Antonio M. Márquez

A CSOV study of the difference between HF and DFT intermolecular interaction energy values: The importance of the charge transfer contribution

Intermolecular interaction energy decompositions using the Constrained Space Orbital Variation (CSOV) method are carried out at the Hartree–Fock level on the one hand and using DFT with usual GGA functionals on the other for a number of model complexes to analyze the role of electron correlation in the intermolecular stabilization energy. In addition to the overall stabilization, the results provide information on the variation, with respect to the computational level, of the different contributions to the interaction energy. The complexes studied are the water linear dimer, the N‐methylformamide dimer, the nucleic acid base pairs, the benzene–methane and benzene‐N2 van der Waals complexes, [Cu+‐(ImH)3]2, where “ImH” stands for the Imidazole ligand, and ImH‐Zn++. The variation of the frozen core energy (the sum of the intermolecular electrostatic energy and the Pauli repulsion energy) calculated from the unperturbed orbitals of the interacting entities indicates that the intramolecular correlation contributions can be stabilizing as well as destabilizing, and that general trends can be derived from the results obtained using usual density functionals. The most important difference between the values obtained from HF and DFT computations concerns the charge transfer contribution, which, in most cases, undergoes the largest increase. The physical meaning of these results is discussed. The present work gives reference calculations that might be used to parametrize new correlated molecular mechanics potentials.

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.

On modelling the interaction of CO on the MgO(100) surface

Abstract In this work we discuss the interaction of an adsorbate on an ionic surface taking as representative example CO on a perfect MgO(100) surface. The main goal is to investigate the different contributions to the interaction and how to model them by using a finite cluster model. To this end we use three different ab initio Hartree-Fock approaches. First, we discuss the convergence properties of the array of point charges used to simulate the Madelung potential. Next, we use large cluster models to show that there is an oscillatory behavior of the interaction energy. We show that rather large clusters are needed to avoid such oscillations. Also, we discuss how to represent these large clusters by using model potentials and compare the results with those obtained from periodic Hartree-Fock calculations. The interaction of CO on MgO is found to be weak and of electrostatic origin, with no noticeable chemical contributions.

On the bonding mechanism of CO to Pt(111) and its effect on the vibrational frequency of chemisorbed CO

Abstract The chemisorption of CO on the atop site of Pt(111) has been simulated by a Pt4 cluster model. Ab initio self consistent field (SCF) and complete active space self consistent field (CASSCF) cluster model wave functions have been obtained for the electronic ground state. Likewise, ab initio SCF wavefunctions have been obtained for two other electronic states. The optimum geometry and vibrational frequencies of chemisorbed CO are reported for the three states. The interaction energy and vibrational shift of chemisorbed CO, with respect to free gas phase CO, have been analyzed for the three electronic states. This analysis is carried out by means of the constrained space orbital variation (CSOV) method. In all cases the bond is found to be dominated by σ donation and π back-donation, known as Blyholders mechanism. This mechanism is further supported by SCF calculations on a larger, Pt13, cluster model. For both clusters, the CSOV analysis of the vibrational frequency definitely shows that, contrary to previous recent studies, a major contribution to the experimentally observed vibrational shift comes from the π back-donation mechanism. However, we found that, contrary to common belief, σ donation also acts to lower the CO frequency and not to increase it. Physical reasons for such unexpected behaviour are given.

Communication: Improving the density functional theory+U description of CeO2 by including the contribution of the O 2p electrons

Density functional theory (DFT) based approaches within the local-density approximation or generalized gradient approximation frameworks fail to predict the correct electron localization in strongly correlated systems due to the lack of cancellation of the Coulomb self-interaction. This problem might be circumvented either by using hybrid functionals or by introducing a Hubbard-like term to account for the on site interactions. This latter DFT+U approach is less expensive and therefore more practical for extensive calculations in solid-state computational simulations. By and large, the U term only affects the metal electrons, in our case the Ce 4f ones. In the present work, we report a systematic analysis of the effect of adding such a U term also to the oxygen 2p electrons. We find that using a set of U(f) = 5 eV and U(p) = 5eV effective terms leads to improved description of the lattice parameters, band gaps, and formation and reduction energies of CeO(2).

Similarities and differences in the Hartree–Fock and density-functional description of the chemisorption bond

Abstract The similarities and differences between the ab initio Hartree–Fock (HF) and density-functional theory (DFT) descriptions of the chemisorption bond have been explored by applying the constrained space orbital variation (CSOV) method to obtain and analyze the HF and DFT total energies of cluster models representing the interaction of CO and NH 3 on Cu(100) and Pt(111). The qualitative picture of the chemisorption bond arising from ab initio HF and DFT quantum-chemical approaches is essentially the same; the relative importance of the different mechanisms remains unchanged. The main quantitative effect of electronic correlation is to increase the inter-unit charge transfer but substrate polarization is also affected. A very important consequence emerges: it is not necessary to revise the physical mechanisms that have been previously proposed from the ab initio Hartree–Fock cluster model approach.

The Rys Quadrature Revisited: A Novel Formulation for the Efficient Computation of Electron Repulsion Integrals over Gaussian Functions

A novel formulation of the Rys quadrature algorithm for the calculation of the electron repulsion integrals over Gaussian basis functions is presented. The new algorithm is specifically designed for high contractions. As for the original Rys quadrature algorithm, the new algorithm is very efficient for high angular momentum functions. In addition it is also equally efficient for low angular momentum functions. The new algorithm takes unique advantage of (1) the numerical Rys quadrature methodology in (2) dealing with charge distributions a la McMurchie–Davidson and in (3) scaling integral blocks as a means of transferring angular momentum a la Gill–Head–Gordon–Pople. An analysis of the algorithm suggests very favorable floating-point operation counts.

Parallel computation of the MP2 energy on distributed memory computers

A parallel distributed implementation of the second‐order Møller‐Plesset perturbation theory method, widely used in quantum chemistry, is presented. Parallelization strategy and performance for the HONDO quantum chemistry program running on a network of Unix computers are also discussed. Superlinear speedups are obtained through a combined use of the CPU and memory of the different processors. Performance for standard and direct algorithms are presented and discussed. A superdirect algorithm that eliminates the communication bottleneck during the integral transformation step is also proposed.

Theoretical values of the enthalpies of formation of the NHx (x = 1,2,3) compounds. Importance of the core-correlation effects

Abstract The enthalpies of formation of the NH x ( x = 1,2,3) compounds were theoretically estimated using the isogyric and hydrogenation reactions as working chemical reactions. Energy differences were computed at seven levels of calculation, using MP4 (with spin projection and post-PMP4 corrections), QCI, CC, and multireferential methods with two extended basis sets. Using NH 3 as the test molecule, we found that accurate results can be obtained with theoretical methods using large basis sets, elaborate correlated wavefunctions, and, above all, with the core-correlation effects explicitly considered. The value obtained for the NH 2 radical is ΔH f,298 K = 43.8 ± 0.6 kcal mol −1 , which is smaller than the recommended JANAF value and the latest experimental values. For the NH species, the value obtained is ΔH f,298 K =86.3±0.8 kcal mol −1 , in excellent agreement with other high-quality results. This last value confirms indirectly the accuracy of our proposed value for the NH 2 radical.

Surface oxygen vacancies in gold based catalysts for CO oxidation

Experimental catalytic activity measurements, diffuse reflectance infrared Fourier spectroscopy, and density functional theory calculations are used to investigate the role and dynamics of surface oxygen vacancies in CO oxidation with O2 catalyzed by Au nanoparticles supported on a Y-doped TiO2 catalyst. Catalytic activity measurements show that the CO conversion is improved in a second cycle of reaction if the reactive flow is composed by CO and O2 (and inert) while if water is present in the flow, the catalyst shows a similar behaviour in two successive cycles. DRIFTS-MS studies indicate the occurrence of two simultaneous phenomena during the first cycle in dry conditions: the surface is dehydroxylated and a band at 2194 cm−1 increases (proportionally to the number of surface oxygen vacancies). Theoretical calculations were conducted in order to explain these observations. On one hand, the calculations show that there is a competition between gold nanoparticles and OH to occupy the surface oxygen vacancies and that the adsorption energy of gold on these sites increases as the surface is being dehydroxylated. On the other hand, these results evidence that a strong electronic transfer from the surface to the O2 molecule is produced after its adsorption on the Au/TiO2 perimeter interface (activation step), leaving the gold particle in a high oxidation state. This explains the appearance of a band at a wavenumber unusually high for the CO adsorbed on oxidized gold particles (2194 cm−1) when O2 is present in the reactive flow. These simultaneous phenomena indicate that a gold redispersion on the surface occurs under reactive flow in dry conditions generating small gold particles which are very active at low temperature. This fact is notably favoured by the presence of surface oxygen vacancies that improve the surface dynamics. The obtained results suggest that the reaction mechanism proceeds through the formation of a peroxo-like complex formed after the electronic transfer from the surface to the gas molecule.