New exchange-Coulomb N2-Ar potential-energy surface and its comparison with other recent N2-Ar potential-energy surfaces

The reliability of five N2-Ar potential-energy surfaces in representing the N2-Ar interaction has been investigated by comparing their abilities to reproduce a variety of experimental results, including interaction second viral coefficients, bulk transport properties, relaxation phenomena, differential scattering cross sections, and the microwave and infrared spectra of the van der Waals complexes. Four of the surfaces are the result of high-level ab initio quantal calculations; one of them utilized fine tuning by fitting to microwave data. To date, these four potential-energy surfaces have only been tested against experimental microwave data. The fifth potential-energy surface, based upon the exchange-Coulomb potential-energy model for the interaction of closed-shell species, is developed herein: it is a combination of a damped dispersion energy series and ab initio calculations of the Heitler-London interaction energy, and has adjustable parameters determined by requiring essentially simultaneous agreement with selected quality interaction second viral coefficient and microwave data. Comparisons are also made with the predictions of three other very good literature potential-energy surfaces, including the precursor of the new exchange-Coulomb potential-energy surface developed here. Based upon an analysis of a large body of information, the new exchange-Coulomb and microwave-tuned ab initio potential-energy surfaces provide the best representations of the N2-Ar interaction; nevertheless, the other potential-energy surfaces examined still have considerable merit with respect to the prediction of specific properties of the N2-Ar van der Waals complex. Of the two recommended surfaces, the new exchange-Coulomb surface is preferred on balance due to its superior predictions of the effective cross sections related to various relaxation phenomena, and to its reliable, and relatively simple, representation of the long-range part of the potential-energy surface. Moreover, the flexibility still inherent in the exchange-Coulomb potential form can be further exploited, if required, in future studies of the N2-Ar interaction.     

Van der waals interactions and decrease of the rotational barrier of methyl-sized rotators: a theoretical study

Rotational barriers of methyl-sized molecular rotators are investigated theoretically using ab initio and empirical force field calculations in molecular models simulating various environmental conditions experienced by the molecular rotors. Calculations on neopentane surrounded by methyl groups suggest that the neopentane's methyl rotational potential energy barrier can be reduced by up to an order of magnitude by locating satellite functional groups around the rotator at a geometry that destabilizes the staggered conformation of the rotator through van der Waals repulsive interactions and reduces the staggered/eclipsed relative energy difference. Molecular mechanics and molecular dynamics calculations indicate that this barrier-reducing geometry can also be found in molecular rotators surface mounted on graphite surfaces or carbon nanotube models. In these models, molecular dynamics simulations show that the rotation of methyl-sized functional groups can be catalyzed by van der Waals interactions, thus making very rigid rotators become thermally activated at room temperature. These results are discussed in the context of design of nanostructures and use of methyl groups as markers for microenvironmental conditions.     

Efficient calculation of van der Waals dispersion coefficients with time-dependent density functional theory in real time: application to polycyclic aromatic hydrocarbons

The van der Waals dispersion coefficients of a set of polycyclic aromatic hydrocarbons, ranging in size from the single-cycle benzene to circumovalene (C(66)H(20)), are calculated with a real-time propagation approach to time-dependent density functional theory (TDDFT). In the nonretarded regime, the Casimir-Polder integral is employed to obtain C(6), once the dynamic polarizabilities have been computed at imaginary frequencies with TDDFT. On the other hand, the numerical coefficient that characterizes the fully retarded regime is obtained from the static polarizabilities. This ab initio strategy has favorable scaling with the size of the system--as demonstrated by the size of the reported molecules--and can be easily extended to obtain higher order van der Waals coefficients.     

Spectroscopic and theoretical characterization of the A2delta-X2pi transition of CH-Ne

The A2delta-X2pi transition of CH-Ne was examined using laser-induced fluorescence and fluorescence depletion techniques. The spectrum was found to be particularly congested due to the large number of bound states derived from the CH(A,n=2)+Ne interaction, and the small energy spacings between these states resulting from the relatively weak anisotropy of the van der Waals bond. High-level ab initio calculations were used to generate two-dimensional potential energy surfaces for CH(X)-Ne and CH(A)-Ne. The equilibrium structures from these surfaces were bent and linear for the X and A states, respectively. Variational calculations were used to predict the bound states supported by the ab initio surfaces. Empirical modification of the potential energy surfaces for the A state was used to obtain energy-level predictions that were in good agreement with the experimental results. Transitions to all of the optically accessible internal rotor states of CH(A,n=2)-Ne were identified, indicating that CH performs hindered internal rotations in the lowest-energy levels of the A and X states. The characteristics of the potential energy surfaces for CH-Ne in the X,A,B, and C states suggest that dispersion and exchange repulsion forces dominate the van der Waals interaction.     

An ab initio study of van der Waals potential energy parameters for silver clusters

We employ ab initio calculations of van der Waals complexes to study the potential energy parameters (C(6) coefficients) of van der Waals interactions for modeling of the adsorption of silver clusters on the graphite surface. Electronic structure calculations of the (Ag(2))(2), Ag(2)-H(2), and Ag(2)-C(6)H(6) complexes are performed using a coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)), M?ller-Plesset second-order perturbation theory (MP2), and spin-component-scaled MP2 (SCS-MP2) methods. Using the atom pair approximation, the C(6) coefficients for silver-silver, silver-hydrogen, and silver-carbon atom systems are obtained after subtracting the energies of quadrupole-quadrupole interactions from the total electronic energy.     

A new scaling procedure to correct semiempirical MEP and MEP-derived properties

Summary The MNDO, AM1, PM3, and ab initio 6–31G^* and 6–31+G^* MEPs for 21 neutral and 12 charged molecules were computed in layers ranging from 1.2 to 2.0 times the van der Waals radii of atoms. Semiempirical and ab initio MEPs for each layer and two groups of layers were compared to gain insight into the relationships between semiempirical and ab initio MEPs. A detailed statistical study allowed us to obtain a new set of scaling coefficients able to correct the semiempirical MEPs to provide better representations of the ab initio values. The corrected semiempirical MEPs were used to obtain electrostatic charges, whose quality was tested by the comparison between semiempirical Coulombic MEPs and ab initio quantum mechanical MEPs.     

Adsorption of benzene on coinage metals: a theoretical analysis using wavefunction-based methods

The interaction of benzene with a Ag(111) surface has been determined using reliable ab initio electronic structure calculations. The results are compared to a recent detailed analysis of the interaction of benzene with copper and gold surfaces, thus making it possible to derive a consistent picture for the electronic structure changes encountered when benzene is brought into contact with the densely packed coinage metal surfaces. To avoid the problems encountered when the presently most frequently employed computational approach, density functional theory (DFT), is applied to adsorbate systems where dispersion (or van der Waals) forces contribute substantially, we use a wavefunction-based approach. In this approach, the weak van der Waals interactions, which are dominated by correlation effects, are described using second-order perturbation theory. The surface dipole moment and the work function changes induced upon adsorption are also discussed.     

Theoretical investigation of the He-I2(E3Πg) ion-pair state: ab initio intermolecular potential and vibrational levels

We present a theoretical study on the potential energy surface and vibrational bound states of the E electronic excited state of the HeI(2) van der Waals system. The interaction energies are computed using accurate ab initio methods and large basis sets. Relativistic small-core effective core potentials in conjunction with a quintuple-zeta quality basis set are employed for the heavy iodine atoms in multireference configuration interaction calculations for the (3)A' and (3)A" states. For the representation of the potential energy surface we used a general interpolation technique for constructing potential surfaces from ab initio data based on the reproducing kernel Hilbert space method. The surface presents global and local minima for T-shaped configurations with well-depths of 33.2 and 4.6 cm(-1), respectively. Vibrational energies and states are computed through variational quantum mechanical calculations. We found that the binding energy of the HeI(2)(E) T-shaped isomer is 16.85 cm(-1), in excellent agreement with recent experimental measurements. In lieu of more experimental data we also report our predictions on higher vibrational levels and we analyze the influence of the underlying surface on them. This is the first attempt to represent the potential surface of such a highly excited electronic state of a van der Waals complex, and it demonstrates the capability of the ab initio technology to provide accurate results for carrying out reliable studies to model experimental data.     

An ab initio investigation of the O(3P)-H2(1sigma(g)+) van der Waals well

We report an ab initio study of the van der Waals region of the O(3P)-H2 potential energy surface based on RCCSD(T) calculations with an aug-cc-pVQZ basis supplemented by bond functions. In addition, an open-shell implementation of symmetry-adapted perturbation theory (SAPT) is used to corroborate the RCCSD(T) calculations and to investigate the relative magnitudes of the various contributions to the van der Waals interaction. We also investigate the effect of the spin-orbit coupling on the position and depth of the van der Waals well. We predict the van der Waals minimum to occur in perpendicular geometry, and located at a closer distance than a secondary well in colinear geometry. The potentials obtained in the present study confirm the previous calculations of Alexander [M. H. Alexander, J. Chem. Phys., 1998, 108, 4467], but disagree with the earlier work of Harding and co-workers [Z. Li, V. A. Apkarian and L. B. Harding, J. Chem. Phys., 1997, 106, 942] as well as with recently refitted surfaces of Brand?o and coworkers [J. Brand?o, C. Mogo and B. C. Silva, J. Chem. Phys., 2004, 121, 8861]. Inclusion of spin-orbit coupling reduces the depth of the van der Waals minimum without causing a change in its position.     

Long-range interactions between like homonuclear alkali metal diatoms

Long-range electrostatic and van der Waals coefficients up to terms of order R(-8) have been evaluated by the sum over states method using ab initio and time-dependent density functional theory. We employ several widely used density functionals and systematically investigate the convergence of the calculated results with basis set size. Static electric moments and polarizabilities up to octopole order are also calculated. We present values for Li(2) through K(2) which are in good agreement with existing values, in addition to new results for Rb(2) and Cs(2). Interaction potential curves calculated from these results are shown to agree well with high level ab initio theory. Preliminary results are reported that demonstrate the applicability of the method to larger alkali clusters.     

Prediction of fluid densities using automatically derived VDW parameters

Automatic parameterization procedures, which are important components in our latest approach of making transferable and extensible general force fields, were demonstrated by deriving parameters from ab initio and empirical data, and subsequently predicting densities for nine molecular fluids. Excellent agreements with available experimental data were obtained for most of the systems studied. The automated procedure for deriving van der Waals parameters, constructed by combining the molecular dynamics simulation and statistical mechanical perturbation theory, is described.     

Ne3-NH3 van der Waals tetramer: rotational spectra and ab initio study of the microsolvation of NH3 with rare gas atoms

Microwave rotational spectra of eleven isotopomers of the Ne(3)-NH(3) van der Waals tetramer were measured using a pulsed jet, Balle-Flygare type Fourier transform microwave spectrometer. The transitions measured fall between 4 and 17 GHz and correspond to the ground internal rotor state of the weakly bound complex. The (20)Ne(3)- and (22)Ne(3)-containing species are symmetric top molecules while the mixed (20)Ne(2)(22)Ne- and (20)Ne(22)Ne(2)-isotopomers are asymmetric tops. For each of the deuterium-containing isotopomers, a tunneling splitting was observed due to the inversion of NH(3) within the tetramer. The (14)N nuclear quadrupole hyperfine structures were resolved and included in the spectroscopic fits of the various isotopomers. The rotational constants obtained from the fits were used to estimate the van der Waals bond lengths of the tetramer while the (14)N nuclear quadrupole coupling contants and the observed inversion tunneling splittings provided information about the internal dynamics of the NH(3) moiety. The experimental results were complemented by the construction of three ab initio potential energy surfaces [CCSD(T)] for the Ne(3)-NH(3) complex, each corresponding to a different internal geometry of NH(3) ( 90 degree angle HNH = 106.67 degrees, 90 degree angle HNH = 113.34 degrees, and 90 degree angle HNH = 120.00 degrees ). The topologies of the surfaces are related to the structures and dynamics of the tetramer. Extensive comparisons are made between the results obtained for the Ne(3)-NH(3) tetramer in this work and previous experimental and ab initio studies of related Rg(n)-NH(3) van der Waals clusters.     

Structures and bonding nature of small monoligated copper clusters (HCN-Cun, n = 1-3) through high-resolution infrared spectroscopy and theory

The structures, C-H stretching frequencies, and dipole moments of HCN-Cun (n = 1-3) clusters are determined through high-resolution infrared spectroscopy. The complexes are formed and probed within superfluid helium droplets, whereby the helium droplet beam is passed over a resistively heated crucible containing copper shot and then through a gas HCN pickup cell. All complexes are found to be bound to the nitrogen end of the HCN molecule and on the "atop site" of the copper cluster. Through the experimental C-H vibrational shifts of HCN-Cun and ab initio calculations, it was found that the HCN-metal interaction changes from a strong van der Waals bond in n = 1 to a partially covalent bond in HCN-Cu3. Comparisons with existing infrared data on copper surfaces show that the HCN-Cun bond must begin to weaken at very large copper cluster sizes, eventually returning to a van der Waals bond in the bulk copper surface case.     

A Gibbs free energy correlation for automated docking of carbohydrates

Thermodynamic information can be inferred from static atomic configurations. To model the thermodynamics of carbohydrate binding to proteins accurately, a large binding data set has been assembled from the literature. The data set contains information from 262 unique protein-carbohydrate crystal structures for which experimental binding information is known. Hydrogen atoms were added to the structures and training conformations were generated with the automated docking program AutoDock 3.06, resulting in a training set of 225,920 all-atom conformations. In all, 288 formulations of the AutoDock 3.0 free energy model were trained against the data set, testing each of four alternate methods of computing the van der Waals, solvation, and hydrogen-bonding energetic components. The van der Waals parameters from AutoDock 1 produced the lowest errors, and an entropic model derived from statistical mechanics produced the only models with five physically and statistically significant coefficients. Eight models predict the Gibbs free energy of binding with an error of less than 40% of the error of any similar models previously published.     

Time-dependent density functional theory calculation of van der Waals coefficient of sodium clusters

In this paper we employ all-electron ab initio time-dependent density functional theory based method to calculate the long range dipole-dipole dispersion coefficient (van der Waals coefficient) C(6) of sodium atom clusters containing even number of atoms ranging from 2 to 20 atoms. The dispersion coefficients are obtained via Casimir-Polder relation [Phys. Rev. 3, 360 (1948)]. The calculations are carried out with two different exchange-correlation potentials: (i) the asymptotically correct statistical average of orbital potential (SAOP) and (ii) Vosko-Wilk-Nusair representation [Can. J. Phys. 58, 1200 (1980)] of exchange-correlation potential within local density approximation. A comparison with the other theoretical results has been performed. We also present the results for the static polarizabilities of sodium clusters and also compare them with other theoretical and experimental results. These comparisons reveal that the SAOP results for C(6) and static polarizability are quite accurate and very close to the experimental results. We examine the relationship between volume of the cluster and van der Waals coefficient, and find that to a very high degree of correlation C(6) scales as the square of the volume. We also present the results for van der Waals coefficient corresponding to cluster-Ar atom and cluster-N(2) molecule interactions.     

Potential-energy surface, van der Waals energy spectrum, and vibronic transitions in s-tetrazine-argon complex

The van der Waals vibrational states and the structure of the vibronic spectrum of s-tetrazine-argon complex have been studied by the ab initio methods. The potential-energy surface of the ground S(0) electronic state of the complex has been constructed by fitting the analytical many-body expansion to a large set of the interaction energy values computed using the second-order M?ller-Plesset perturbation theory combined with the standard aug-cc-pVDZ basis set. The equilibrium structure of the complex found is that with argon located above the tetrazine ring at a distance of 3.394 A. The calculated dissociation energy of 354 cm(-1) is compatible with the experiment. The van der Waals energy spectrum calculated from the potential-energy surface is explained analyzing a correlation with a simpler energy spectrum of benzene-argon. A new assignment of the S(0)-S(1) vibronic spectrum is proposed on the basis of the rigorous selection rules, vibrational energy levels in S(0) and S(1) states and vibronic transition intensities calculated from the electronic transition dipole moment surfaces.