Carlos Sosa

A theoretical study of the ionic dissociation of HF, HCl, and H2S in water clusters

The ionic dissociation of HF, HCl, and H2S in water is examined using density functional theory (DFT), Hartree–Fock (HF), and Mo/ller–Plesset theory to second order (MP2). The calculations show that HF, HCl, and H2S form fully dissociated stable clusters with four water molecules. Each cluster appears to be stabilized by the formation of six hydrogen bonds. These calculations also indicate that a minimum of four water molecules are required to form stable structures in which positive and negative ions coexist in the cluster. The hydrogen transfer between the acid and water molecules is very similar to the mechanism proposed for hydrogen transfer in water solutions. The binding energies of the hydrated hydrofluoric acid, hydrated hydrochloric acid, and hydrated hydrogen disulfide estimated with B‐LYP are 37.51, 41.17, and 20.68 kcal/mol, respectively.

Density functional description of transition structures using nonlocal corrections. Silylene insertion reactions into the hydrogen molecule

The insertion reactions of SiH2 , SiHF, and SiF2 into the hydrogen molecule have been investigated using density functional methods. Local spin density (LSD) calculations were performed with the LSD exchange functional and with the Vosko, Wilk, and Nusair correlation energy functional (VWN). Nonlocal spin density corrections (NLSD) were estimated with the exchange functional of Becke and the correlation energy functional of Perdew (B–P); Becke and the correlation energy functional of Lee, Yang, and Parr (B–LYP); and Perdew and Wang (PW) generalized gradient exchange‐correlation energy functional. Reactants, transition structures, and products were fully optimized at the LSD and NLSD levels. For each of these reactions, relative energies have been calculated with density functional methods and also at the quadratic configuration interaction with single, double, and triple excitations [QCISD(T)]/6‐31G(2df,pd) level. Vibrational frequencies were also computed with local and nonlocal approximations as well as...

Theory and implementation of the MBPT density matrix. An application to one-electron properties

Abstract A perturbation-independent response density matrix has been derived and implemented for many-body perturbation theory. This density contains all orbital relaxation terms, which allows for fast, efficient computation of correlated one-electron response properties. As such, it eliminates the need for finite-field calculations of first-order properties. The method is applied to H 2 O using an extended basis set for the MBPT (2), MBPT (3), and SDQ-MBPT (4) levels of theory. The relationship of the response density to an average density matrix is discussed.

THEORETICAL STUDY OF THE MECHANISM OF RECOMBINATIVE HYDROGEN DESORPTION FROM THE MONOHYDRIDE PHASE OF SI(100) : THE ROLE OF DEFECT MIGRATION

Density functional theory with nonlocal corrections is used together with cluster models to examine various pathways for H2 desorption from the Si(100)2×1 surface. The barrier calculated for direct desorption of H2 from the doubly‐occupied dimer is appreciably larger than the experimentally observed activation energy at submonolayer coverages. We propose a mechanism in which surface defects are converted into dihydride (SiH2) species from which H2 desorption occurs. The barrier calculated for this process (57 kcal/mol) is in excellent agreement with the measured activation energy. The barrier for defect migration is predicted to be only 14 kcal/mol, so that a single defect can account for the desorption of H2 from a large number of monohydride sites. Single‐point calculations for several of the optimized structures are carried out using the quadratic configuration interaction (QCI) method. The reaction energies and barrier heights calculated with the QCI and density functional theory (DFT) methods are in ...

Fullerene derivatives. Comparative theoretical study of C60O and C60CH2

Abstract Semi-empirical, ab initio Hartree—Fock, and density functional studies (including gradient corrections) on the structures and stabilities of C 60 CH 2 are reported. Two low-energy isomers are found for both compounds which are roughly comparable in energy. These are obtained by bridging across the two different CC bonds in C 60 . The interconversion pathway between the two isomers has been studied for C 60 O. The corresponding energy barrier is large suggesting that it should be possible to isolate both isomers.

Ab initio molecular orbital calculations on F+H2→HF+H and OH+H2→H2O+H using unrestricted Møller-Plesset perturbation theory with spin projection

Abstract The reactions of fluorine atom and hydroxyl radical with molecular hydrogen have been studied by molecular orbital methods using the 6-311G (d, p) and 6-311 + + G (2df, 2pd) basis sets with the unrestricted Hartree-Fock approach and with Moller-Plesset perturbation theory up to fourth order, with and without spin projection. The positions of transition states were optimized with a grid search at the MP n and PMP n levels using the 6-311G(d, p) basis. The projected MP2, MP3 and MP4 barriers are 1–1.5 kcal/mol lower and 0.03–0.05 A closer to the reactants than the unprojected calculations. At the PMP4SDTQ/ 6-311 + + G(2df, 2pd) level, the classical barriers are 2.6 kcal/mol for F+H 2 and 5.7 kcal/mol for OH+H 2 .

Selection of the reduced virtual space for correlated calculations. An application to the energy and dipole moment of H2O

Abstract Many-body perturbation theory and coupled-cluster calculations for the energy and dipole moment of H 2 O are performed using different techniques to reduce the virtual space. The optimized virtual orbital space (OVOS) method is compared with frozen natural orbitals (FNO), defined from a relaxed density matrix, and just SCF orbitals to generate an optimized reduced virtual space. It is shown that OVOS as well as FNO orbitals recover more than 90% of the correlated energy even when the virtual space has been reduced by almost 60%. In the full occupied space, the OVOS method is slightly preferable to FNO results in intermediate sized virtual spaces. By eliminating core correlations effectively perfect agreement is found between the FNO orbitals and OVOS.

Peptide and protein folding

Abstract Ab initio peptide folding, and its role in the reductionistic approach towards the understanding of protein folding are discussed from the points of view of past, present and possible future developments. It is believed that after the initial holistic approach, we are now at a new epoch, which will be dominated by reductionism. New quantitative mathematical models will be the result of the reductionistic approach that will lead toward a new, more sophisticated holistic era.

Peptide models XXIV: An ab initio study on N-formyl-L-prolinamide with trans peptide bond. The existence or non-existence of αL and ϵL conformations

Abstract N-formyl-L-prolinamide was subjected to geometry optimization at three levels of theory: HF/3-21G, HF/6-31G (d) and B3LYP/6-31G (d). At all three levels of computation the global minimum was γ L (inverse γ -Turn) backbone conformation with two ring-puckered forms “UP” and “DOWN”. At HF/3-21G level of theory three backbone conformations were found γ L , ϵ L , and α L . At higher levels of theory the ϵ L , and α L conformations disappeared. The “UP” and “DOWN” ring-puckered forms, in the γ L backbone conformation, led to practically identical vibrational spectra at the B3LYP/6-31G (d) level of theory.

Evidence of the existence of dissociated water molecules in water clusters

The dissociation of water molecules in the water cluster was investigated using ab initio methods and density functional theory. A stable minimum energy configuration of a cluster containing H3O+ and OH− ions was located for a water cluster with five water molecules, (H2O)5. There are six hydrogen bonds in the dissociated water cluster to form the minimum energy structure. A similar structure with H3O+ and OH− ions was also found for a (H2O)8 cluster.