Index of content:
Volume 123, Issue 18, 08 November 2005
- Theoretical Methods and Algorithms
Multipole-based integral estimates for the rigorous description of distance dependence in two-electron integrals123(2005); http://dx.doi.org/10.1063/1.2079967View Description Hide Description
We derive multipole-based integral estimates (MBIE) as rigorous and tight upper bounds to four-center two-electron integrals in order to account for the distance decay between the charge distributions, which is missing in the Schwarz screening commonly used in ab initio methods. Our screening criteria are valid for all angular momenta and can be formulated for any order of multipoles. We have found the expansion limited to dipoles to be sufficiently tight for estimating the integrals in Hartree-Fock and density-functional theories, while the screening effort is negligible. For, e.g., a DNA fragment with 1052 atoms and 10 674 basis functions the exchange part is faster by a factor of 2.1 as compared to the Schwarz screening both within our linear exchange scheme, whereas a smaller factor of 1.3 is gained for the Coulomb part within the continuous fast multipole method. Most importantly, our new MBIE screening is perfectly suited to exploit the strong distance decay of electron-correlation effects of at least in atomic-orbital-based formulations of correlation methods.
123(2005); http://dx.doi.org/10.1063/1.2079987View Description Hide Description
We derive rigorous multipole-based integral estimates (MBIE) in order to account for the distance dependence occurring in atomic-orbital (AO) formulations of electron correlationtheory, where our focus is on AO-MP2 theory within a Laplace scheme. We find for the exact transformed integral products an extremely early onset of a linear-scaling behavior and a very small number of significant products. To preselect the significant integral products we adapt our MBIE method as rigorous upper bound. In this way it is possible to exploit the favorable scaling behavior observed and to reduce the scaling of estimated products asymptotically to linear, without sacrificing accuracy or reliability. By separating Coulomb- and exchange-type contractions only half-transformed integrals need to be computed. Furthermore, our scheme of rigorously preselecting transformed integral products via MBIE seems to offer particularly interesting perspectives for a direct formation of half- or fully transformed integrals by using multipole expansions and auxiliary basis sets.
123(2005); http://dx.doi.org/10.1063/1.2102898View Description Hide Description
Transition state theory(TST) is revisited, as well as evolutions upon TST such as variational TST in which the TST dividing surface is optimized so as to minimize the rate of recrossing through this surface and methods which aim at computing dynamical corrections to the TSTtransitionrate constant. The theory is discussed from an original viewpoint. It is shown how to compute exactly the mean frequency of transition between two predefined sets which either partition phase space (as in TST) or are taken to be well-separated metastable sets corresponding to long-lived conformation states (as necessary to obtain the actual transitionrate constants between these states). Exact and approximate criterions for the optimal TST dividing surface with minimum recrossing rate are derived. Some issues about the definition and meaning of the free energy in the context of TST are also discussed. Finally precise error estimates for the numerical procedure to evaluate the transmission coefficient of the TST dividing surface are given, and it is shown that the relative error on scales as when is small. This implies that dynamical corrections to the TSTrate constant can be computed efficiently if and only if the TST dividing surface has a transmission coefficient which is not too small. In particular, the TST dividing surface must be optimized upon (for otherwise is generally very small), but this may not be sufficient to make the procedure numerically efficient (because the optimal dividing surface has maximum , but this coefficient may still be very small).
Two-dimensional cross-spectral correlation analysis and its application to time-resolved Fourier transform emission spectra of transient radicals123(2005); http://dx.doi.org/10.1063/1.2074147View Description Hide Description
A spectralanalysis method, based on the generalized two-dimensional (2D) vibrational spectra correlation analysis, is developed for deciphering the correlation among the spectral peaks of two differentspectra. This 2D cross-spectral correlation (2DCSC) analysis is aimed at revealing the vibrational features associated with a common species in two spectra, each obtained from a system containing multiple species with at least one common species. The cross-spectral correlation is based on the premise that the spectral features of the same species should have the same time and frequency responses toward similar perturbations. The effectiveness of the cross-spectral correlation analysis is first illustrated with model systems, with spectral peaks decaying linearly or exponentially with time, before being applied to analyzingtime-resolvedemission spectra obtained, by a Fourier transform IR spectrometer, for samples consisting of the vibrationally excited transient cyanooxomethyl radical (OCCN). 2DCSC among the three different sets of time-resolvedspectra collected following the photodissociation of three different precursor molecules of OCCN, respectively, allows the identification of the CN and CO stretching modes of this radical.
123(2005); http://dx.doi.org/10.1063/1.2104447View Description Hide Description
Over the past few years, it has been shown in various studies on small molecules with only a few electrons that the density-matrix renormalization group (DMRG) method converges to results close to the full configuration-interaction limit for the total electronic energy. In order to test the capabilities of the method for molecules with complex electronic structures, we performed a study on the potential-energy curves of the ground state and the first excited state of symmetry of the cesium hydride molecule. For cesium relativistic effects cannot be neglected, therefore we have used the generalized arbitrary-order Douglas-Kroll-Hess protocol up to tenth order, which allows for a complete decoupling of the Dirac Hamiltonian. Scalar-relativistic effects are thus fully incorporated in the calculations. The potential curves of the cesium hydride molecule feature an avoided crossing between the ground state and the first excited state, which is shown to be very well described by the DMRG method. Compared to multireference configuration-interaction results, the potential curves hardly differ in shape, for both the ground state and the excited state, but the total energies from the DMRG calculations are in general consistently lower. However, the DMRG energies are as accurate as corresponding coupled clusterenergies at the equilibrium distance, but convergence to the full configuration-interaction limit is not achieved.
123(2005); http://dx.doi.org/10.1063/1.2110165View Description Hide Description
The performance of fixed node diffusion Monte Carlo (FNDMC) for weakly interacting molecules is investigated. The effect of Gaussian basis sets on the asymptotic description of the molecular orbitals which is crucial for a successful importance sampling is analyzed for the example of the hydrogen atom. We find that accurate reference binding energies of the water, the ammonia, and the T-shaped as well as the parallel-displaced benzene dimer are correctly reproduced by FNDMC. The binding energies for the benzene dimers are and , respectively. The description of the methane dimer which has the smallest binding energy and a quite large intermolecular distance requires a more flexible basis set of diffuse quadruple- quality in order to prevent sampling errors.
123(2005); http://dx.doi.org/10.1063/1.2104387View Description Hide Description
The performance of the standard hierarchy of ab initio models—that is, Hartree–Fock theory, second-order Møller–Plesset theory,coupled-cluster singles-and-doubles theory, and coupled-cluster singles-doubles-approximate-triples theory—in combination with correlation-consistentbasis sets is investigated for equilibrium geometries of molecules containing second-row elements. From an analysis on a collection of (yielding statistical samples of 41 bond distances and 13 bond angles), the statistical errors (mean deviation, mean absolute deviation, standard deviation, and maximum absolute deviation) are established at each level of theory. The importance of core correlation is examined by comparing calculations in the frozen-core approximation with calculations where all electrons are correlated.
Calculations of two-photon charge-transfer excitations using Coulomb-attenuated density-functional theory123(2005); http://dx.doi.org/10.1063/1.2104367View Description Hide Description
In this work, we show that an implementation of Coulomb-attenuated density-functional theory leads to considerably better prospects than hitherto for modeling two-photon absorption cross sections for charge-transfer species. This functional, which corrects for the effect of poor asymptotic dependence of commonly used functionals, essentially brings down the widely different results for larger charge-transfer species between Hartree-Fock and density-functional theory (DFT)-B3LYP into a closer range. The Coulomb-attenuated functional, which retains the best aspects of the Hartree-Fock and DFT-B3LYP methods, proves to be very promising for further modeling design of multiphotonmaterials with technical applications.
The interaction of and with noble metal surfaces: Electronic level alignment and the origin of the interface dipole123(2005); http://dx.doi.org/10.1063/1.2107647View Description Hide Description
The electronic interaction of two molecules, the aromatic benzene and the saturated hydrocarbon cyclohexane with a Cu(111) surface, have been determined using precise, ab initio electronic structure calculations. For the interaction of these adsorbates with the substrate, we present a detailed analysis and decomposition of various individual chemical mechanisms that contribute. A novel aspect of this analysis is the use of charge-density difference contour plots to graphically display the chemistry. A wave-function-based approach was used in order to avoid problems when the presently most commonly employed approach, density-functional theory, is applied to weakly chemisorbed molecules, where the interaction is dominated by van der Waals forces. The present information are not only relevant with regard to understanding the chemistry going on when molecules are adsorbed on a Cusurface but also have important consequences with regard to charge injection in molecular electronic devices, e.g., organic field-effect transistors and organic light-emitting diodes.
- Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
123(2005); http://dx.doi.org/10.1063/1.2044707View Description Hide Description
By using the HCN/CNH isomerizationreaction as an illustrative vehicle of chemical reactions on multisaddle energy landscapes, we give explicit visualizations of molecular motions associated with a straight-through reaction tube in the phase space inside which all reactive trajectories pass from one basin to another, with eliminating recrossing trajectories in the configuration space. This visualization provides us with a chemical intuition of how chemical species “walk along” the reaction-rate slope in the multidimensional phase space compared with the intrinsic reaction path in the configuration space. The distinct nonergodic features in the two different HCN and CNH wells can be easily demonstrated by a section of Poincaré surface of section in those potential minima, which predicts in a priori the pattern of trajectories residing in the potential well. We elucidate the global phase-space structure which gives rise to the non-Markovian dynamics or the dynamical correlation of sequential multisaddle chemical reactions. The phase-space structure relevant to the controllability of the product state in chemical reactions is also discussed.
123(2005); http://dx.doi.org/10.1063/1.2079867View Description Hide Description
We present the ab initiopotential-energysurfaces of the NH–NH complex that correlate with two NH molecules in their electronic ground state. Three distinct potential-energysurfaces, split by exchange interactions, correspond to the coupling of the and electronic spins of the monomers to dimer states with , 1, and 2. Exploratory calculations on the quintet , triplet , and singlet states and their exchange splittings were performed with the valence bond self-consistent-field method that explicitly accounts for the nonorthogonality of the orbitals on different monomers. The potential surface of the quintet state, which can be described by a single Slater determinant reference function, was calculated at the coupled cluster level with single and double excitations and noniterative treatment of the triples. The triplet and singlet states require multiconfiguration reference wave functions and the exchange splittings between the three potential surfaces were calculated with the complete active space self-consistent-field method supplemented with perturbative configuration interaction calculations of second and third orders. Full potential-energysurfaces were computed as a function of the four intermolecular Jacobi coordinates, with an aug-cc-pVTZ basis on the N and H atoms and bond functions at the midpoint of the intermolecular vector . An analytical representation of these potentials was given by expanding their dependence on the molecular orientations in coupled spherical harmonics, and representing the dependence of the expansion coefficients on the intermolecular distance by the reproducing kernel Hilbert space method. The quintet surface has a van der Waals minimum of depth at for a linear geometry with the two NH electric dipoles aligned. The singlet and triplet surfaces show similar, slightly deeper, van der Waals wells, but when is decreased the weakly bound NH dimer with and converts into the chemically bound diimide (also called diazene) molecule with only a small energy barrier to overcome.
123(2005); http://dx.doi.org/10.1063/1.2101456View Description Hide Description
A time-of-flight (TOF) ion mass spectrum in coincidence with threshold photoelectrons was measured in the photon energy region between the first and second dissociation limits of to examine the decay processes of the state. The measured TOF spectrum reveals that fragment ions are produced through dissociation of the repulsive state, which can be formed by the decay process of the state accompanied with emission of photons. The potential-energy curve of the state is deduced with detailed analysis of the observed TOF spectrum, in which the radiative lifetime of the state was also derived to be . Additionally, evidence of the dissociation process of ions was obtained in the same photon energy region, where the dominant channel is .
123(2005); http://dx.doi.org/10.1063/1.2107587View Description Hide Description
The effect of different basis sets for calculation of the spectroscopic constants of the ground state of sulfur monochloride was analyzed using scalar relativistic multireference configuration interaction with single and double excitations plus Davidson correction. Then the generally contracted all-electronic correlation-consistent polarized valence quintuple zeta basis sets were selected to compute the electronic states of including 12 valence and 9 Rydberg states. The spin-orbit coupling effect was calculated via the state interaction approach with the full Breit-Pauli Hamiltonian. This effect splits these states into states. Potential-energy curves of all these states are plotted with the help of the avoided crossing rule between the electronic states of the same symmetry. The structural properties of these states are analyzed. Spectroscopic constants of bound excited states that have never been observed in experiment are obtained. The transition dipole moments and the Franck-Condon factors of several transitions from low-lying bound excited states to the ground state were also calculated.
123(2005); http://dx.doi.org/10.1063/1.2104532View Description Hide Description
Exclusively selective OD bonddissociation of HOD has been demonstrated by the ultraviolet photodissociation at through the fourth overtone state of the OD stretching mode . Branching ratio between the OH and OD bonddissociation channels has been determined by detecting H and D atoms, utilizing a resonance-enhanced multiphoton ionization (REMPI) process. The OD bonddissociation has been solely observed with the branching ratio , which has been determined by the detection limit for the H atom. Time-dependent wave-packet calculations reveal two important features for the highly selective OD bonddissociation: (1) strong local-mode character of the state and (2) limitation of the total excitation energy lower than the saddle point between the OH and OD dissociation channels in the state. Additionally, the recoil velocity and angular distribution of the nascent D atom are roughly evaluated by analyzing the Doppler-resolved REMPI spectrum. Based on these results, the dynamics of the selective OD dissociation has been discussed in detail.
123(2005); http://dx.doi.org/10.1063/1.2104467View Description Hide Description
We present a theoretical study of the structure and electronic properties of positively charged and fullerenes. Electronic energies and optimum geometries have been obtained using density-functional theory with the B3LYP functional for exchange and correlation. We have found that closed- and semiclosed-shell ions (, 5, and 10) preserve the original icosahedral symmetry of neutral . For other charges, significant distortions have been obtained. The fullerenes are, in general, less symmetric, being the closest to the spherical shape. Most fullerenes follow Hund’s rule for spin multiplicity, while most fullerenes are more stable with the lowest spin multiplicity. The calculated ionization potentials for both kinds of fullerenes increase almost linearly with charge, except in the vicinity of and . We have also explored the region of the potential-energy surface of that leads to asymmetric fission. Minima and transition states corresponding to the last steps of the fission process have been obtained. This has led us to conclude that, for , emission is the preferred fragmentation channel, whereas, for higher values, emission of two charged atomic fragments is more favorable. The corresponding fission barrier vanishes for .
- Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation
123(2005); http://dx.doi.org/10.1063/1.2090259View Description Hide Description
The infrared vibrational spectra of amorphous solid water thin films doped with at reveal a strong continuous absorbance in the range. This so-called Zundel continuum is the spectroscopic hallmark for aqueous protons. The extensive ionic dissociation of at such low temperature suggests that the reaction enthalpy remains negative down to . These observations support the interpretation that dilute aqueous solutions behave as weak acids largely due to the large positive reaction entropy resulting from the structure making character of the hydrated fluoride ion.
123(2005); http://dx.doi.org/10.1063/1.2102808View Description Hide Description
Molecular-dynamics simulations for linear quadrupoleliquids are presented. The study is carried out for two different molecular lengths at constant density and a number of temperatures and quadrupole moments. All the simulated thermodynamic states correspond to the condensed phases and some of them show typical features of a solid structure. Furthermore, a change on the preferred intermolecular orientation in the liquid phase is observed from a shifted parallel molecular arrangement to a perpendicular orientation as the quadrupole raises. This change depends on the quadrupole moment as well as on the molecular length and is put in relation with the solid structure of different “diatomic” molecules such as nitrogen, ethane, and acetylene. The appearance of a plastic solid phase at low quadrupole moment and density is also justified. A thoroughly discussion about the availability of classical perturbation theories for this kind of systems is presented.
Investigating pressure effects on structural and dynamical properties of liquid methanol with many-body interactions123(2005); http://dx.doi.org/10.1063/1.2039079View Description Hide Description
Molecular-dynamics simulations utilizing a many-body potential was used to study the pressure dependence of structural and dynamical properties for liquid methanol. The liquid density as a function of pressure agreed well with experiment, and a combination of radial and angular distribution functions were used to analyze molecular structure. From these distribution functions, it was observed that hydrogen bond strength increased with increasing pressure. This observation coincided with an increase in the molecular dipole as a function of pressure, having a significant effect on the observed increased hydrogen bond strength. Also, methanols were found to more strongly favor exactly two hydrogen bonds, with fewer methanols of zero, one, or three hydrogen bonds present at higher pressures. Furthermore, a majority of the compression with increased pressure was found to occur in regions perpendicular to the methanol hydrogen-oxygen bond vector. This was the case despite hydrogen-oxygen nonbonded distances between hydrogen bonding species being shorter, but their stiffer oxygen-hydrogen-(nonbonded) oxygen angle offsets this, resulting in their oxygen-oxygen distances being relatively unaffected. The methanol translational diffusion decreased significantly with increased pressure, while the rotational diffusion decreased at a similar magnitude around the oxygen-hydrogen and oxygen-carbon bond vectors, despite having very different overall diffusion. Finally, the hydrogen bond lifetime increased significantly with pressure, owing to the increased hydrogen bond strength, and the slower translational and rotational dynamics.
123(2005); http://dx.doi.org/10.1063/1.2121648View Description Hide Description
Pure water experimental and simulation results are combined to predict the thermodynamics of cavity formation, spanning atomic to macroscopic length scales, over the entire ambient liquid temperature range. The resulting cavityequation of state is used to quantify dewetting excess contributions to cavity formation thermodynamics and construct a thermodynamic perturbation theory of hydrophobic hydration. Predictions are compared with large cavity simulations and experimental rare-gas hydration thermodynamics data (for , , , , , and ). Key findings include the strong temperature dependence of the critical length scale for hydrophobic dewetting and the evaluation of fundamental solute-solvent interaction contributions to rare-gas hydration chemical potentials.
Collision as a way of forming bimetallic nanoclusters of various structures and chemical compositions123(2005); http://dx.doi.org/10.1063/1.2104487View Description Hide Description
In the present work, a new way to obtain bimetallic nanoclusters of different structures and chemical compositions is proposed, which is based on computer simulations. Collision processes between two metal clusters of different natures are simulated through molecular-dynamics simulations using many-body potentials. Diverse diffusion mechanisms and structures can be observed, depending on the metals combined and the initial kinetic energies. The nanostructures we have found are core-shell , alloyed , and three-shell onionlike (.