Ab initio group model potentials including electron correlation effects

A method for determination of ab initio group model potentials, with the intention of describing the effects of a whole molecule or a chemical group within a density functional theory framework, is reported. The one-electron part of the Kohn–Sham equations is modified by incorporation of a Coulomb operator, which accounts for the classical electron interaction arising from the group. Exchange and correlation effects are introduced by a suitable modification of the exchange-correlation functionals. The strong orthogonality condition, usually required by the theory of separability of many electron systems, is written in terms of first order reduced density matrices. In order to check the method a group model potential for H2O (environment) was obtained and employed in the calculation of HF⋯H2O and H2O⋯H2O complexes using several functionals. Equilibrium intergroup distances and binding energies are compared with all-electron calculations.Dirección General de Enseñanza Superior (DGES). España PB98-1125Junta de Andalucía FQM13

Mechanism of Cu deposition on the α−Al2O3 (0001) surface

The growth mechanism of the Cu/α−Al2O3 (0001) interface is studied by first-principles molecular-dynamics simulations as a function of the transition-metal coverage (θ) and the temperature of the system. On the anhydrous surface growth of Cu(0) 3D clusters is predicted. On the partially hydroxylated surface, a Cu(I) monolayer, relatively stable upon the temperature rising, is first observed (θ<1/3  ML). Increasing Cu loading leads to Cu(I)/Cu(0) mixed phases that when heated aggregate into 3D particles increasing the number of Cu(0) atoms, in agreement with the Auger spectra of Kelber et al.Ministerio de Ciencia y Tecnología MAT2002-057

Effect of on-site Coulomb repulsion term U on the band-gap states of the reduced rutile (110) TiO2 surface

We present a study concerning the effect of the on site d-d Coulomb interaction energy U on the band-gap states of nonstoichiometric rutile 110 TiO2 surface. As well known, the excess electrons resulting from the formation of oxygen vacancies localize on the Ti 3d orbitals forming band-gap states. Local density approximation LDA does not give a correct description of these band-gap states, either with or without gradient corrections. The failure of LDA is often attributed to an inadequate treatment of electron correlation in systems with localized orbitals and is commonly corrected with an empirical local Coulomb repulsion term, i.e., the LDA+U method. This study provides a completely general strategy to estimate the U value in this kind of systems, illustrated here for reduced 110 TiO2 surface, well characterized from experiments. From ab initio embedded cluster configuration interaction calculations, combined with the effective Hamiltonian theory, a value of U of 5.5±0.5 eV is obtained, in good agreement with those reported for this system from x-ray photoemission spectroscopy experiments U=4.5±0.5 eV . It is observed that when the ab initio estimate of U is injected into the periodic LDA+U calculations, a correct description of the gap states is obtained from the periodic LDA+U calculations. Additionally, the results indicate that the position of these states on the band gap strongly depends on the level at which lattice relaxation is taken into account, with significant differences between the density of states curves at the LDA+U level obtained using the optimal generalized gradient approximation or LDA+U geometries. These results suggest that this combined strategy could be a useful tool for those systems where electron correlation plays a key role, and no experimental data are available for the on-site Coulomb repulsio

Interaction potentials from periodic density-functional theory calculations: Molecular-dynamics simulations of Au clusters deposited on the TiN (001) surface

Molecular-dynamics simulations of gold particles deposited on a TiN 001 surface have been accounted for through classical pair potentials describing the atom force field. The interaction between Ti–N, Ti–Ti, N–N, Au–Au, Au–Ti, and Au–N pairs was estimated by following a procedure in which the interaction energy between two sets of atoms is estimated from density-functional calculations performed with periodic boundary conditions using plane waves as basis set. The pair potentials were expressed as the sum of two contributions: long range in a Coulomb form and a short-range term, which included the rest of the energy contributions. Simulations of the TiN 001 isolated surface reproduced the already described surface relaxation, with a rippling parameter in agreement with that found from a purely first-principles approach. Simulations of gold deposition on such surfaces showed the formation of metal clusters with well-defined fcc structure and epitaxially grown.Ministerio de Ciencia y Tecnología de España MAT2005-01872Junta de Andalucía FQM-13

Electronic structure of porphyrin-based metal– organic frameworks and their suitability for solar fuel production photocatalysis

Metal–organic frameworks (MOFs) can be exceptionally good catalytic materials thanks to the presence of active metal centres and a porous structure that is advantageous for molecular adsorption and confinement. We present here a first-principles investigation of the electronic structure of a family of MOFs based on porphyrins connected through phenyl-carboxyl ligands and AlOH species, in order to assess their suitability for the photocatalysis of fuel production reactions using sunlight. We consider structures with protonated porphyrins and those with the protons exchanged with late 3d metal cations (Fe2+, Co2+, Ni2+, Cu2+, Zn2+), a process that we find to be thermodynamically favorable from aqueous solution for all these metals. Our band structure calculations, based on an accurate screened hybrid functional, reveal that the bandgaps are in a favorable range (2.0 to 2.6 eV) for efficient adsorption of solar light. Furthermore, by approximating the vacuum level to the pore centre potential, we provide the alignment of the MOFs' band edges with the redox potentials for water splitting and carbon dioxide reduction, and show that the structures studied here have band edges positions suitable for these reactions at neutral pH.Royal Society for an International Exchange Scheme grantVia our membership of the UK's HPC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work made use of the facilities of ARCHER, the UK's national high-performance computing services, which are funded by the Office of Science and Technology through EPSRC's High End Computing ProgrammeEuropean Research Council through an ERC Starting Grant (ERC2011-StG-279520-RASPA)MINECO (CTQ2013-48396-P)Andalucía Region (FQM-1851)University of Granad

The constrained space orbital variation analysis for periodic ab initio calculations

The constrained space orbital variation (CSOV) method for the analysis of the interaction energy has been implemented in the periodic ab initio CRYSTAL03 code. The method allows for the partition of the energy of two interacting chemical entities, represented in turn by periodic models, into contributions which account for electrostatic effects, mutual polarization and charge transfer. The implementation permits one to carry out the analysis both at the Hartree-Fock and density functional theory levels, where in the latter the most popular exchange-correlation functionals can be used. As an illustrating example, the analysis of the interaction between CO and the MgO (001) surface has been considered. As expected by the almost fully ionic character of the support, our periodic CSOV results, in general agree with those previously obtained using the embedded cluster approach, showing the reliability of the present implementation.Ministerio de Educación y Ciencia de España MAT2005-1872Secretaría de la Educación Pública (SEP)-Consejo Nacional de Ciencia y Tecnología (CONACYT). Gobierno de México SEP-2004-CO1-4698

A Proposal for a Modified Moller-Plesset Perturbation Theory

A modified version of the Moller-Plesset approach for obtaining the correlation energy associated to a Hartree-Fock ground state is proposed. The method is tested in a model of interacting fermions that allows for an exact solution. Using up to third order terms improved results are obtained, even more accurate in the limit of loosely bound particles. This result suggests the possible convenience of the scheme for the study of chemical bound problems.Comment: 10 pages, 1 figur

Hydrogen bonds in lead halide perovskites: insights from ab initio molecular dynamics

Hydrogen bonds (HBs) play an important role in the rotational dynamics of organic cations in hybrid organic/inorganic halide perovskites, affecting the structural and electronic properties of the perovskites. However, the properties and even the existence of HBs in these perovskites are not well established. We investigate HBs in perovskites MAPbBr$_3$ (MA$^+$=CH$_3$NH$_3^+$), FAPbI$_3$ (FA$^+$= CH(NH$_2$)$_2^+$), and their solid solution (FAPbI$_3$)$_{7/8}$(MAPbBr$_3$)$_{1/8}$, using ab initio molecular dynamics and electronic structure calculations. We consider HBs donated by X-H fragments (X=N, C) of the organic cations and accepted by the halides (Y=Br, I), and characterize their properties based on pair distribution functions and on a combined distribution function of hydrogen-acceptor distance with donor-hydrogen-acceptor angle. By analyzing these functions, we establish geometric criteria for HB existence based on hydrogen-acceptor distance $d(H-Y)$ and donor-hydrogen-acceptor angle $\measuredangle(X-H-Y)$. The distance condition is defined as $d(H-Y)<0.3$ nm, for N-H-donated HBs, and $d(H-Y)<0.4$ nm for C-H-donated HBs. The angular condition is $135{^\circ}\le\measuredangle(X-H-Y)\le 180{^\circ}$ for both types of HBs. At the simulated temperature (350 K), the HBs dynamically break and form. We compute time correlation functions of HB existence and HB lifetimes, which range between 0.1 and 0.3 picoseconds at that temperature. The analysis of HB lifetimes indicates that N-H--Br bonds are relatively stronger than N-H--I bonds, while C-H--Y bonds are weaker. To evaluate the impact of HBs on vibrational spectra, we present the power spectra, showing that peaks associated with N-H stretching modes in perovskites are redshifted and asymmetrically deformed compared with the peaks of isolated cations.Comment: 19 pages, 9 figure

Revealing the role of Pb 2+ in the stability of organic–inorganic hybrid perovskite CH 3 NH 3 Pb 1− x Cd x I 3: an experimental and theoretical study

This paper presents the synthesis of organic–inorganic hybrid perovskite CH3NH3Pb1−xCdxI3. The effect of incorporating Cd2+ or Pb2+ on the stability of the perovskite structure was analysed from a theoretical and experimental viewpoint. The XRD results showed that the tetragonal perovskite structure was formed for x values of up to 0.5, which seems to indicate that the presence of a considerable amount of Pb2+ is necessary to stabilise the structure. In turn, UV-Vis spectroscopy showed how the presence of Cd2+ led to a reduction in the optical band gap of the perovskite structure of up to 9% for CH3NH3Pb0.5Cd0.5I3 with regard to the MAPbI3 structure. Moreover, periodic-DFT calculations were performed to understand the effect of the increased concentration of Cd on the structural and electronic properties of MAPbI3 perovskites. The analysis of both the ELF and the non-covalent interaction (NCI) index show the important role played by the Pb2+ ions in stabilizing this kind of hybrid perovskite structures. Finally, the DOS analysis confirmed the experimental results obtained using UV-Vis spectroscopy. The theoretical band gap values decreased as the concentration of Cd increased.Junta de Andalucía (P09- FQM-04938)Fondo Europeo de Desarrollo Regional (FEDER)Centro Informático Científico de Andalucía (CICA