Jorge Garza

Strength of the NH···OC and CH···OC Bonds in Formamide andN-Methylacetamide Dimers

The structures of the dimers of formamide and N-methylacetamide have been calculated at the ab initio electronic structure theory level, 2nd order Miller-Plesset perturbation theory (MP2) with augmented correlation consistent basis sets. Five unique structures were optimized for the formamide dimers at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels. At the optimized geometries obtained with the aug-cc-pVTZ basis set, MP2 energies were evaluated with the aug-cc-pVQZ basis set allowing an extrapolation of the energies to the complete basis set limit. Four structures were found for the N-methylacetamide dimer at the MP2/aug-cc-pVDZ level and single point energies were calculated at the MP2/aug-cc-pVTZ level. In both systems, the basis set superposition error (BSSE) was estimated with the counterpoise method. The strength of the N-HiiiO=C bond has a mean value of 7.1 kcal/mol in the formamide dimers and a mean value of 8.6 kcal/mol in the N-methylacetamide dimers. The difference in hydrogen bond strengths is attributed to differences in basicity at the carbonyl oxygen receptor site. In several dimers C-HiiiO=C hydrogen bonds play an important role in stabilizing these intermolecular complexes increasing the interaction energy by 1.1 to 2.6 kcal/mol per interaction.

The optimized effective potential and the self-interaction correction in density functional theory: Application to molecules

The Krieger, Li, and Iafrate approximation to the optimized effective potential including the self-interaction correction for density functional theory has been implemented in a molecular code, NWChem, that uses Gaussian functions to represent the Kohn and Sham spin–orbitals. The differences between the implementation of the self-interaction correction in codes where planewaves are used with an optimized effective potential are discussed. The importance of the localization of the spin–orbitals to maximize the exchange-correlation of the self-interaction correction is discussed. We carried out exchange-only calculations to compare the results obtained with these approximations, and those obtained with the local spin density approximation, the generalized gradient approximation and Hartree–Fock theory. Interesting results for the energy difference (GAP) between the highest occupied molecular orbital, HOMO, and the lowest unoccupied molecular orbital, LUMO, (spin–orbital energies of closed shell atoms and mo...

Confined helium atom low-lying S states analyzed through correlated Hylleraas wave functions and the Kohn-Sham model

Calculation including the electron correlation effects is reported for the ground 1 1S and lowest triplet 1 3S state energies of the confined helium atom placed at the center of an impenetrable spherical box. While the adopted wave-functional treatment involves optimization of three nonlinear parameters and 10, 20, and 40 linear coefficients contained in wave functions expressed in a generalized Hylleraas basis set that explicitly incorporates the interelectronic distance r12, via a Slater-type exponent and through polynomial terms entering the expansion, the Kohn-Sham model employed here uses the Perdew and Wang exchange-correlation functional in its spin-polarized version within the local-density approximation (LDA) with and without the self-interaction correction. All these calculations predict a systematic increase in the singlet-triplet energy splitting toward the high confinement regime, i.e., when the box radius is reduced. By using the variational results as benchmark, it is found that the LDA underestimates the singlet-triplet energy splitting, whereas the self-interaction correction overestimates such a quantity.

Orbital energy analysis with respect to LDA and self-interaction corrected exchange-only potentials

The self-interaction correction of Perdew and Zunger with the optimized effective potential using the Krieger–Li–Iafrate approximation is analyzed for atomic and molecular systems in the exchange-only context. Including the self-interaction correction (SIC) orbital by orbital shows that the appropriate asymptotic behavior of the exchange potential can be achieved if just the contribution of the highest occupied molecular orbital (HOMO) is considered. However, if a good description of the exchange potential in the valence region is required, and consequently a good description of the HOMO energy, then all electrons of the valence shell must be taken into account. In contrast, the lowest unoccupied molecular orbital (LUMO) is described adequately if just the HOMO SIC contribution is employed. In addition, if the lowest occupied orbital is also considered in the SIC approximation, there is an improvement in the description of the exchange potential in inner regions of an atom. When all electrons in an atom o...

Mean excitation energy, static polarizability, and hyperpolarizability of the spherically confined hydrogen atom

Calculations of mean excitation energy, Im, static polarizability, α, and hyperpolarizability, γ, using the variation perturbation procedure are reported for the spherically confined hydrogen atom. The electric response properties α and γ have been found to strongly depend upon the radius of confinement. The hyperpolarizabilty changes sign and becomes negative under strong confinement.

The role of the local-multiplicative Kohn–Sham potential on the description of occupied and unoccupied orbitals

The optimum local-multiplicative exchange potential was found using as input the Hartree–Fock electron density, for the molecular systems: H2, LiH, HF, NH3, CH4, H2O, N2, CO, F2, C2H2 and C2H4. The Zhao and Parr method was used to obtain the local-multiplicative potential where the kinetic energy is minimized using a constrained-search formulation of density functional theory. Two orbital sets were compared, those obtained with the nonlocal Hartree–Fock potential and those obtained with the local-multiplicative potential, both sets yielding the same electron density. As expected, the highest occupied molecular orbital (HOMO) energy was similar in both orbital sets. In contrast, the virtual orbital energies, and in particular the lowest unoccupied molecular orbital (LUMO), exhibited considerable differences. The Hartree–Fock LUMO energy goes to zero in a complete basis set limit and to nearly zero with reasonably large basis sets (e.g., augmented triple zeta) with sufficient diffuse functions added. The LU...

DFT reactivity indices in confined many-electron atoms

The density functional descriptors of chemical reactivity given by electronegativity, global hardness and softness are reported for a representative set of spherically confined atoms of IA, IIA, VA and VIIIA series in the periodic table. The atomic electrons are confined within the impenetrable spherical cavity defined by a given radius of confinement satisfying the Dirichlet boundary condition such that the electron density vanishes at the radius of confinement. With this boundary condition the non-relativistic spin-polarized Kohn-Sham equations were solved. The electronegativity in a confined atom is found to decrease as the radius of confinement is reduced suggesting that relative to the free state the atom loses its capacity to attract electrons under confined conditions. While the global hardness of a confined atom increases as the radius of confinement decreases, due to the accompanying orbital energy level crossing, it does not increase infinitely. At a certain confinement radius, the atomic global hardness is even reduced due to such crossover. General trends of the atomic softness parameter under spherically confined conditions are reported and discussed.

Static dipole polarizability of shell-confined hydrogen atom

Using the Sternheimer perturbation-numerical procedure, calculations of static dipole polarizability are reported for the shell-confined hydrogen atom as defined by two impenetrable concentric spherical walls. Unusually high polarizability states are predicted for the hydrogen atom as the inner sphere radius is increased to larger values inside the outer sphere of a constant radius. Implications of this model in mimicking internal compression leading to the metallic behaviour of the shell-confined hydrogen atoms are discussed.

Shell structure in free and confined atoms using the density functional theory

Abstract The average local electrostatic potential function, defined as the electrostatic potential divided by the electron density, is used to study the shell structure in free and confined atoms within Kohn–Sham density functional theory. Several exchange-correlation functionals have been used to calculate the average potential function. It was observed that the self-interaction correction significantly alters the shell structure along the large radial distances. Many electron atoms confined in a sphere exhibit a gradual loss of the shell structure as the confinement is increased. The loss of structure can be characterized by the sphere radius r c and in the limit r c →0, the electron gas behavior is obtained.

The association of neutral systems linked by hydrogen bond interactions: a quantitative electrochemical approach

The electrochemical characterization of the neutral–neutral association by hydrogen bonds was performed on the basis of voltammetric current measurements. The diffusion coefficient of the electroactive compound is modified by effect of association, this provoke important variations in the voltammetric current peak. As an example of the weak hydrogen bond between neutral complexes, it was determined that 1,4-benzoquinone (Q) and benzoic acid (HBz) can associate with a 1:1 stoichiometry with a conditional association constant between 10 and 15 M � 1 . The Q(HBz) complex geometry was optimized using density functional theory and Moller–Plesset perturbation theory. In both theories, the most stable geometry is flat and exhibits two hydrogen bond interactions: O–H ��� O and C–H ��� O interactions. The binding energy at our best level of theory was )7.7 kcal/mol, that supports the stability of the 1:1 Q–HBz complex and which is accord with the values of the conditional association constant obtained from the electrochemical method here described. 2002 Elsevier Science B.V. All rights reserved.