a highly accurate prediction of structural and optoelectronic properties. However, the traditional methods of DFT failed to predict optoelectronic properties satisfactorily. Therefore, it will be necessary to examine methods containing different percentages of Hartree-Fock exchange and correlation in order to find the most suitable functionals. DFT and Time-Dependent-DFT (TD-DFT) calculations was carried out using four different functionals approximations incorporating a different amount of Hartree Fock exchange (B3LYP, BHandHLYP, CAM-B3LYP and LCωPBE), in order to evaluate their accuracies to predict the geometrical, optoelectronic and charge transfer properties of four triphenylamine-based dyes for Dye-Sensitized Solar Cells (DSSCs) applications. The functional hybrid B3LYP was the best among adopted functional that reproduced the geometrical, optoelectronic and charge transfer properties. On the other hand, it has been shown that the Hartree-Fock exchange percentage for BHandHLYP, significantly improved TD-DFT results in the case of organic dyes. Moreover, the corrected long-range functionals (CAM-B3LYP and LC-wPBE) present valuable tools for giving results of comparable precision with experimental optical data. In terms of the choice of the most appropriate functional for computational calculation, the obtained results can be useful for future DSSC applications. DOI: http://dx.doi.org/10.17807/orbital.v14i1.1682

The Influence of the Electronic Structure Method on Intersystem Crossing Dynamics. The Case of Thioformaldehyde.

The ability of different electronic structure methods to correctly describe intersystem crossing dynamics is evaluated, using thioformaldehyde as a test case. Mischievously, all methods considered-ranging from the multireference methods MRCISD, MS-CASPT2, or SA-CASSCF, to the single-reference methods ADC(2), CC2, and TDDFT in different flavors-provide the same state ordering and energies of the low-lying singlet and triplet electronic excited states within an acceptable error of 0.2-0.3 eV. However, the outcome of the nonadiabatic simulations after excitation to the lowest S1 (1 nπ*) state are dramatically different. While MS-CASPT2, ADC(2), BP86, and PBE do not transfer population to the triplet states within 500 fs-consistent with experimental evidence-SA-CASSCF, B3LYP, and BHHLYP predict intersystem crossing yields between 3% and 21% within the same time. The different excited state dynamics can be rationalized by inspecting potential energy profiles along the C-S bond stretch mode and single-triplet energy gaps. It is found that already at a C-S bond length of 1.9 Å, all the single-reference methods struggle to describe the correct asymptotic behavior of the potentials. Moreover, some methods, including SA-CASSCF, obtain incorrect 1 nπ*-3 ππ* energy gaps, leading to compensation of errors (ADC(2), BP86, PBE), or wrong dynamics (SA-CASSCF, B3LYP, BHHLYP). Only the accurate MRCISD and MS-CASPT2 methods are able to describe the C-S bond correctly and thus able to deliver the correct potential energy surfaces and dynamics for the right reason. A correlation with the amount of Hartree-Fock exchange in the density functional and the ease to access the 3 ππ* state from the 1 nπ* are able to explain the different behavior observed for GGA and hybrid functionals. It is thus illustrated that even in the case of a simple molecule, like CH2S, the sole assessment of vertical excitation energies as reliability predictors for nonadiabatic dynamics is inadequate. The reason is that ISC does not occur at the FC geometry, but rather at distorted geometries where the singlet-triplet gaps become small. Hence, a characterization of the potential energy surfaces beyond the Franck-Condon region is mandatory.

Investigation of the Electronic Excited-State Equilibrium Geometries of Three Molecules Undergoing ESIPT: A RI-CC2 and TDDFT Study.

Energy minima on the potential energy surfaces of the ground and excited states have been characterized for three photoactive molecules that undergo excited-state intramolecular proton transfer: 3-hydroxychromone, N-salicylideneaniline, and 2-(2-hydroxyphenyl)benzothiazole. Both the CC2 method and the TDDFT methodology with different exchange-correlation (XC) functionals differing by the amount of Hartree-Fock (HF) exchange have been employed. Besides the analysis of the structures along the reaction paths, this study has compared the TDDFT and CC2 results to provide guidelines for selecting the best XC functionals. Several geometrical parameters as well as the excitation energies are found to vary monotonically with the amount of HF exchange. Systematically, this study has addressed the ground-state geometries, those of the excited states, and their variations upon excitation, showing that the M06 XC functional provides the closest agreement with the CC2 results. Still, large differences of geometries have been observed between the different levels of approximation, mostly for the excited states: (i) Not all methods locate the same number of minima, (ii) the bond length variations upon excitation might be reversed, and (iii) the H-bond network can be modified from one level to another, changing the keto/enol character. Moreover, TDDFT/M06 and B3LYP-35 vertical excitation energies are in good agreement with the CC2 values. All in all, these results call for being cautious when using these optimized geometries for predicting the spectroscopic signatures of these compounds to understand the processes that take place during photoexcitation.

Role of Exact Exchange and Relativistic Approximations in Calculating 19F Magnetic Shielding in Solids Using a Cluster Ansatz.

Calculations of 19F magnetic shielding in various materials are presented. In calculations on gas-phase molecules, the variation of magnetic shielding with the amount of Hartree-Fock exchange (HFX) in the functional demonstrates that excellent agreement with experiment is obtained with an admixture of 50%, here denoted PBE0 (50%). Calculations at the PBE, PBE0 (25%), and PBE0 (50%) levels on 10 crystalline organofluorines and 15 crystalline inorganic fluorides, in which a cluster ansatz is used to model the lattice environment, were performed. For fluorine-containing aromatics, increasing the admixture of HFX results in the prediction of larger magnetic-shielding spans, whereas increasing the admixture of HFX in calculations for CFCl3 decreases the span. In calculations of 19F magnetic shielding of the inorganic fluorides, the use of sufficiently large clusters of inorganic fluorides results in accuracies similar to those calculated for the organofluorines. Relativistic effects on the magnetic shielding of inorganic fluorides, modeled with ZORA at both the scalar and spin-orbit levels, are dominated by the scalar terms that increase the shielding of most 19F sites over the non-relativistic results. These effects appear to scale with the atomic number of the cation. For most elements of the sixth row (Cs, Ba, La, and Pb), the scalar relativistic contribution to the magnetic shielding is in the range of 20-77 ppm. For elements of group XII (Zn, Cd, and Hg) bonded to fluorine, the scalar relativistic contribution results in deshielding of the 19F site.

Hidden complexities in the reaction of H2O2 and HNO revealed by ab initio quantum chemical investigations.

Nitroxyl (HNO) and hydrogen peroxide have both been implicated in a variety of reactions relevant to environmental and physiological processes and may contribute to a unique, unexplored, pathway for the production of nitrous acid (HONO) in soil. To investigate the potential for this reaction, we report an in-depth investigation of the reaction pathway of H2O2 and HNO forming HONO and water. We find the breaking of the peroxide bond and a coupled proton transfer in the first step leads to hydrogen nitryl (HNO2) and an endogenous water, with an extrapolated NEVPT2 (multireference perturbation theory) barrier of 29.3 kcal mol-1. The first transition state is shown to possess diradical character linking the far peroxide oxygen to the bridging, reacting, peroxide oxygen. The energy of this first step, when calculated using hybrid density functional theory, is shown to depend heavily on the amount of Hartree-Fock exchange in the functional, with higher amounts leading to a higher barrier and more diradical character. Additionally, high amounts of spin contamination cause CCSD(T) to significantly overestimate the TS1 barrier with a value of 36.2 kcal mol-1 when using the stable UHF wavefunction as the reference wavefunction. However, when using the restricted Hartree-Fock reference wavefunction, the TS1 CCSD(T) energy is lowered to yield a barrier of 31.2 kcal mol-1, in much better agreement with the NEVPT2 result. The second step in the reaction is the isomerization of HNO2 to trans-HONO through a Grotthuss-like mechanism accepting a proton from and donating a proton to the endogenous water. This new mechanism for the isomerization of HNO2 is shown to have an NEVPT2 barrier of 23.3 kcal mol-1, much lower than previous unimolecular estimates not including an explicit water. Finally, inclusion of an additional explicit water is shown to lower the HNO2 isomerization barrier even further.

Assessment of different basis sets and DFT functionals for the calculation of structural parameters, vibrational modes and ligand binding energies of Zr4O2(carboxylate)12 clusters

Different basis sets and functionals in the framework of Density Functional Theory (DFT) were assessed for an accurate prediction of structural parameters and vibrational modes of the cluster Zr4O2(methacrylate)12. A basis set of at least triple-ξ quality is necessary to accurately model the geometry and the IR spectra. Bond distances are best reproduced with hybrid functionals, while IR frequencies are best described by the generalized gradient approximation family of functionals. LDA functionals give the highest GGA functionals the lowest ligand binding energies. Increasing the amount of Hartree Fock exchange in the hybrid functionals results in higher binding energies. Binding energies of ligands in different positions of Zr4O2(methacrylate)12 were calculated. The ligand binding energy decreases when the H atoms in an acetate ligand are successively replaced by F atoms with concomitant lengthening of the Zr–O bond. A chelating acetylacetonate ligand shows a higher ligand binding energy than chelating carboxylate ligands.

Evaluation of modern DFT functionals and G3n-RAD composite methods in the modelization of organic singlet diradicals.

The evaluation of four high-level composite methods based on the modification of Gaussian-3 (G3) theory for radicals and 18 exchange-correlation density functionals, including modern long-range and dispersion-corrected functionals, in the modelization of singlet diradicals has been performed in this work. Structural parameters and properties such as singlet-triplet gaps, electron affinities, ionization potentials, dipole moments, enthalpies of formation, and bond dissociation energies have been calculated in a set of six well-characterized singlet diradicals, and benchmarked against experimental data and wavefunction-based CASSCF/CASPT2 calculations. The complexity of the open-shell singlet ground state is revealed in the difficulties to properly represent the diradical character reported by some DFT functionals, specially those that do not comprise a certain amount of Hartree-Fock exchange in their formulation. We find that STGs, EAs, dipole moments, and thermochemical properties are, in general, satisfactorily calculated, while for IPs larger deviations with respect to the experiments are found in all cases. The best overall performance is accounted for by hybrid functionals, including some of the long-range corrected functionals, but also pure functionals, comprising the kinetic energy density in their formulation, are found to be competent. Composite methods perform satisfactorily, especially G3(MP2)-RAD and G3X(MP2)-RAD, which calculate singlet-triplet gaps and electron affinities more accurately. On the other hand, G3-RAD and G3X-RAD provide better ionization potentials. This study emphasizes that the use of recently developed functionals, within the broken symmetry approximation, is an appropriate tool for the simulation of organic singlet diradicals, with similar accuracy compared to more expensive composite methods. Nevertheless, suitable selection of the methodology is still crucial for the accomplishment of accurate results.

π-Delocalization and the vibrational spectroscopy of conjugated materials: computational insights on Raman frequency dispersion in thiophene, furan, and pyrrole oligomers.

The symmetric C═C stretching frequency (ν(Я)) of conjugated polymers and oligomers is a sensitive spectroscopic reporter of molecular structure and material morphologies; however, thorough understanding of how structure affects this frequency is lacking because computational investigations of this relationship have been undertaken with limited approaches. We present a comprehensive computational investigation of the structure-dependent Raman spectroscopy of oligothiophenes, oligofurans, and oligopyrroles in their ground electronic states using density functional theory. We assessed how various functionals (BLYP, B3LYP, BHLYP, and CAM-B3LYP) impact predictions of length-dependent trends in ν(Я). The amount of Hartree-Fock exchange in a functional is critical for accurately treating π-delocalization and polarizability and hence the structure-dependent Raman behavior. BLYP and B3LYP fail to accurately predict trends in ν(Я) with oligomer length because they over-represent delocalization; in contrast, the range-corrected CAM-B3LYP functional produces the same trends observed experimentally for oligomers in solution and in the solid phase. Through comparisons with a simple mechanical model, we demonstrate that the length- and conformation-dependent spectroscopy of oligothiophenes results from a delicate balance between delocalization-induced softening of ν(Я) and the coupling of oscillators that increase ν(Я). These findings are used to address how variations in inter- and intramolecular order impact the Raman spectroscopy of polythiophenes.

Description of plasmon-like band in silver clusters: the importance of the long-range Hartree-Fock exchange in time-dependent density-functional theory simulations.

Absorption spectra of Ag20 and Ag55(q) (q = +1, -3) nanoclusters are investigated in the framework of the time-dependent density functional theory in order to analyse the role of the d electrons in plasmon-like band of silver clusters. The description of the plasmon-like band from calculations using density functionals containing an amount of Hartree-Fock exchange at long range, namely, hybrid and range-separated hybrid (RSH) density functionals, is in good agreement with the classical interpretation of the plasmon-like structure as a collective excitation of valence s-electrons. In contrast, using local or semi-local exchange functionals (generalized gradient approximations (GGAs) or meta-GGAs) leads to a strong overestimation of the role of d electrons in the plasmon-like band. The semi-local asymptotically corrected model potentials also describe the plasmon as mainly associated to d electrons, though calculated spectra are in fairly good agreement with those calculated using the RSH scheme. Our analysis shows that a portion of non-local exchange modifies the description of the plasmon-like band.

Assessment of Quantum Mechanical Methods for Copper and Iron Complexes by Photoelectron Spectroscopy.

Broken-symmetry density functional theory (BS-DFT) calculations are assessed for redox energetics [Cu(SCH3)2]1–/0, [Cu(NCS)2]1–/0, [FeCl4]1–/0, and [Fe(SCH3)4]1–/0 against vertical detachment energies (VDE) from valence photoelectron spectroscopy (PES), as a prelude to studies of metalloprotein analogs. The M06 and B3LYP hybrid functionals give VDE that agree with the PES VDE for the Fe complexes, but both underestimate it by ∼400 meV for the Cu complexes; other hybrid functionals give VDEs that are an increasing function of the amount of Hartree–Fock (HF) exchange and so cannot show good agreement for both Cu and Fe complexes. Range-separated (RS) functionals appear to give a better distribution of HF exchange since the negative HOMO energy is approximately equal to the VDEs but also give VDEs dependent on the amount of HF exchange, sometimes leading to ground states with incorrect electron configurations; the LRC-ωPBEh functional reduced to 10% HF exchange at short-range give somewhat better values for both, although still ∼150 meV too low for the Cu complexes and ∼50 meV too high for the Fe complexes. Overall, the results indicate that while HF exchange compensates for self-interaction error in DFT calculations of both Cu and Fe complexes, too much may lead to more sensitivity to nondynamical correlation in the spin-polarized Fe complexes.

Understanding the Density Functional Dependence of DFT-Calculated Electronic Couplings in Organic Semiconductors.

We present an analysis of the magnitude of density functional theory (DFT)-calculated intermolecular electronic couplings (transfer integrals) in organic semiconductors to give insight into the impact that the choice of functional has on the value of this parameter, which is particularly important in the context of charge transport. The major factor determining the magnitude of the calculated transfer integrals is the amount of nonlocal Hartree-Fock (HF) exchange within a given functional, with the transfer integrals increasing by up to a factor of 2 when going from 0 to 100% HF exchange for a series of conventional functionals. We underline that these variations in the transfer integrals are in fact to be expected, with the computed transfer integrals evolving linearly with the amount of HF exchange. We also use a long-range corrected functional to tune the contributions of (semi)local and nonlocal HF exchanges and highlight their respective roles as a function of intermolecular separation.

Hybrid density functional study of the structural, bonding, and electronic properties of bismuth vanadate

The structure and property prediction of metal oxides can significantly be improved by incorporating exact Hartree-Fock (HF) exchange into density functional theory (DFT), which is the so-called hybrid DFT. We explored the impact of HF exchange inclusion on the predicted structural, bonding, and electronic properties of bismuth vanadate (BiVO${}_{4}$), with particular attention to the difference between its monoclinic and tetragonal scheelite phases. The applied exchange-correlation (xc) functionals include the gradient corrected Perdew-Burke-Ernzerhof (PBE) and the PBE-HF hybrid functionals with HF exchange amounts of 10$%$, 25$%$, and 50$%$. We find that the PBE-HF25$%$ yields a monoclinic structure in very close agreement with the experimentally determined structure, while the PBE-HF50$%$ tends to overestimate the monoclinic distortion and the PBE/PBE-HF10$%$ can hardly identify a distinct monoclinic configuration at ambient conditions. Electronic structure analysis reveals that the increasing monoclinic distortion with the amount of HF exchange is related to the enhancement of hybridization between Bi 6$s$-O 2$p$ antibonding states and unoccupied Bi 6$p$ states. The bonding mechanisms and band structures of the monoclinic and tetragonal phases of BiVO${}_{4}$ were also investigated, and we discuss how the predictions are sensitive to the xc functional choice.

Which DFT Functional Performs Well in the Calculation of Methylcobalamin? Comparison of the B3LYP and BP86 Functionals and Evaluation of the Impact of Empirical Dispersion Correction

Controversy remains regarding the suitable density functionals for the calculation of vitamin B(12) systems that contain cobalt. To identify the optimum functionals, geometry optimization calculations were performed on a full-size model of methylcobalamin (MeCbl) using the B3LYP, B3LYP-D, BP86, and BP86-D methods in conjunction with the 6-31G* basis set. Single-point energy evaluations were also performed with the 6-311+G(2d,p) basis set. Consistent with previous studies, the BP86-optimized geometry showed fairly good agreement with the experimental geometry. Various factors that may influence the homolytic bond dissociation energy (BDE) of the Co-C bond of MeCbl were systematically evaluated with these methods. Our analysis demonstrated that dispersion was the largest correction term that influenced the magnitude of BDE. Previous studies have shown that B3LYP significantly underestimates BDE, whereas BP86 gives BDE values that are fairly close to the experimental values (36-37 kcal/mol). The same trend in the relative magnitudes of the BDEs was observed in the present calculations. However, BP86 underestimated the BDE for a full model of MeCbl. When the amount of Hartree-Fock exchange in the B3LYP functional was reduced to 15% and the dispersion correction was made (i.e., B3LYP*-D), the calculated BDE was in good accord with experimental values. B3P86-D also performed well. A detailed analysis was undertaken to determine which atoms in cobalamin have large dispersion interactions with a methyl fragment of MeCbl.

Franck–Condon simulation of the A 1B2 → X 1A1 dispersed fluorescence spectrum of fluorobenzene and its rate of the internal conversion

By using three different hybrid exchange-correlation functionals containing 20%, 35%, and 50% of exact Hartree-Fock (HF) exchange of the density functional theory and its time-dependent extension plus the Hartree-Fock and the configuration interaction of single excitation methods, equilibrium geometries, and their 30 vibrational-normal-mode frequencies of the ground S(0)((1)A(1)) and the first excited S(1)((1)B(2)) states of fluorobenzene (FB) were calculated. The dispersed fluorescence spectrum and internal conversion (IC) rate of the A(1)B(2)→X(1)A(1) transition were simulated by Franck-Condon (FC) calculations within the displaced harmonic oscillator approximation plus anharmonic and distorted corrections. The simulated spectral profile is primarily described by the Franck-Condon progression from the ring-breathing modes ν(9) and ν(10) which belong to totally symmetry modes. Anharmonic corrections simultaneously improve the intensity order of 9(1)(0) and 10(1)(0) bands and diminish 1(1)(0) transition that is fairly strong in harmonic simulations. It is concluded that the amount of Hartree-Fock exchange does impact the geometries and vibrational frequencies of FB molecule, but not the relative intensities of the transitions. It is anharmonic corrections that make the relative intensities of the transitions in good agreement with experimental results. Distorted corrections could assign most of the dominant overtones of out-of-plane nontotally symmetry modes, and the results agree well with the experimental assignments. Furthermore, it was found that the internal conversion rate is dominated by three promoting modes that are computed with lowing symmetry to C(1). By choosing dephasing width as 10 cm(-1) that is consistent with spectral simulation, we obtained the lifetimes of the A(1)B(2)→X(1)A(1) de-excitation as 11 and 19 ns, respectively, from TD(B3LYP) and HF∕CIS calculations in comparison with the experimental value 14.75 ns.

Comparative Study on the Performance of Hybrid DFT Functionals in Highly Correlated Oxides: The Case of CeO2 and Ce2O3.

The outstanding catalytic properties of cerium oxides rely on the easy Ce(3+) ↔ Ce(4+) redox conversion, which however constitutes a challenge in density functional based theoretical chemistry due to the strongly correlated nature of the 4f electrons present in the reduced materials. In this work, we report an analysis of the performance of five exchange-correlation functionals (HH, HHLYP, PBE0, B3LYP, and B1-WC) implemented in the CRYSTAL06 code to describe three properties of ceria: crystal structure, band gaps, and reaction energies of the CeO2 → Ce2O3 process. All five functionals give values for cell parameters that are in fairly good agreement with experiment, although the PBE0 hybrid functional is found to be the most accurate. Band gaps, 2p-4f-5d in the case of CeO2 and 4f-5d in the case of Ce2O3, are found to be, in general, overestimated and drop off when the amount of Hartree-Fock exchange in the exchange-correlation functional decreases. In contrast, the reaction energies are found to be underestimated, and increase when the amount of HF exchange lowers. Overall, at its standard formulation, the B1-WC functional seems to be the best choice as it provides good band gaps and reaction energies, and very reasonable crystal parameters.

The axial methionine ligand may control the redox reorganizations in the active site of blue copper proteins

Structural and energetic reorganizations in redox reaction of type 1 copper proteins are studied by density functional and ab initio molecular orbital calculations. Model complexes of the active site with varying number of ligands, from Cu(SCH(3))(0/+) to Cu(SCH(3))(Im)(2)(S(CH(3))(2))(0/+), where Im denotes imidazole, are investigated. Following the findings of structural instability in Cu(I)(SCH(3))(Im)(2) and its stabilization by the addition of the axial methionine (Met) ligand model, the structure and energetics are examined as functions of the Cu-S(Met) distance in the range of 2.1-3.3 Å. The reorganization energies in both redox states exhibit a minimum at the Cu-S(Met) distance of ∼2.4 Å, whereas the ionization potential increases monotonically. The changes of reorganization energies correlate well with one of the Cu-N(His) distances rather than the Cu-S(Cys) distance. The estimated Arrhenius factor for oxidation of plastocyanin by P700(+) (in photosystem I) changes by an order of magnitude when the Cu-S(Met) distance fluctuates between 2.4 and 3.0 Å, whereas the factor for reduction of plastocyanin by cytochrome f is nearly constant. Together with the data from our previous classical molecular dynamics simulation of solvated protein, we argue that the electron transfer rate is affected, and thus may be controlled, by the fluctuation of a weakly bound axial Met ligand. We also present the assessment of various exchange-correlation functionals, including those with the long-range correction, against the CCSD(T) reference and on the basis of a perturbative adiabatic connection model. For Cu(SCH(3)) and Cu(SCH(3))(Im), simple correlations have been found between the reorganization energies and the amount of Hartree-Fock exchange.

A comparative ab initio study of neutral and charged kink solitons on conjugated carbon chains

The ground state of odd-N polyynic oligomers CNH2 features kink solitons in carbon-carbon bond-length alternation (BLA) patterns. We perform a systematic first-principles computational study of neutral and singly charged kinks in long oligomers addressing relationships between BLA patterns, electron energy gaps, and accompanying distributions of spin and charge densities, both in vacuum and in the screening solvent environment. A quantitative comparison is made of the results derived with four different ab initio methods: from pure density-functional theory to pure Hartree–Fock (HF) and including two popular hybrid density functionals, B3LYP and BHandHLYP. A clear correlation is demonstrated between the derived spatial extent of kinks and the amount of HF exchange used in the functionals. For charged kinks, we find a substantial difference in the behavior of charge and spin densities.