Volume 133, Issue 2, 14 July 2010
Index of content:
, , and -phases from density functional theory calculations" title="Electronic and magnetic structure of bulk cobalt: The , , and -phases from density functional theory calculations" />
The geometric, electronic and magnetic properties of the three metallic cobalt phases: , , and have been theoretically studied using periodic density functional calculations with generalized gradient approximation (GGA) and plane wave basis set. These results have been compared with those obtained with approach which have shown a noticeable improvement with regard to experimental data. For instance, the cohesive energy values predicted by GGA are overestimated by , whereas underestimate them by 14%–17%. On the other hand, magnetic moment values are underestimated in GGA while are overestimated for approach by almost the same amount. Besides, the introduction of U parameter gives rise to an electronic redistribution in the d-band structure, which leads to variations in the magnetic properties. Moreover, a higher attention has been paid in the study of the electronic and magnetic properties of the -phase that has not described previously. These studies show that this phase posses special properties that could lead to an unusual behavior in magnetic or catalytic applications.
- Theoretical Methods and Algorithms
A numerical test of a high-penetrability approximation for the one-dimensional penetrable-square-well model133(2010); http://dx.doi.org/10.1063/1.3455330View Description Hide Description
The one-dimensional penetrable-square-well fluid is studied using both analytical tools and specialized Monte Carlo simulations. The model consists of a penetrable core characterized by a finite repulsive energy combined with a short-range attractive well. This is a many-body one-dimensional problem, lacking an exact analytical solution, for which the usual van Hove theorem on the absence of phase transition does not apply. We determine a high-penetrability approximation complementing a similar low-penetrability approximation presented in previous work. This is shown to be equivalent to the usual Debye–Hückel theory for simple charged fluids for which the virial and energy routes are identical. The internal thermodynamic consistency with the compressibility route and the validity of the approximation in describing the radial distribution function is assessed by a comparison against numerical simulations. The Fisher–Widom line separating the oscillatory and monotonic large-distance behaviors of the radial distribution function is computed within the high-penetrability approximation and compared with the opposite regime, thus providing a strong indication of the location of the line in all possible regimes. The high-penetrability approximation predicts the existence of a critical point and a spinodal line, but this occurs outside the applicability domain of the theory. We investigate the possibility of a fluid-fluidtransition by the Gibbs ensemble Monte Carlo techniques, not finding any evidence of such a transition. Additional analytical arguments are given to support this claim. Finally, we find a clusteringtransition when Ruelle’s stability criterion is not fulfilled. The consequences of these findings on the three-dimensional phase diagrams are also discussed.
Performance of multireference and equation-of-motion coupled-cluster methods for potential energy surfaces of low-lying excited states: Symmetric and asymmetric dissociation of water133(2010); http://dx.doi.org/10.1063/1.3451074View Description Hide Description
Multireference (MR), general-model-space (GMS), state-universal (SU)coupled-cluster (CC) method that considers singly (S) and doubly (D) excited cluster amplitudes relative to the reference configurations spanning the model space (GMS SU CCSD), as well as its externally corrected (ec) version -CCSD that employs -reference MR CISD as an external source of higher-than-pair cluster amplitudes in a -reference GMS CCSD, are employed to investigate low-lying states of the water molecule. The emphasis is on a generation of several low lying states belonging to the same symmetry species. Cuts of the potential energy surface (PES) corresponding to the breaking of a single OH bond and leading to the fragments, as well as the simultaneous breaking of both bonds into the are considered. Relying on a simple ab initio model that enables a comparison with the exact full configuration interaction energies, the performance of the GMS-based methods is assessed in the whole relevant range of internuclear separations. It is shown that the ec -CCSD version provides best results for both the singlet and the triplet states considered. The same cuts of the PES are then explored using a realistic aug-cc-pVTZ basis set. For triplets, the use of high-spin references is to be preferred.
A tractable and accurate electronic structure method for static correlations: The perfect hextuples model133(2010); http://dx.doi.org/10.1063/1.3456001View Description Hide Description
We present the next stage in a hierarchy of local approximations to complete active space self-consistent field (CASSCF) model in an active space of one active orbital per active electron based on the valence orbital-optimized coupled-cluster (VOO-CC) formalism. Following the perfect pairing (PP) model, which is exact for a single electron pair and extensive, and the perfect quadruples (PQ) model, which is exact for two pairs, we introduce the perfect hextuples (PH) model, which is exact for three pairs. PH is an approximation to the VOO-CC method truncated at hextuples containing all correlations between three electron pairs. While VOO-CCDTQ56 requires computational effort scaling with the 14th power of molecular size, PH requires only sixth power effort. Our implementation also introduces some techniques which reduce the scaling to fifth order and has been applied to active spaces roughly twice the size of the CASSCF limit without any symmetry. Because PH explicitly correlates up to six electrons at a time, it can faithfully model the static correlations of molecules with up to triple bonds in a size-consistent fashion and for organic reactions usually reproduces CASSCF with chemical accuracy. The convergence of the PP, PQ, and PH hierarchy is demonstrated on a variety of examples including symmetry breaking in benzene, the Cope rearrangement, the Bergman reaction, and the dissociation of fluorine.
Symmetry-adapted cluster and symmetry-adapted cluster-configuration interaction method in the polarizable continuum model: Theory of the solvent effect on the electronic excitation of molecules in solution133(2010); http://dx.doi.org/10.1063/1.3456540View Description Hide Description
In this paper we present the theory and implementation of the symmetry-adapted cluster (SAC) and symmetry-adapted cluster-configuration interaction (SAC-CI) method, including the solvent effect, using the polarizable continuum model (PCM). The PCM and SAC/SAC-CI were consistently combined in terms of the energy functional formalism. The excitation energies were calculated by means of the state-specific approach, the advantage of which over the linear-response approach has been shown. The single-point energy calculation and its analytical energy derivatives are presented and implemented, where the free-energy and its derivatives are evaluated because of the presence of solute-solvent interactions. We have applied this method to s-trans-acrolein and metylenecyclopropene of their electronic excitation in solution. The molecular geometries in the ground and excited states were optimized in vacuum and in solution, and both the vertical and adiabatic excitations were studied. The PCM-SAC/SAC-CI reproduced the known trend of the solvent effect on the vertical excitation energies but the shift values were underestimated. The excited state geometry in planar and nonplanar conformations was investigated. The importance of using state-specific methods was shown for the solvent effect on the optimized geometry in the excited state. The mechanism of the solvent effect is discussed in terms of the Mulliken charges and electronic dipole moment.
133(2010); http://dx.doi.org/10.1063/1.3457157View Description Hide Description
An understanding of the electrostatic interactions that exist between charged particles of dielectric materials has applications that span much of chemistry, physics, biology, and engineering. Areas of interest include cloud formation, ink-jet printing, and the stability of emulsions. A general solution to the problem of calculating electrostatic interactions between charged dielectric particles is presented. The solution converges very rapidly for low values of the dielectric constant and is stable up to the point where particles touch. Through applications to unspecified particles with a range of size and charge ratios, the model shows that there exist distinct regions of dielectric space where particles with the same sign of charge are strongly attracted to one another.
A uniform source-and-sink scheme for calculating thermal conductivity by nonequilibrium molecular dynamics133(2010); http://dx.doi.org/10.1063/1.3463699View Description Hide Description
A uniform source-and-sink (USS) scheme, which combines features of the reverse [F. Müller-Plathe, J. Chem. Phys.106, 6082 (1997)] and improved relaxation [B. Y. Cao, J. Chem. Phys.129, 074106 (2008)] methods, is developed to calculate the thermal conductivity by nonequilibrium molecular dynamics (NEMD). The uniform internal heat source and sink are realized by exchanging the velocity vectors of individual atoms in the right half and left half systems, and produce a periodically quadratic temperature profile throughout the system. The thermal conductivity can be easily extracted from the mean temperatures of the right and left half systems rather than by fitting the temperature profiles. In particular, this scheme greatly increases the relaxation of the exited localized phononmodes which often worsen the calculation accuracy and efficiency in most other NEMD methods. The calculation of the thermal conductivities of solid argon shows that the simple USS scheme gives accurate results with fast convergence.
133(2010); http://dx.doi.org/10.1063/1.3455708View Description Hide Description
A global search for possible LiF clusterstructures is performed up to . The method is based on simulated annealing, where all the energies are evaluated on the ab initio level. In addition, the threshold algorithm is employed to determine the energy barriers for the transitions among these structures, for the cluster, again on the ab initio level, and the corresponding tree graph is obtained.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
Proton formation dynamics in the process via the and Rydberg states of HCl investigated by three-dimensional velocity mapping133(2010); http://dx.doi.org/10.1063/1.3427541View Description Hide Description
HCl in the bulk gas phase at a pressure of has been excited via selected Q-lines of the two-photon transition band systems [Q(8)], [Q(8), Q(7)] and [Q(2–6)]. Concerning the excitation, subsequent photon absorption is known to yield , , and . Vibrationally excited can be photodissociated to , and excited atoms can be easily photoionized by absorption of a fourth photon, respectively. Using three-dimensional velocity map imaging, the spatial proton velocity distributions resulting from these processes for these particular transitions were studied for the first time. Kvaran et al. [J. Chem. Phys.131, 044324 (2009); J. Chem. Phys.129, 164313 (2008)] recently reported a substantial increase in the formation of chlorine and hydrogen ions in single rovibrational transitions of the and band systems using mass resolved resonance enhanced multiphoton ionization spectroscopy and explained this by the vicinity of single rovibrational levels of the state for which photorupture is the main feature. Thus, the known dissociation dynamics of the state should also leave their fingerprint in the spatial proton velocity distribution emerging from the photodissociation of those states. Accordingly, we found a strong increase in the ion signal for the Q(5) line of the transition, the extra signal resulting from dissociation into and the ion pair. No increase for the photodissociation channel or dissociation into has been observed. Furthermore, distributions from the Q transitions of the band system were found to show the two features previously ascribed to the “gateway” state , i.e., autoionization into and nonadiabatic dissociation into . The band system only showed significant proton formation for the Q(8) line. The speed distribution is the same as for the Q(8,7) lines of the transition while the excitation history is conserved in the angular distribution confirming the resonance interpretation.
Measuring the conformational properties of 1,2,3,6,7,8-hexahydropyrene and its van der Waals complexes133(2010); http://dx.doi.org/10.1063/1.3455341View Description Hide Description
Rotationally resolved fluorescence excitation spectra of the origin band transitions of two conformers of 1,2,3,6,7,8-hexahydropyrene (HHP) have been recorded in the gas phase. The band at has been assigned as the origin band of the chair conformer and the band at has been assigned as the origin band of the boat conformer on the basis of differences in their rotational constants. In addition, rotationally resolved fluorescence excitation spectra of single Ar and van der Waals complexes of chair-HHP have been observed. Analyses of these results indicate that the weakly attached argon atom (water molecule) is located on top of the plane of the bare molecule at from its center of mass in the electronic state. No complexes of boat-HHP were detected.
133(2010); http://dx.doi.org/10.1063/1.3459128View Description Hide Description
Rotationally resolved electronic spectra of the vibrationless origin and of eight vibronic bands of 5-methoxyindole (5MOI) have been measured and analyzed using an evolutionary strategy approach. The experimental results are compared to the results of ab initio calculations. All vibronic bands can be explained by absorption of a single conformer, which unambiguously has been shown to be the anti-conformer from its rotational constants and excitation energy. For both anti- and syn-conformers, a gap larger than is calculated, making the vibronic coupling between both states very small, thereby explaining why the spectrum of 5MOI is very different from that of the parent molecule, indole.
133(2010); http://dx.doi.org/10.1063/1.3455208View Description Hide Description
Electronic absorption spectra of linear were recorded in 6 K neon matrices following their mass-selective deposition. Four new electronic band systems are identified; the strongest lies in the UV and the second most intense is located in the visible range. The known absorption is an order of magnitude weaker than . Transitions to the and states are also discussed. The wavelengths of the electronic systems obey a linear relation as a function of the size of the cations, similar to other carbon chains. The transition in the UV of neutral has also been identified upon photobleaching of the cations trapped in the matrices.
133(2010); http://dx.doi.org/10.1063/1.3455220View Description Hide Description
Reactions between small group 6 transition metal suboxide clusters, ( or ; ; ) and both and CO were studied in gas phase using mass spectrometric analysis of high-pressure, fast flow reaction products. Both and show evidence of sequential oxidation by of the form, for the more reduced species. Similar evidence is observed for the trimetallic clusters, although appears uniquely unreactive. Lower mass resolution in the range precludes definitive product mass assignments, but intensity patterns suggest the continued trend of sequential oxidation of the more reduced end of the oxide series. Based on thermodynamic arguments, cluster oxidation by is possible if . Although simple bond energy analysis suggests that tungsten oxides may be more reactive toward compared to molybdenum oxides, this is not born out experimentally, suggesting that the activation barrier for the reduction of by tungsten suboxide clusters is very high compared to analogous molybdenum suboxide clusters. In reactions with CO, suboxides of both metal-based oxides show CO addition, with the product distribution being more diverse for than for . No evidence of cluster reduction by CO is observed.
A new accurate and full dimensional potential energy surface of based on a triatomics-in-molecules analytic functional form133(2010); http://dx.doi.org/10.1063/1.3454658View Description Hide Description
In this work a reliable full nine-dimensional potential energy surface for studying the dynamics of is constructed, which is completely symmetric under any permutation of the nuclei. For this purpose, we develop a triatoms-in-molecules method as an extension of the more common diatoms-in-molecules one, which allows a very accurate description of the asymptotic regions by including correctly the charge-induced dipole and quadrupole interactions. Moreover, this treatment provides a semiquantitative description of all the topological features of the global potential compared with coupled cluster results. In particular, the hop of the proton between two fragments produces a double well in the potential. This resonant structure involving the five atoms produces a stabilization, lowering the barrier, and the triatoms-in-molecules yields to a barrier significantly higher than the ab initio results. Therefore, to improve the triatomics-in-molecules potential surface, two five-body terms are added, which are fitted to more than 110 000 coupled-cluster ab initio points. The global potential energy surface thus obtained in this work has an overall root mean square error of 0.079 kcal/mol for energies below 27 kcal/mol above the global well. The features of the potential are described and compared with previous available surfaces.
133(2010); http://dx.doi.org/10.1063/1.3456538View Description Hide Description
We report the results of ab initio calculations on the spectral system of . The present study is closely related to the recent comprehensive experimental and theoretical work by Wei et al. [J. Chem. Phys.129, 134307 (2008)]. By means of the state-average complete active space self-consistent field and multireference configuration interaction approach, we computed the vertical excitation energies to the low-lying doublet electronic species, potential surfaces and spin-orbit constants for the and states, as well as the components of the electric dipole moment for the transition between these two species. Using these data, we calculated the vibronic energy levels, the spin-orbit structure of the spectrum, and the vibronic transition moments of the system. The results of the present study for the state agree with those derived from experimental findings by Wei et al., they elucidate the vibronic and spin-orbit structure in the species, and offer predictions for experimental searches of heretofore unobserved electronic states.
Ultraviolet photochemistry of buta-1,3- and buta-1,2-dienes: Laser spectroscopic absolute hydrogen atom quantum yield and translational energy distribution measurements133(2010); http://dx.doi.org/10.1063/1.3462951View Description Hide Description
Using pulsed H-atom Lyman-laser-induced fluorescence spectroscopy along with a photolytic calibration approach, absolute H-atom product quantum yields of and were measured under collision-free conditions for the 193 nm gas-phase laser flash photolysis of buta-1,3- and buta-1,2-diene at room temperature, which demonstrate that nascent H-atom formation is of comparable importance for both parent molecules. Comparison of the available energy fraction, and , released as product translational energy with results of impulsive and statistical energy partitioning modeling calculations indicates that for both, buta-1,3- and buta-1,2-diene, H-atom formation is preceded by internal conversion to the respective electronic ground state potential energy surfaces. In addition, values of and for the previously unknown Lyman- (121.6 nm) radiation photoabsorption cross sections of buta-1,3- and buta-1,2-diene in the gas-phase were determined.
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
133(2010); http://dx.doi.org/10.1063/1.3456545View Description Hide Description
Combining first-principles calculations and non-Condon charge transfer rates proposed by us recently [Y. Zhao and W. Z. Liang, J. Chem. Phys.130, 034111 (2009)], we investigated non-Condon effect on charge carriermobility of organic semiconductor dithiophene-tetrathiafulvalene (DT-TTF) crystal. The first-principles results reveal that only several high-frequency intramolecular vibrational modes dominate the reorganization energy, and the nuclear-coordinate dependence of electronic coupling prefers to perform an exponential or Gaussian property for most intermolecular modes rather than a linear one as assumed in conventional models. Furthermore, the electronic coupling of an isolated DT-TTF dimer is indeed affected by the surrounding molecules. The predicted non-Condon mobilities with use of the obtained structure parameters are always greater than those from Condon approximation, and the non-Condon dynamic disorder is not important for DT-TTF, which is also confirmed by molecular dynamics simulation. More interestingly, the bandlike property can be predicted under the hopping mechanism when the nuclear tunneling is incorporated.
Collective excitations in supercritical fluids: Analytical and molecular dynamics study of “positive” and “negative” dispersion133(2010); http://dx.doi.org/10.1063/1.3442412View Description Hide Description
The approach of generalized collective modes is applied to the study of dispersion curves of collective excitations along isothermal lines of supercritical pure Lennard-Jones fluid. An effect of structural relaxation and other nonhydrodynamic relaxation processes on the dispersion law is discussed. A simple analytical expression for the dispersion law in the long-wavelength region of acoustic excitations is obtained within a three-variable viscoelasticmodel of generalized hydrodynamics. It is shown that the deviation from the linear dependence in the long-wavelength region can be either “positive” or “negative” depending on the ratio between the high-frequency (elastic) and isothermal speed of sound. An effect of thermal fluctuations on positive and negative dispersion is estimated from the analytical solution of a five-variable thermoviscoelastic model that generalizes the results of the viscoelastic treatment. Numerical results are reported for a Lennard-Jones supercritical fluid along two isothermal lines with different densities and discussed along the theoretical expressions derived.
133(2010); http://dx.doi.org/10.1063/1.3462962View Description Hide Description
Raman spectra within the range have been recorded as a function of temperature for different ionic liquids based on imidazolium cations. A correlation has been found between fragility and the temperature dependence of the strength of fast relaxational motions. Understanding quasielastic scattering as the relaxational contribution to ionic mean-squared displacement elucidates some effects on ionic liquids’ fragility resulting from modifications in the chemical structure.
133(2010); http://dx.doi.org/10.1063/1.3456987View Description Hide Description
We present a theoretical model for calculating the relaxivity of the water protons due to complexes trapped inside nanovesicles, which are permeable to water. The formalism is applied to the characterization of apoferritin systems [S. Aime et al., Angew. Chem., Int. Ed.41, 1017 (2002); O. Vasalatiy et al., Contrast Media Mol. Imaging1, 10 (2006)]. The very high relaxivity due to these systems is attributed to an increase of the local viscosity of the aqueous solution inside the vesicles and to an outer-sphere mechanism which largely dominates the inner-sphere contribution. We discuss how to tailor the dynamic parameters of the trapped complexes in order to optimize the relaxivity. More generally, the potential of relaxivity studies for investigating the local dynamics and residence time of exchangeable molecules in nanovesicles is pointed out.
Mode-dependent dispersion in Raman line shapes: Observation and implications from ultrafast Raman loss spectroscopy133(2010); http://dx.doi.org/10.1063/1.3464332View Description Hide Description
Ultrafast Raman loss spectroscopy (URLS) enables one to obtain the vibrational structural information of molecular systems including fluorescent materials. URLS, a nonlinear process analog to stimulated Raman gain, involves a narrow bandwidth picosecond Raman pump pulse and a femtosecond broadband white light continuum. Under nonresonant condition, the Raman response appears as a negative (loss) signal, whereas, on resonance with the electronic transition the line shape changes from a negative to a positive through a dispersive form. The intensities observed and thus, the Franck–Condon activity (coordinate dependent), are sensitive to the wavelength of the white light corresponding to a particular Raman frequency with respect to the Raman pump pulse wavelength, i.e., there is a mode-dependent response in URLS.