Index of content:
Volume 127, Issue 5, 07 August 2007
- Theoretical Methods and Algorithms
127(2007); http://dx.doi.org/10.1063/1.2759209View Description Hide Description
We report a plane wave-projector augmented wave implementation of the recently proposed exchange-only range separated hybrid (RSHX) density functional [Gerber and Ángyán, Chem. Phys. Lett.415, 100 (2005)] and characterize its performance in the local density approximation (RSHXLDA) for a set of archetypical solid state systems, as well as for some transition metal oxides. Lattice parameters, bulk moduli, band gaps, and magnetic moments of the transition metal oxides have been calculated at different values of the range separation parameter and compared with results obtained with standard local density approximation (LDA), gradient corrected (PBE), and hybrid (HSE) functionals. The RSHX functional, which has the main feature of providing a correct asymptotic behavior of the exchange potential, has a tendency to improve the description of structural parameters with respect to local and generalized gradient approximations. The band gaps are too strongly opened by the presence of the long-range Hartree-Fock exchange in all but wide-gap systems. In the difficult case of transition metal oxides, the gap is overestimated, while magnetic moments and lattice constants are slightly underestimated. The optimal range separation parameter has been found around , slightly lower than the value of , recommended earlier for molecular systems.
127(2007); http://dx.doi.org/10.1063/1.2751159View Description Hide Description
A solution to the long-standing problem of developing numerically stable optimized effective potential (OEP) methods based on Gaussian basis sets is presented by introducing an approach consisting of an exact exchange OEP method with an accompanying construction and balancing scheme for the involved auxiliary and orbital Gaussian basis sets that is numerically stable and that properly represents an exact exchange Kohn-Sham method. The method is a purely analytical method that does not require any numerical grid, scales like Hartree-Fock or B3LYP procedures, is straightforward to implement, and is easily generalized to take into account orbital-dependent density functionals other than the exact exchange considered in this work. Thus, the presented OEP approach opens the way to the development and application of novel orbital-dependent exchange-correlation functionals. It is shown that adequately taking into account the continuum part of the Kohn-Sham orbital spectrum is crucial for numerically stable Gaussian basis set OEP methods. Moreover, it is mandatory to employ orbital basis sets that are converged with respect to the used auxiliary basis representing the exchange potential. OEP calculations in the past often did not meet the latter requirement and therefore may have led to erroneously low total energies.
Linear-scaling method for calculating nuclear magnetic resonance chemical shifts using gauge-including atomic orbitals within Hartree-Fock and density-functional theory127(2007); http://dx.doi.org/10.1063/1.2749509View Description Hide Description
Details of a new density matrix-based formulation for calculating nuclear magnetic resonancechemical shifts at both Hartree-Fock and density functional theory levels are presented. For systems with a nonvanishing highest occupied molecular orbital–lowest unoccupied molecular orbital gap, the method allows us to reduce the asymptotic scaling order of the computational effort from cubic to linear, so that molecular systems with 1000 and more atoms can be tackled with today’s computers. The key feature is a reformulation of the coupled-perturbed self-consistent field (CPSCF) theory in terms of the one-particle density matrix (D-CPSCF), which avoids entirely the use of canonical MOs. By means of a direct solution for the required perturbed density matrices and the adaptation of linear-scaling integral contraction schemes, the overall scaling of the computational effort is reduced to linear. A particular focus of our formulation is to ensure numerical stability when sparse-algebra routines are used to obtain an overall linear-scaling behavior.
Rapid motion capture of mode-specific quantum wave packets selectively generated by phase-controlled optical pulses127(2007); http://dx.doi.org/10.1063/1.2753834View Description Hide Description
Rapid motion capture of phase-controlled wave packets was realized using a sensitive wave-packet spectrometer, which was previously developed by the present authors. Two-dimensional Fourier-transformed spectrograms obtained by the wave-packet spectrometer provide us full information about the wave-packet motion on both excited- and ground-statepotential surfaces. Vibrational wave packet associated with a twisting mode in a DTTCI molecule was observed to be dependent on the pulse chirp, and was generated in the excited state preferably with negatively chirped excitation. The result indicates that the excited-statewave packet can be driven along a favorable configuration coordinate by using phase-controlled femtosecond pulses. The present method is essential to adaptive coherent-control application.
Quantum molecular dynamics of hydrogen bonded complexes of rigid molecules using the semiclassical initial value representation in Cartesian coordinates127(2007); http://dx.doi.org/10.1063/1.2755963View Description Hide Description
Semiclassical initial value representation calculations are performed for the constrained water dimer in Cartesian coordinates. The study represents the first application of a previously reported method [Issak and Roy, J. Chem. Phys.123, 084103 (2005); 126, 024111 (2007)] to a molecular cluster.Bound stateenergies are calculated for a dimer of rigid water molecules as well as its deuterated form . The results show that the approach fares well with respect to accuracy in capturing quantum effects in intermolecular interactions.
127(2007); http://dx.doi.org/10.1063/1.2759202View Description Hide Description
Density-functional theory requires ever better exchange-correlation (xc) functionals for the ever more precise description of many-body effects on electronic structure. Universal constraints on the xc energy are important ingredients in the construction of improved functionals. Here we investigate one such universal property of xc functionals: the Lieb-Oxford lower bound on the exchange-correlation energy, , where . To this end, we perform a survey of available exact or near-exact data on xc energies of atoms, ions, molecules, solids, and some model Hamiltonians (the electron liquid, Hooke’s atom, and the Hubbard model). All physically realistic density distributions investigated are consistent with the tighter limit . For large classes of systems one can obtain class-specific (but not fully universal) similar bounds. The Lieb-Oxford bound with is a key ingredient in the construction of modern xc functionals, and a substantial change in the prefactor will have consequences for the performance of these functionals.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
127(2007); http://dx.doi.org/10.1063/1.2752171View Description Hide Description
Aerosol nucleation events have been observed at a variety of locations worldwide, and may have significant climatic and health implications. Binary homogeneous nucleation (BHN) of and is the foundation of recently proposed nucleation mechanisms involving additional species such as ammonia, ions, and organic compounds, and it may dominate atmospheric nucleation under certain conditions. We have shown in previous work that BHN can be treated as a quasi-unary nucleation (QUN) process involving in equilibrium with vapor, and we have developed a self-consistent kinetic model for nucleation. Here, the QUN approach is improved, and an analytical expression yielding QUN rates is derived. Two independent measurements related to monomer hydration are used to constrain the equilibrium constants for this process, which reduces a major source of uncertainty. It is also shown that the capillarity approximation may lead to a large error in the calculated Gibbs free energy change for the evaporation of molecules from small clusters, which affects the accuracy of predicted BHN nucleation rates. The improved QUN model—taking into account the recently measured energetics of small clusters—is thermodynamically more robust. Moreover, predicted QUN nucleation rates are in better agreement with available experimental data than rates calculated using classical BHN theory.
Accuracy of recent potential energy surfaces for the interaction. I. Virial and bulk transport coefficients127(2007); http://dx.doi.org/10.1063/1.2753483View Description Hide Description
A new exchange-Coulomb semiempirical model potential energy surface for the interaction has been developed. Together with two recent high-level ab initiopotential energy surfaces, it has been tested for the reliability of its predictions of second-virial coefficients and bulk transport phenomena in binary mixtures of He and . The agreement with the relevant available measurements is generally within experimental uncertainty for the exchange-Coulomb surface and the ab initiosurface of Patel et al. [J. Chem. Phys.119, 909 (2003)], but with slightly poorer agreement for the earlier ab initiosurface of Hu and Thakkar [J. Chem. Phys.104, 2541 (1996)].
127(2007); http://dx.doi.org/10.1063/1.2752803View Description Hide Description
The diatomic molecules SiPb and GePb were for the first time identified by producing high temperature vapors of the constituent pure elements in a “double-oven-like” molecular-effusion assembly. The partial pressures of the atomic, heteronuclear, and homonuclear gaseous species observed in the vapor, namely, Si, Ge,Pb, SiPb, GePb, , , and , were mass-spectrometrically measured in the overall temperature ranges (Ge–Pb) and (Si–Pb). The dissociation energies of the new species were determined by second- and third-law analyses of both the direct dissociationreactions and isomolecular exchange reactions involving homonuclear molecules. The selected values of the dissociation energies at are and , respectively, for SiPb and GePb, and the corresponding enthalpies of formation are and . The ionization efficiency curves of the two species were measured, giving the following values for the first ionizationenergies: (SiPb) and (GePb). A computational study of the species SiPb and GePb was also carried out at the CCSD(T) level of theory using the relativistic electron core potential approach. Molecular parameters, adiabatic ionizationenergies, adiabatic electron affinities, and dissociation energies of the title species were calculated, as well as the enthalpy changes of the exchange reactions involving the other Pb-containing diatomics of group 14. Finally, a comparison between the experimental and theoretical results is presented, and from a semiempirical correlation the unknown dissociation energies of the SiSn and PbC molecules are predicted as and , respectively.
127(2007); http://dx.doi.org/10.1063/1.2751499View Description Hide Description
The collision-induced reaction of with HCl has been studied by use of classical dynamics procedures at collision energies using empirical potential parameters. The principal reaction pathway on the potential energy surface is the formation of with the maximum reaction cross section,, occurring at . At lower energies, the cross section for the charge transfer process is comparable to that for formation, but at higher energies, it is larger by a factor of 2. The cross section of the formation is an order of magnitude smaller than that of . For both and formations, the reaction threshold is . The formation takes place immediately following the turning point in a direct-mode mechanism, whereas an indirect-mode mechanism operates in the formation of . Both and formations come mainly from the perpendicular configuration, , at the turning point. Product vibrational excitation is found to be strong in both and .
127(2007); http://dx.doi.org/10.1063/1.2756534View Description Hide Description
The - and -type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-, ortho-, HD–OCS, para-, and ortho-, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum, regardless of whether the hydrogen rotational angular momentum is as in para-, ortho-, and HD or as in ortho- and para-. This indicates quenching of the hydrogen angular momentum for the ortho- and para- species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two species. The species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para- relative to ortho- increases exponentially with decreasing normal component in gas mixtures, making the observation of para- in the presence of the more strongly bound ortho- dependent on using lower concentrations of . The determined rotational constants are , , and for para-; , , and for ortho-; , , and for HD–OCS; , , and for ortho-; and , , and for para-.
127(2007); http://dx.doi.org/10.1063/1.2756538View Description Hide Description
The reaction of propionyl radical with oxygen has been studied using the full coupled clustertheory with the complete basis set. This is the first time to gain a conclusive insight into the reaction mechanism and kinetics for this important reaction in detail. The reaction takes place via a chemical activation mechanism. The barrierless association of propionyl with oxygen produces the propionylperoxy radical, which decomposes to form the hydroxyl radical and the three-center -lactone predominantly or the four-center -propiolactone. The oxidation of propionyl radical to carbon monoxide or carbon dioxide is not straightforward rather via the secondary decomposition of -lactone and -propiolactone. Kinetically, the overall rate constant is almost pressure independent and it approaches the high-pressure limit around tens of torr of helium. At temperatures below , the rate constant shows negative temperature dependence. The experimental yields of the hydroxyl radical can be well reproduced, with the average energy transferred per collision at 213 and (helium bath gas). At low pressures, together with the hydroxy radical, -lactone is the major product, while -propiolactone only accounts for about one-fifth of -lactone. At the high-pressure limit, the production of the propionylperoxy radical is dominant together with a fraction of the isomers. The infrared spectroscopy or the mass spectroscopy techniques are suggested to be employed in the future experimental study of the reaction.
127(2007); http://dx.doi.org/10.1063/1.2759214View Description Hide Description
In this work, we have extended our previous high resolution study of the vacuum ultraviolet emission spectrum of the molecule [M. Roudjane, et al.J. Chem. Phys.125, 214305 (2006)] up to in order to investigate the band system. The analysis of the spectrum has been carried out by means of a complex spectrum visual identification code IDEN [V. I. Azarov, Phys. Scr.44, 528 (1991);48, 656, (1993)] and supported by theoretical calculations using ab initio data [L. Wolniewicz, J. Chem. Phys.103, 1792 (1995);99, 1851 (1993);G. Staszewska and L. Wolniewicz, J. Mol. Spectrosc.212, 208 (2002);L. Wolniewicz and G. Staszewska, 220, 45 (2003)] which provided level energies and transition probabilities. More than 1480 new emission lines have been observed and 109 bands belonging to the system have been identified between 84.1 and . Except for the bands that were reported in absorption [I. Dabrowski and G. Herzberg, Can. J. Phys.52, 1110 (1974)], all the bands are reported here for the first time. The analysis led to the determination of 111 rovibronic energy levels in the state, of which 31 with higher rotational numbers are new. Observed perturbations are accounted for through a set of coupled equations involving the four excited electronic states, , , and and including nonadiabatic couplings. The solution of this set provides the percent contribution of these four states to each of the observed rovibronic level.
Density functional analysis of the structural evolution of clusters and its influence on the melting characteristics127(2007); http://dx.doi.org/10.1063/1.2759215View Description Hide Description
Recent experimental results have reported surprising variations in the shapes of the heat capacity curves and melting temperatures of galliumclusters in the size range of 30–55 atoms [G. A. Breaux et al., J. Am. Chem. Soc.126, 8628 (2004)]. In the present work, we have carried out an extensive density functional investigation on ten selected clusters in the above mentioned size range. In particular, we have analyzed the ground state geometry and the nature of bonding in these clusters using electron localization function. We demonstrate that the existence or otherwise of a large island of atoms bonded with similar strength (i.e., the local order) in the ground state geometry is responsible for the variation in the shape of the heat capacity curve. We attribute the observed higher melting temperatures of some of the clusters (viz., ) to the presence of a distinct core and strong covalent bonds between the core and surface atoms. The present work clearly demonstrates that it is possible to understand the general trends observed in the heat capacity curves across the entire series on the basis of the analysis of their ground state.
Unraveling the structure of hydrogen bond stretching mode infrared absorption bands: An anharmonic density functional theory study on 7-azaindole dimers127(2007); http://dx.doi.org/10.1063/1.2759213View Description Hide Description
The structure of the linear infrared absorptionspectrum of the N–H stretching mode in 7-azaindole dimers is analyzed by quartic anharmonic vibrational force field calculations based on density functional theory. It is demonstrated that a multiple Fermi resonancemodel including contributions from 12 fingerprint vibrational modes, most of them containing considerable contributions of N–H bending motions, combined with a single low-frequency mode satisfactorily explains the complex line shape of N–H stretching mode absorption band.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
127(2007); http://dx.doi.org/10.1063/1.2752155View Description Hide Description
The phase diagram for a system of model anisotropic particles with six attractive patches in an octahedral arrangement has been computed. This model for a relatively narrow value of the patch width where the lowest-energy configuration of the system is a simple cubic crystal. At this value of the patch width, there is no stable vapor-liquid phase separation, and there are three other crystalline phases in addition to the simple cubic crystal that is most stable at low pressure. First, at moderate pressures, it is more favorable to form a body-centered-cubic crystal, which can be viewed as two interpenetrating, and almost noninteracting, simple cubic lattices. Second, at high pressures and low temperatures, an orientationally ordered face-centered-cubic structure becomes favorable. Finally, at high temperatures a face-centered-cubic plastic crystal is the most stable solid phase.
127(2007); http://dx.doi.org/10.1063/1.2754266View Description Hide Description
A mode coupling theory (MCT) expression for the self-diffusion coefficient follows simply when the soft fluctuating intermolecular forces are projected along a collective densitylike variable. The projected forces separate into two parts: from the gradient of the direct correlation function (dcf), and from the short range forces. The time correlation function of the dcf-derived forces is related to the excess entropy, as shown by Ali [J. Chem. Phys.124, 144504 (2006)], and this relationship is evaluated for two variations of MCT. As for hard spheres, the derivation of an analogous MCT is beset by a number of singularities that kinetic theory could not remove. A justifiable MCT for hard sphere fluids may not exist.
127(2007); http://dx.doi.org/10.1063/1.2756838View Description Hide Description
Reversible diffusion limited cluster aggregation of hard spheres with rigid bonds was simulated and the self-diffusion coefficient was determined for equilibrated systems. The effect of increasing attraction strength was determined for systems at different volume fractions and different interaction ranges. It was found that the slowing down of the diffusion coefficient due to crowding is decoupled from that due to cluster formation. The diffusion coefficient could be calculated from the cluster size distribution and became zero only at infinite attraction strength when permanent gels are formed. It is concluded that so-called attractive glasses are not formed at finite interaction strength.
Comparison between the Landau–Teller and flux-flux methods for computing vibrational energy relaxation rate constants in the condensed phase127(2007); http://dx.doi.org/10.1063/1.2753155View Description Hide Description
The calculation of vibrational energy relaxation (VER) rate constants in the condensed phase is usually based on the Landau–Teller formula, which puts them in terms of the Fourier transform, at the vibrational frequency, of the autocorrelation function of the force exerted on the relaxing mode by the bath modes. An alternative expression for the VER rate constant puts it in terms of the autocorrelation function of the vibrational energy flux. In this paper, we compare the predictions obtained via those two methods in the case of iodine in liquid xenon. We find that the computational cost underlying both methods is comparable and that they predict similar VER rates. However, while the calculation of the VER rate via the Landau–Teller formula is somewhat more direct, the predictions obtained via the flux-flux formula are in somewhat better agreement with the VER rates obtained from nonequilibrium molecular dynamics simulations.
Water properties and potential of mean force for hydrophobic interactions of methane and nanoscopic pockets studied by computer simulations127(2007); http://dx.doi.org/10.1063/1.2749250View Description Hide Description
We consider model systems consisting of a methane molecule and hemispherical pockets of subnanometer radii whose walls are made of hydrophobic material. The potential of mean force for process of translocation of the methane molecule from bulk water into the pockets’ interior is obtained, based on an explicit solventmolecular dynamics simulations. Accompanying changes in water density around the interacting objects and spatial distribution of solvent’s potential energy are analyzed, allowing for interpretation of details of hydrophobic interactions in relation to hydrophobic hydration properties. Applicability of surface area–based models of hydrophobic effect for systems of interest is also investigated. A total work for the translocation process is not dependent on pocket’s size, indicating that pocket desolvation has little contribution to free energy changes, which is consistent with the observation that solvent density is significantly reduced inside “unperturbed” pockets. Substantial solvent effects are shown to have a longer range than in case of a well investigated methane pair. A desolvation barrier is present in a smaller pocket system but disappears in the larger one, suggesting that a form of a “hydrophobic collapse” is observed.