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The transition temperature $T_\textrm{c}$ of unconventional superconductivity is often tunable. For a monolayer of FeSe, for example, the sweet spot is uniquely bound to titanium-oxide substrates. By contrast for La$_{2-\mathrm{x}}$Sr$_\mathrm{x}$CuO$_4$ thin films, such substrates are sub-optimal and the highest $T_\textrm{c}$ is instead obtained using LaSrAlO$_4$. An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in $T_\mathrm{c}$ and how can we tune them? Here we demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of La$_2$CuO$_4$ thin films can be enhanced by compressive strain. Our experiments and theoretical calculations establish that the substrate producing the largest $T_\textrm{c}$ under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates., 15 pages, 7 figures and 2 tables (including Supplementary Information)

The modification of metal oxide surfaces with organic moieties has been widely studied as a method of preparing organic-inorganic hybrid materials for various applications. Among the inorganic oxides, ion-exchangeable layered perovskites are particularly interesting, because of their appealing electronic and reactive properties. In particular, their protonated interlayer surface can be easily functionalized with organic groups allowing the production of stable hybrid materials. As a further step in the design of new inorganic-organic hybrid proton conductors, a combined experimental and theoretical study of two intercalated compounds (propanol and imidazole) in HLaNb2O7 is presented here. A generally very good agreement with the available experimental data is found in reproducing both structural features and C-13-NMR chemical shifts, and marked differences between the two considered intercalated compounds are evidenced, with possible important outcomes for proton conduction. Notably, the free imidazole molecules are easily protonated by the acidic protons present in the interlayer spacing, thus inhibiting an efficient charge transport mechanism. In order to overcome this problem, a model system has been considered, where the imidazoles are bound to the end of a butyl chain, the whole being intercalated between two perovskite layers. The obtained theoretical data suggest that, in such a system, proton transfer between two adjacent imidazoles is a barrierless process. These results could then open new perspectives for such hybrid proton conductors.

A comprehensive theoretical investigation of the MLaNb2O7 (M = H, Li, Na, K, Rb, and Cs) series of ion-exchangeable layered perovskite is presented. These perovskites are in particular interesting in view of their potential applications as inorganic supports for the design of new hybrid inorganic-organic proton conductors. In particular, their structural and electronic properties have been investigated by periodic calculations in the framework of Density Functional Theory, using different exchange-correlation functionals. A general very good agreement with the available experimental (XRD, NPD, and EXAFS) data has been found. The structure of the protonated HLaNb2O7 form has also been further clarified and a new tetragonal space group is proposed for this compound, better reproducing the experimental cell parameters and yielding to a more realistic picture of the system. The electronic investigation highlighted that all the compounds considered are very similar to each other and that the interaction between interlayer cations and perovskite slabs is purely ionic, except for the proton that is, instead, covalently bound.

Time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies of s-tetrazine, both in the gas-phase and in aqueous solution. The model density functional (PBE0) is obtained by combining the Perdew–Burke–Erzenrhof (PBE) generalized gradient functional with a predetermined amount of exact exchange, while the polarizable continuum model (PCM) is used to mime solvent effects on electronic transitions. Our results in the gas-phase show that the PBE0 functional provides accurate excitations both to valence and to low-lying Rydberg states. At the same time, the experimental solvent shifts in aqueous solution are well reproduced when the solute and the first solvation shell are embedded by a continuum solvent. These results show the potentialities of the combined TDDFT/PCM approach for the study of UV spectra of aromatic compounds.

The geometry, conformational behaviour and magnetic properties of 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, an important spin carrier in molecular magnetic materials, have been studied by a combined experimental and theoretical approach. The available structural data have been completed by the electronic spin resonance spectrum in aqueous solution, and we have utilised a hybrid Hartree–Fock/Kohn–Sham method coupled to a polarizable continuum representation of the solvent. The computations show that the molecule has a non-planar conformation in the gas phase and in solution, but that the molecular magnetic properties are significantly tuned by environmental effects.

Density functional theory calculations were performed on uranyl complexed with nitrate and monoamide ligands (L) [UO(2)(NO(3))(2)·2L]. The obtained results show that the complex stability is mainly governed by two factors: (i) the maximization of the polarizability of the coordinating ligand and (ii) the minimization of the steric hindrance effects. Furthermore, the electrostatic interaction between ligands and uranium(vi) was found to be a crucial parameter for the complex stability. These results pave the way to the definition of (quantitative) property/structure relationships for the in silico screening of monoamide ligands with improved extraction efficiency of uranium(vi) in nitrate acidic solution.

A new method for the expansion of potential energy surfaces has been developed exploiting the peculiar properties of Hahn polynomials, a class of orthogonal polynomials of a discrete variable which generalize 3j vector coupling coefficients of angular momentum algebra. The method has been tested for the Henon–Heiles potential, a typical model for coupled oscillators, and applied to the representation of the potential energy surface of malonaldehyde, a prototype system for intramolecular proton transfer in polyatomic molecules. The representation is obtained by fitting the polynomial expansion to a set of points calculated by the density functional theory method on a hyperspherical effective three-center coordinate system, in view of perspective quantum dynamical calculations of the proton transfer process.

The ability of density functional models in dealing with polymerization mechanisms has been investigated by comparison with high level post-Hartree–Fock methods. Ethylene insertion and chain transfer reactions have been compared for a model of the active species suggested for the Jordan aluminum catalyst [{R′C(NR″)2}AIR]+. Conventional density functional approaches (BP86) show a strong bias in favor of chain transfer reactions via hydrogen transfer. The B1LYP model provides improved energy barriers. The aluminum model used strongly favors chain transfer over insertion, this preference being further enhanced by lengthening the growing polymer chain. These findings cast some doubts about currently accepted models of the active species in the Jordan catalyst.

We analyze the performance of a density functional model obtained by combining the Perdew–Burke–Erzenrhof (PBE) generalized gradient functional with a predetermined amount of exact exchange for predicting vertical electronic excitation energies within a time-dependent approach. Four molecules, namely, CO, H2CO, (CH3)2CO, and C2H4, have been chosen as benchmark cases. Our results show that this model (PBE0) provides accurate excitations both to valence and Rydberg states. Furthermore, the results are numerically close to those obtained using asymptotically correct exchange-correlation functionals. The performance of the PBE0 model for predicting excitation energies in larger molecules is assessed for benzene, pyridine, and naphthalene. Here, the PBE0 model provides results which are in fairly good agreement with experimental data and of similar quality to those obtained by more sophisticated (and time-consuming) post-Hartree–Fock methods.

We analyze the performances of a new density functional model (PBE0) obtained combining the so-called PBE generalized gradient functional with a predefined amount of exact exchange, in evaluating molecular dipole polarizabilities. Five molecules, namely H2O, NH3, HF, H2S and CO, have been chosen as test cases in the evaluation of dipole moments and polarizabilities. Our results show that the PBE0 model outperforms other current density functionals. The performance of this model has been assessed for a set of 12 small molecules. Here the PBE0 results are in good agreement with experimental data and close to those obtained by sophisticated (and more time-demanding) post-Hartree–Fock methods. Average dipole polarizabilities have also been computed for a set of aliphatic and aromatic hydrocarbons, including medium- and large-size molecules, for which post-HF calculations are too expensive.

A spin-unrestricted approach to time-dependent density functional theory is applied to calculate vertical excitation energies of some small and medium-size organic radicals, including nitromethyl, benzyl, anilino and phenoxyl. The computational model is obtained combining the Perdew–Burke–Erzenrhof (PBE) generalized gradient functional with a predetermined amount of exact exchange. Our results show that this method (PBE0) provides accurate excitations both to valence and low-lying Rydberg states, even in the presence of substantial contributions from double excitations.

Methods rooted in the adiabatic connection theorem offer a significant improvement over standard density functional approximations. Among them, the most successful model is the so-called B3LYP functional, which includes three empirical parameters optimized by a best fit to atomization energies. We introduce a related functional, B1LYP, in which the ratio between Hartree-Fock and density functional exchange is determined a priori from purely theoretical considerations and no further parameters are present. The numerical results obtained on a standard molecular data set and on some ‘delicate’ chemical systems indicate that this model has essentially the same performance as the B3LYP model, but within a more satisfactory theoretical framework.

A parallel study by different conventional and hybrid density functional methods has been performed for the whole series of binary cationic hydrides of first row transition metals. The results show that hybrid methods represent a significant improvement over standard density functionals, reaching an accuracy comparable to that of the most sophisticated post-Hartree-Fock approaches. among the different hybrid methods, the B3LYP variant seems to provide the most balanced results. In all cases converged results are obtained with basis sets small enough to be applied in the study of large systems.

A comparative post‐Hartree–Fock study has been performed on cyclopropyl and oxiranyl radicals in order to ascertain the role of the oxygen atom in modifying the hyperfine structure and height of the barrier governing inversion at the radical center. The structural parameters and harmonic force fields obtained for the parent molecules using second‐order many‐body perturbation theory with a large basis set are in good agreement with experiment. The same approach points out significant distortions upon breaking of a CH bond and a larger pyramidality for the radical center in oxiranyl with respect to cyclopropyl. Also inversion barriers of both radicals are in remarkable agreement with experimental estimates. Isotropic hyperfine parameters in good agreement with those obtained from electron spin resonance spectra can be computed only when using purposely tailored basis sets in the framework of a coupled cluster approach and taking into account vibrational averaging effects induced by the inversion motion. Int...

The structure and EPR parameters of fluorinated vinyl radicals have been investigated by the density functional approach. Comparison with conventional post-Hartree—Fock methods shows that DF results are competitive with MP2 results for geometries and harmonic frequencies and with coupled cluster computations of hyperfine couplings. The results are in good agreement with experimental data and offer a comprehensive picture of substituent effects on geometric and electronic characteristics. The accuracy of the results and the feasibility of this kind of computation for large systems indicate that DF methods can be particularly useful to those researchers seeking cogent relationships between structure and spectroscopy.