Abstract: The (ϕ, ψ) energy surface of blocked alanine (N-acetyl–N′-methyl alanineamide) was calculated at the Hartree-Fock (HF)/6-31G* level using ab initio molecular orbital theory. A collection of six electrostatic models was constructed, and the term electrostatic model was used to refer to (1) a set of atomic charge densities, each unable to deform with conformation; and (2) a rule for estimating the electrostatic interaction energy between a pair of atomic charge densities. In addition to two partial charge and three multipole electrostatic models, this collection includes one extremely detailed model, which we refer to as nonspherical CPK. For each of these six electrostatic models, parameters—in the form of partial charges, atomic multipoles, or generalized atomic densities—were calculated from the HF/6-31G* wave functions whose energies define the ab initio energy surface. This calculation of parameters was complicated by a problem that was found to originate from the locking in of a set of atomic charge densities, each of which contains a small polarization-induced deformation from its idealized unpolarized state. It was observed that the collective contribution of these small polarization-induced deformations to electrostatic energy differences between conformations can become large relative to ab initio energy differences between conformations. For each of the six electrostatic models, this contribution was reduced by an averaging of atomic charge densities (or electrostatic energy surfaces) over a large collection of conformations. The ab initio energy surface was used as a target with respect to which relative accuracies were determined for the six electrostatic models. A collection of 42 more complete molecular mechanics models was created by combining each of our six electrostatic models with a collection of seven models of repulsion + dispersion + intrinsic torsional energy, chosen to provide a representative sample of functional forms and parameter sets. A measure of distance was defined between model and ab initio energy surfaces; and distances were calculated for each of our 42 molecular mechanics models. For most of our 12 standard molecular mechanics models, the average error between model and ab initio energy surfaces is greater than 1.5 kcal/mol. This error is decreased by (1) careful treatment of the nonspherical nature of atomic charge densities, and (2) accurate representation of electrostatic interaction energies of types 1—2 and 1—3. This result suggests an electrostatic origin for at least part of the error between standard model and ab initio energy surfaces. Given the range of functional forms that is used by the current generation of protein potential functions, these errors cannot be corrected by compensating for errors in other energy components. © 1995 by John Wiley & Sons, Inc.

Abstract: The effective surface charge obtained by fitting the Poisson-Boltzmann approximation to measured data is related to the actual surface charge. Evaluation of the formally exact expression, which depends upon the wall-ion direct correlation function, gives a precise measure of the amount of ion condensation in the planar double layer. Results are presented for 0.001, 0.01, and 0.1 M monovalent and divalent restricted primitive model electrolytes using the singlet hypernetted chain closure with the first bridge diagram. In general, the apparent surface charge is less than the actual surface charge, due to the influence of ion-ion correlations. Thus, the Poisson-Boltzmang approximation underestimates the actual surface charge and overestimates the amount of counterion binding. At high surface charge densities and apparent surface charge saturates, and there is a maximum surface charge density that a given electrolyte will appear to support. At surface charge densities beyond this and high concentrations (e.g. 0.1 M divalent, 75 %1* per unit surface charge), charge reversal occurs, due to overscreening by the counterions in the first layer adjacent to the surface.

Abstract: A potential energy function is developed to represent the interaction of small monovalent cations, Li+, Na+, and K+, with the backbone of polypeptides. The results are based on ab initio calculations up to the 6-31G* level of the interactions of the ions with acetamide and N-methylacetamide. Basis set superposition errors are corrected with the counterpoise method. A systematic overestimate of the bond polarities is taken into account by an empirical scaling procedure that uses the ratio of the experimental to ab initio dipole moment. The calculated binding energies obtained with this procedure show consistent convergence with different basis sets and are in good agreement with experimental data on cation–water and cation–dimethylformamide systems. Investigations of the calculated ab initio potential energy surface indicate that the cation–peptide interaction is dominated by electrostatics and includes a nonnegligible contribution from polarization of the peptide group by the ion. The induced polarization results in a steeper-than-Coulombic interaction and cannot be described by fixed ion–peptide partial charges electrostatics. Atomic polarizabilities located on the atoms of the ligand molecule are introduced to account for the induced polarization in the empirical energy function. A ∼1/r4 attractive interaction appears in the potential function. The resulting radial and angular dependence of the potential energy surface is well reproduced. © 1995 by John Wiley & Sons, Inc.

Abstract: Molecular mechanics parameters for the homologous series of fluorine-substituted methanes, CH{sub x}F{sub 4-o} are derived from ab initio Hartree-Fock surfaces. Ab initio intermolecular potential energy surfaces are calculated using a 6-31+G basis set and include correlation using second-order Moller-Plesset perturbation theory. A least squares fit of the ab initio surface to a standard molecular mechanics potential function, including Lennard-Jones interactions plus partial charges, is then performed. The thermodynamic properties of the resulting molecular mechanics potential are calculated using conventional molecular dynamics simulations and compared to experimental results. Additional fine-tuning of the molecular mechanics potential is then performed to optimize the agreement with experimental results. The effect of including high-energy configurations in the fit is systematically investigated. 28 refs., 12 figs., 14 tabs.

Abstract: Supramolecular systems in which photoinduced charge separation is accomplished by a sequence of electron transfer steps can be highly efficient in terms of quantum yield and lifetime of the long-range charge separated Optimal design of such systems requires precise knowledge of the factors determining the kinetics and thermodynamics of the individual steps. Our studies are aimed at obtaining such precise data from experiments using simple organic molecules in which two electron donors and a photoexcitable electron acceptor are linked together in a linear arrangement. The excited state charge separation occurs in two steps: D~-D,-A* (LE)

Abstract: We report a molecular dynamics model of the monomeric liquid dimethyl ether. The united atom approach is used to treat CH{sub 3} groups as point source centers. Partial charges are derived from the experimental dipole moment. Harmonic force constants are used for intramolecular interactions, and their values are so chosen that the model`s fundamental frequencies agree with experimental results. Because we are interested in solvation properties, the model contains flexible molecules, allowing molecular distortion and internal dynamical quantities. We report radial distribution functions and the static structure factors as well as some dynamical quantities such as the dynamical structure factor, infrared absorption, and Raman scattering spectra. Calculated results agree reasonably well with experimental and other simulation results. 25 refs., 8 figs., 1 tab.

Abstract: We have studied a mechanism for the stabilization of monolayer graphite on Si-terminated SiC(111) through a charge transfer which occurs between them by use of the first-principles molecular-orbital cluster method. The charge-transfer features change with atom configuration. At the commensurate site, the charge transfer hardly takes place, whereas at its neighbors a relatively large electronic charge transfers from the substrate to the overlayer. At the onset of the charge transfer an s orbital plays a crucial role such that the incommensurately (commensurately) sited carbon atom has a larger (smaller) s orbital component to give rise to larger (smaller) electronegativity.

Abstract: Atom-centered partial charges are calculated for the Fe-heme in cytochrome P45Ocam for use in molecular dynamics simulations of polar substrates bound in the active site of the enzyme. Charges are fit to the electrostatic potential produced by ab initio UHF wavefunctions for an Feporphine model. Basis set dependence of these charges is observed using the LANL1DZ, LANL2DZ and augmented 6-3IG levels of theory. Upon geometry optimization of the enzyme, these charge sets cause varying degrees of distortion of the porphyrin from its crystallographically observed conformation. Scaling the charges calculated from the augmented 6-31G basis by 75% reduces the heme distortion while preserving reasonable interactions with a polar substrate. A comparison of the calculated charges with other published values is presented.

Abstract: The influence of discrepancies between analytical expressions for charge changing cross section on the ionization state of swift heavy ions interacting with hot and dense plasmas is analyzed within the framework of our new average correlated hydrogenic atom model. Making use of our Classical Trajectory Monte-Carlo results, we show that the partial charge transfer cross section into the projectile atomic levels has the same importance as the total charge transfer cross section.

Abstract: A model is proposed for the elementary Coulomb interaction and a formula is given for the Coulomb interaction energy of elementary charges. It is postulated that the electron charge is elementary and that a free charge cannot be a fraction of the electron charge. An explanation is given for the mass of charged leptons and some hadrons.

Abstract: A theoretical treatment of the B3+ + He charge exchange process is performed by means of a semi-classical formalism using ab initio potential energy curves and couplings. A good agreement with experiment is obtained for the partial and total capture cross-sections. The orientation effects are examined.

Abstract: Atom-centered partial charges are calculated for the Fe-heme in cytochrome P450cam for use in molecular dynamics simulations of polar substrates bound in the active site of the enzyme. Charges are fit to the electrostatic potential produced by ab initio UHF wavefunctions for an Fe-porphine model. Basis set dependence of these charges is observed using the LANL1DZ, LANL2DZ and augmented 6–31G levels of theory. Upon geometry optimization of the enzyme, these charge sets cause varying degrees of distortion of the porphyrin from its crystallographically observed conformation. Scaling the charges calculated from the augmented 6–31G basis by 75% reduces the heme distortion while preserving reasonable interactions with a polar substrate. A comparison of the calculated charges with other published values is presented.

Abstract: X-ray absorption Ni K-edge spectra are recorded for the following systems: Ni metal, Ni(NO3)2 • 6H20, Ni3(PO4)2•6H20, NiSO4 • 6H 2 0, NiCO3, NiO and La2NiO4 using a Cauchois-type bent crystal spectrograph. The chemical shift of these systems is correlated with the partial charge determined using Sanderson's method. On the basis of regression analysis a relation ΔE = A0 + Ai q + A2 q2 -±3q3 + A4 q4 between the chemical shift, DE, and the partial charge, q, has been suggested. The discrepancy in the shift of NiO and La2NiO4 has also been discussed.

Abstract: The charge density induced by a point charge immersed in an electron gas, in the presence of a magnetic field is studied using linear response theory, for a range of metallic densities and fields up to 4*105 T. The response function, inclusive of an approximate field factor for exchange and correlation effects, is first presented in the current density functional theory formalism. Numerical methods are then described. Results for the embedding energy of the point charge and non-spherical deformation of the charge density are reported. They are discussed in the perspective of building a statistical model of electronic structure in the presence of strong magnetic fields.

Abstract: Ab initio molecular orbital calculations have been employed to study the rearrangement of (α-methylazo)alkyl isocyanates to 1,2,5-trisubstituted 1,2-dihydro-1,2,4-triazol-3-ones. The migration of three different substituents (methyl, ethyl and isopropyl) has been investigated. Geometries of stationary points on the potential energy hypersurface were optimized at the HF/6-31G* level of theory. Second order Moeller Plesset perturbation theory with the 6-31G* basis set was applied in order to correct for correlation effects. Selected geometries were reoptimized at the MP2/6-31G* level. HF/6-31G*, MP2/6-31G*//HF/6-31G* and MP2/6-31G* energies predict migratory aptitudes in the order isopropyl > ethyl > methyl. In the transition states, partial charges obtained from natural population analysis indicate strong electron deficiency at the nitrogen atom being the target for migration. The transition states show partial carbocation character of the migrating group with respect to charge distribution and geometry. An alternative reaction pathway, namely 1,2-shift of a methyl group to the adjacent nitrogen of the isocyanato function leading to formation of 2,4,5-trimethyl-2,4-dihydro-1,2,4-triazol-3-one, has been investigated, but can be excluded due to the much higher activation energy required.

Abstract: According to Gimarc's principle of topological chargestabilization, heteroatomic molecules are topologically stabilized when more electronegative atoms are placed in those positions where atom-atom connectivity and the electron-filling level provide the highest electron charge in the reference hydrocarbon frame. Recently, we showed that the relative atomic moments of energy (the frequencies of atomic self-returning walks) in such uniform molecular skeletons are equal to the respective squared principal eigenvector coefficients. We show here that for conjugated heteroryclic molecules these partial atomic charges follow closely the patterns mirrored by topological charge stabilization and, by producing a nonuniform charge distribution in alternant molecules, enable the broader application of this principle to such molecules.