The first π → π* transition in a number of cyanine dyes was studied using both time dependent and time independent density functional methods using a coupled cluster (CC2) method as the benchmark scheme. On the basis of 10 different functionals, it was concluded that adiabatic time dependent density functional theory (ATDDFT) almost independently of the functional gives rise to a singlet-triplet separation that is too large by up to 1 eV, leading to too high singlet energies and too low triplet energies. This trend is even clearer when the Tamm-Dancoff (TD) approximation is introduced and can in ATDDFT/TD be traced back to the representation of the singlet-triplet separation by a HF-type exchange integral between π and π*. The time independent DFT methods (ΔSCF and RSCF-CV-DFT) afford triplet energies that are functional independent and close to those obtained by ATDDFT. However, both the singlet energies and the singlet-triplet separations increases with the fraction α of HF exchange. This trend can readily be explained in terms of the modest magnitude of a KS-exchange integral between π and π* in comparison to the much larger HF-exchange integral. It was shown that a fraction α of 0.5 affords good estimates of both the singlet energies and the singlet-triplet separations in comparison to several ab initio benchmarks.

We present here simulated magnetic circular dichroism (MCD) spectra of MTAP (M=Mg, Ni, Zn) and MPc (M=Mg, Zn) where TAP=tetraazaporphyrin and Pc=phthalocyanine. The study is based on magnetically perturbed time-dependent density functional theory (MP-TDDFT) and a newly implemented method for the calculation of A and B terms from the theory of MCD. It follows from our investigation that the MCD spectrum for the MTAP and MPc systems in the Q-band region consists of a single positive A term augmented by a positive B term, in agreement with experiment where available. The Q band can be fully characterized in terms of the 2a1u-->2eg one-electron excitation. For the aza systems MgTAP and ZnTAP, the simulated MCD spectra in the Soret region are dominated by the two one-electron excitations 2a2u-->2eg and 1a2u-->2eg and has the appearance of a positive A term (with values between 1.33-1.55, depending on the MTAP system) made asymmetric by a negative B term, in good agreement with experiment. We find, in agreement with all available experimental findings on MPc (M=Mg, Zn) type systems, that the MCD spectra in the Soret region are dominated by two transitions with positive A/ D-term values and two negative B/ D-term values. The major contribution to the two transitions comes from the 2a2u-->2eg and 1a2u-->2eg one-electron excitations. It appears that the ratio of A/ B for the term parameters is underestimated by theory.

We have introduced a new method for the calculation of spin-exchange between weakly interacting electron spins on different metal centers. The method is based on spin-flip constricted variational density functional theory (SF-CV-DFT). The application of SF-CV-DFT to two trinuclear systems [Cu3(L)(μ3-O)](4+) and [(L')3]Cu3(μ-OH)3](3+) revealed that SF-CV-DFT affords exchange coupling constants that are similar to the values obtained by the traditional broken-symmetry (BS) scheme for the same functional. The BHandHLYP functional affords for both systems the best fit to experiment and results from high-level theory in the case of BS-DFT as well as SF-CV(2)-DFT. All methods and functionals predict [Cu(L)(μ3-O)](4+) to be ferromagnetic and the [(L')3Cu3(μ-OH)3](3+) system to be antiferromagnetic. The SF-CV(2)-DFT method is not only able to evaluate exchange coupling constants, it can in addition calculate the full multiplet spectrum with complete use of spatial symmetry. Further, in its restricted formulation, calculations can be carried out with use of full spin-symmetry without spin-contamination.

Evaluation of the excitation energies in CuCl42− and plastocyanin was carried out with three different density functional theory (DFT) methods: time‐dependent DFT (TDDFT), ΔDFT, and Δself consistent field (SCF)‐DFT. Use was made of two local (local density approximation and BP86), two hybrid (B3LYP and PBE0), and one long‐range corrected (LC‐BP86) functionals to probe the effect of the exact exchange on the excitation energies. Some well‐known deficiencies observed in TDDFT were explained on the basis of lacking orbital relaxation and two‐electron terms not considered in adiabatic TDDFT. Δself consistent field (SCF)‐DFT reproduces well the position of the charge‐transfer excitations and yields results in best agreement with experiment regardless of the system or functional used. We conclude that the orbital relaxation and proper account of higher‐order terms neglected in adiabatic TDDFT are of great importance for the simulation of excitation spectra. © 2014 Wiley Periodicals, Inc.

Solute carrier family 4 (SLC4) transporters mediate the transmembrane transport of HCO 3 − , CO 3 2− , and Cl − necessary for pH regulation, transepithelial H + /base transport, and ion homeostasis. Substrate transport with varying stoichiometry and specificity is achieved through an exchange mechanism and/or through coupling of the uptake of anionic substrates to typically co-transported Na + . Recently solved outward-facing structures of two SLC4 members (human anion exchanger 1 [hAE1] and human electrogenic sodium bicarbonate cotransporter 1 [hNBCe1]) with different transport modes (Cl − /HCO 3 − exchange versus Na + -CO 3 2− symport) revealed highly conserved three-dimensional organization of their transmembrane domains. However, the exact location of the ion binding sites and their protein–ion coordination motifs are still unclear. In the present work, we combined site identification by ligand competitive saturation mapping and extensive molecular dynamics sampling with functional mutagenesis studies which led to the identification of two substrate binding sites (entry and central) in the outward-facing states of hAE1 and hNBCe1. Mutation of residues in the identified binding sites led to impaired transport in both proteins. We also showed that R730 in hAE1 is crucial for anion binding in both entry and central sites, whereas in hNBCe1, a Na + acts as an anchor for CO 3 2− binding to the central site. Additionally, protonation of the central acidic residues (E681 in hAE1 and D754 in hNBCe1) alters the ion dynamics in the permeation cavity and may contribute to the transport mode differences in SLC4 proteins. These results provide a basis for understanding the functional differences between hAE1 and hNBCe1 and may facilitate potential drug development for diseases such as proximal and distal renal tubular acidosis.