Optimized Gaussian basis sets for use with relativistic effective (core) potentials: K,Ca, Ga—Kr

Correlation-consistent valence basis sets were developed for the third-row main block elements (K, Ca, Ga—Kr) for use with relativistic effective core potentials. These basis sets are somewhat larger than double-zeta in size, with polarization functions, and are balanced for use in both Hartree–Fock and correlation calculations. Spin–orbit splittings for atoms and molecules are calculated and compared to experiment. These calculations use the approximate spin–orbit operator from the relativistic effective core potentials. The use of these results in the calculation of accurate thermochemical data is discussed. © 1997 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Complex interactions between molecular ions in solution and their effect on protein stability

Protein stability in ionic solutions depends on the delicate balance between protein-ion and ion-ion interactions. For molecular ions containing multiple charged groups, the role of ion-ion interactions is particularly important. In this study, we show how the interplay between homo- and heteroion pairing influences protein stability using polyarginine salts as a model system. For the chloride salts, protein thermostability decreases as the size of the peptide increases, indicating enhanced binding to the protein. Moreover, it indicates reduced homoion pairing between Gdm(+) and carboxylate groups that is largely responsible for aggregation suppression, rather than denaturation, in monomeric arginine solutions. However, for the sulfate salts, strong heteroion pairing between the Gdm(+) groups and the sulfate counterions compensates for the loss of homoion pairing and, in return, leads to enhanced thermostability and a dramatically reduced (up to 10-30 times) rate of protein aggregation. Molecular dynamics simulations reveal how this ion pairing enhances conformational stability and, at the same time, reduces protein association. This study provides insight into complex ion effects on protein stability and serves as an example of how these intrasolvent interactions can be leveraged to enhance protein stability.     

Self consistent tight binding model for dissociable water

We report results of development of a self consistent tight binding model for water. The model explicitly describes the electrons of the liquid self consistently, allows dissociation of the water and permits fast direct dynamics molecular dynamics calculations of the fluid properties. It is parameterized by fitting to first principles calculations on water monomers, dimers, and trimers. We report calculated radial distribution functions of the bulk liquid, a phase diagram and structure of solvated protons within the model as well as ac conductivity of a system of 96 water molecules of which one is dissociated. Structural properties and the phase diagram are in good agreement with experiment and first principles calculations. The estimated DC conductivity of a computational sample containing a dissociated water molecule was an order of magnitude larger than that reported from experiment though the calculated ratio of proton to hydroxyl contributions to the conductivity is very close to the experimental value. The conductivity results suggest a Grotthuss-like mechanism for the proton component of the conductivity.     

On regular bipartite-preserving supergraphs

For a graphG with chromatic numberχ(G) ? 2 and maximum degree Δ(G), there exists anr-regular graphH, for everyr ? Δ(G), such thatG is an induced subgraph ofH and_χ(H) =_χ (G). In the case whereG is bipartite, the minimum order of such a graphH is determined.     

Theoretical investigation into the nature of the second vertical ionic state of the water dimer

Ab initio molecular orbital calculations have been carried out to resolve questions concerning the location of the second ionization in the water dimer. The results indicate that it is from the proton acceptor molecule of the water dimer.     

Molecular orbital studies of Al2F6 and Al2Cl6 using a minimal basis set

Ab initio minimal basis LCAO-SCF molecular orbital (MO ) calculations have been carried out on various structures of the AIF₃ and AlCl₃ dimers. A bridged D_2h structure is found to be the most stable of the structures investigated for Al₂F₆. A similar structure was obtained for Al₂Cl₆ in agreement with the experimental evidence.     

Effect of atomic layer deposition coatings on the surface structure of anodic aluminum oxide membranes

Anodic aluminum oxide (AAO) membranes were characterized by UV Raman and FT-IR spectroscopies before and after coating the entire surface (including the interior pore walls) of the AAO membranes by atomic layer deposition (ALD). UV Raman reveals the presence of aluminum oxalate in bulk AAO, both before and after ALD coating with Al2O3, because of acid anion incorporation during the anodization process used to produce AAO membranes. The aluminum oxalate in AAO exhibits remarkable thermal stability, not totally decomposing in air until exposed to a temperature >900 degrees C. ALD was used to cover the surface of AAO with either Al2O3 or TiO2. Uncoated AAO have FT-IR spectra with two separate types of OH stretches that can be assigned to isolated OH groups and hydrogen-bonded surface OH groups, respectively. In contrast, AAO surfaces coated by ALD with Al2O3 display a single, broad band of hydrogen-bonded OH groups. AAO substrates coated with TiO2 show a more complicated behavior. UV Raman results show that very thin TiO2 coatings (1 nm) are not stable upon annealing to 500 degrees C. In contrast, thicker coatings can totally cover the contaminated alumina surface and are stable at temperatures in excess of 500 degrees C.     

Mechanisms of lithium transport in amorphous polyethylene oxide

We report calculations using a previously reported model of lithium perchlorate in polyethylene oxide in order to understand the mechanism of lithium transport in these systems. Using an algorithm suggested by Voter, we find results for the diffusion rate which are quite close to experimental values. By analysis of the individual events in which large lithium motions occur during short times, we find that no single type of rearrangement of the lithium environment characterizes these events. We estimate the free energies of the lithium ion as a function of position during these events by calculation of potentials of mean force and thus derive an approximate map of the free energy as a function of lithium position during these events. The results are consistent with a Marcus-like picture in which the system slowly climbs a free energy barrier dominated by rearrangement of the polymer around the lithium ions, after which the lithium moves very quickly to a new position. Reducing the torsion forces in the model causes the diffusion rates to increase.     

Theoretical studies of O2−:(H2O)n clusters

The interaction of superoxide ion O₂^? with up to four water molecules [O₂^?: (H₂O)_n, n = 1, 2, 4] has been investigated using ab initio molecular orbital theory. The binding energy of O₂^?: H₂O is calculated to be ?20.6 kcal/mol in good agreement with gas phase experimental data. At the MP3/6-31G^* level the O₂^?:H₂O complex has a C_2v structure with a double (cyclic) hydrogen bond between O₂^? and H₂O. A C_s structure with a single hydrogen bond is only 0.7 kcal/mol less stable. Interaction of H₂O with the doubly occupied π^* orbital of O₂^? is preferred slightly over interaction with the singly occupied π^* orbital. Natural bond orbital analysis suggests that both electrostatic and charge transfer interactions are important in anionic complexes. The charge transfer occurs predominantly in the O₂^? → H₂O direction and is important in determining the relative stabilities of the different structures and states. Singly and doubly hydrogen-bonded structures for the O₂^?: (H₂O)₂ and O₂^?: (H₂O)₄ clusters were found to be similar in stability and the increase in binding of the cluster becomes smaller as each additional water molecule is added to the cluster.     

An assessment of density functional methods for studying molecular adsorption in cluster models of zeolites

The performance of six density functional theory (DFT) methods has been tested for a zeolite cluster containing three tetrahedral atoms (3T) and the complexes it forms with water and methanol molecules. The DFT methods (BLYP, BP86, BPW91, B3LYP, B3P86, B3PW91) give results in good agreement with second-order perturbation theory (MP2). The results in this paper provide evidence of the suitability of DFT methods for studying hydrogen-bonded adsorption complexes in zeolites. Generally, the hybrid DFT methods are in closer agreement with experiment and MP2 than the pure DFT methods for geometrical parameters. The only exception is the Z⁻ geometry, perhaps due to its anionic character. All DFT methods give results in good overall agreement with MP2 for intramolecular geometrical parameters of the adsorption complexes, intramolecular vibrational frequencies, and adsorption energies. The B3LYP method gives intermolecular geometries and intermolecular vibrational frequencies which are closest to those obtained from the MP2 method. Thus, the B3LYP method seems to be the best choice for a density functional treatment of molecular adsorption in zeolite systems.