Influence of structure on electron correlation effects and electron-water dispersion interactions in anionic water clusters

Electronic structure calculations at the level of second-order M?ller-Plesset perturbation theory have been performed on anionic water clusters, (H2O)n(-), in the n = 14-33 size regime. The contribution to the electron binding energy that arises from electron correlation is found to be significantly larger for cavity-bound electrons than it is for surface-bound electrons, even for surface states with electron binding energies well above 1 eV. A decomposition of the correlation energy into interactions between pairs of Boys-localized molecular orbitals is used to demonstrate that the larger correlation energy found in the cavity isomers arises from electron-water dispersion interactions, and that the dispersion interaction is larger in cavity-bound isomers because the unpaired electron penetrates well beyond the first solvation shell. In contrast, a surface-bound electron exhibits virtually no penetration into the interior of the cavity. To obtain a qualitatively accurate picture of this phenomenon, one must plot molecular orbitals using isoprobability surfaces rather than arbitrarily-selected isocontours.     

Local response dispersion method. II. Generalized multicenter interactions

Recently introduced local response dispersion method [T. Sato and H. Nakai, J. Chem. Phys. 131, 224104 (2009)], which is a first-principles alternative to empirical dispersion corrections in density functional theory, is implemented with generalized multicenter interactions involving both atomic and atomic pair polarizabilities. The generalization improves the asymptote of intermolecular interactions, reducing the mean absolute percentage error from about 30% to 6% in the molecular C(6) coefficients of more than 1000 dimers, compared to experimental values. The method is also applied to calculations of potential energy curves of molecules in the S22 database [P. Jure?ka et al., Phys. Chem. Chem. Phys. 8, 1985 (2006)]. The calculated potential energy curves are in a good agreement with reliable benchmarks recently published by Molnar et al. [J. Chem. Phys. 131, 065102 (2009)]. These improvements are achieved at the price of increasing complexity in the implementation, but without losing the computational efficiency of the previous two-center (atom-atom) formulation. A set of different truncations of two-center and three- or four-center interactions is shown to be optimal in the cost-performance balance.     

The 6-31B(d) basis set and the BMC-QCISD and BMC-CCSD multicoefficient correlation methods

Three new multicoefficient correlation methods (MCCMs) called BMC-QCISD, BMC-CCSD, and BMC-CCSD-C are optimized against 274 data that include atomization energies, electron affinities, ionization potentials, and reaction barrier heights. A new basis set called 6-31B(d) is developed and used as part of the new methods. BMC-QCISD has mean unsigned errors in calculating atomization energies per bond and barrier heights of 0.49 and 0.80 kcal/mol, respectively. BMC-CCSD has mean unsigned errors of 0.42 and 0.71 kcal/mol for the same two quantities. BMC-CCSD-C is an equally effective variant of BMC-CCSD that employs Cartesian rather than spherical harmonic basis sets. The mean unsigned error of BMC-CCSD or BMC-CCSD-C for atomization energies, barrier heights, ionization potentials, and electron affinities is 22% lower than G3SX(MP2) at an order of magnitude less cost for gradients for molecules with 9-13 atoms, and it scales better (N6 vs N,7 where N is the number of atoms) when the size of the molecule is increased.     

Accurate and efficient corrections for missing dispersion interactions in molecular simulations

In simulations, molecular dispersion interactions are frequently neglected beyond a cutoff of around 1 nm. In some cases, analytical corrections appropriate for isotropic systems are applied to the pressure and/or the potential energy. Here, we show that in systems containing macromolecules, either of these approaches introduce statistically significant errors in some observed properties; for example, the choice of cutoff can affect computed free energies of ligand binding to proteins by 1 to 2 kcal/mol. We review current methods for eliminating this cutoff-dependent behavior of the dispersion energy and identify some situations where they fail. We introduce two new formalisms, appropriate for binding free energy calculations, which overcome these failings, requiring minimal computational effort beyond the time required to run the original simulation. When these cutoff approximations are applied, which can be done after all simulations are completed, results are consistent across simulations run with different cutoffs. In many situations, simulations can be run with even shorter cutoffs than typically used, resulting in increased computational efficiency.     

On-line method for measurement of the dispersion coefficient of radionuclide migration using a column method

An on-line method to obtain breakthrough curves from a conservative tracer generated in crushed rock columns has been introduced. The breakthrough curve can be used to evaluate some important hydrologic parameters for studying radionuclide migration in groundwater system. These parameters include the dispersion coefficient, average flow yelocity, effective porosity, and retardation factor of the columns tested. A conservative radiotracer,¹³¹I, was used to generate the breakthrough curves, and linear regression analysis was applied to obtain the optimum value of dispersion coefficient. The effects of the injected volume of radioactive tracer, average flow velocity, and effective diameter of packed material on the dispersion coefficient as well as the stability of the packed material, and their in-situ application are discussed.     

A London-type formula for the dispersion interactions of endohedral A@B systems

A London-type formula is derived for endohedral systems. It involves the static dipole polarisability, alpha(1)(A) of the inner system, A, and a new type of dipole polarisability, alpha(-2)(B) with an r(-2) radial operator, for the outer system, B. The new formula has no explicit dependence on the radius, R, of B. The predicted interaction energies are compared against MP2 supermolecular calculations for A@C(60), A = He-Xe, Zn, Cd, Hg, and CH(4).     

Experimental support for the role of dispersion forces in aromatic interactions

Herein a core scaffold of 1-phenylnaphthalenes and 1,8-diphenylnaphthalenes with different substituents on the phenyl rings was used to study substituent effects on parallel-displaced aromatic π???π interactions. The energetics of the interaction was evaluated in gas phase based on the standard molar enthalpies of formation, at T=298.15?K, for the compounds studied; these values were derived from the combination of the results obtained by combustion calorimetry and Knudsen/Quartz crystal effusion. A homodesmotic gas-phase reaction scheme was used to quantify and compare the intramolecular interaction enthalpies in various substituted 1,8-diphenylnaphthalenes. The application of this methodology allowed a direct evaluation of aromatic interactions, and showed that substituent effects on the interaction enthalpy cannot be rationalized solely on classical electrostatic grounds, because no correlation with the σ(meta) or σ(para) Hammett constants was observed. Moreover, the results obtained indicate that aromatic π???π interactions are significantly enhanced by substitution, in a way that correlates with the ability of the interacting aryl rings to establish dispersive interactions. A combined experimental and computational approach for calculation of the true aromatic π???π interaction energies in these systems, free of secondary effects, was employed, and corroborates the rationale derived from the experimental results. These findings clearly emphasize the role of dispersion and dilute the importance of electrostatic forces on this type of interactions.     

A numerical method for computing dispersion constants

We reformulate and discuss a previously proposed variational numerical technique for the computation of dispersion coefficients. The method extends the Full CI idea to the perturbation equation for the intermolecular interaction, by expanding the perturbative solution in a small number of tensor products of suitably chosen Full CI vectors. Some new expansion vectors are proposed and their convergence properties are tested by performing computations on HF and H₂O. Last, a natural state analysis of the solution is performed via an orthogonal transformation of the original expansion vectors and it is found that a single couple of natural states strongly dominates the expansion.     

Raman dispersion and intermolecular interactions in unsubstituted thiophene oligomers

We present a critical analysis of the Raman spectra of unsubstituted oligothiophenes and rediscuss the well-known Raman dispersion of conjugated systems explicitly considering intermolecular interactions. Temperature-dependent Raman spectra and DFT calculations for dimers of different chain lengths show that the effect of intermolecular interactions on the frequency and intensity of carbon-carbon (CC) stretching modes is non-negligible. This effect has not been considered in previous works and might explain many spectral features of this class of compounds which are not completely interpreted by the usual models. Both intensities and frequencies are significantly affected by intermolecular interactions showing that molecular self-organization should be taken into account in future spectroscopic studies of conjugated molecules. In particular, the interactions among molecules cause an upward shift of the frequency of the R mode (amplitude mode) which can explain the lack of frequency dispersion with conjugation length of oligothiophenes, as experimentally observed for solid-state samples at room temperature.     

Long‐range corrected density functionals combined with local response dispersion: A promising method for weak interactions

Density functional theory, in general, is considered to underestimate the weak van der Waals type of intermolecular interactions. We optimized parameters of the local response dispersion (LRD) method applied to the long‐range corrected exchange‐correlation functionals (LC‐BOP12+LRD and LCgau‐BOP+LRD) on the interaction energy for the complexes in the recently compiled S66 database and found to be comparable with the high‐level wave function‐based methods reported in ?ezá? et al. (J. Chem. Theory Comput. 2011 , 7, 2427). Our calculations with the S66 intermolecular complexes at equilibrium geometries suggests that the LC‐BOP12+LRD and LCgau‐BOP+LRD are well‐balanced and lower cost alternatives to the methods reported in the database. Further, test on the S66X8 database (with eight nonequilibrium points) and the HBC6 and NBC10 database shows LC+LRD method with newly optimized parameters is a promising candidate for dealing such weak interactions. Finally, the new parameterized LC+LRD method was tested on X40 benchmark halogenated complexes.Copyright © 2013 Wiley Periodicals, Inc.     

Polarizability effects and dispersion interactions in alkene-Br2 pi-complexes

Weakly bound molecular complexes play an important role in chemistry, physics, and biodisciplines. The preequilibrium pi-complexes of various alkenes with bromine have been examined quantitatively, and a direct relationship between association constants (KF) of these pi-complexes and polarizability of the olefins was found. The stability of the Br2-olefin pi complexes is affected by both the donor ionization potential and the polarizability of the olefin, and an equation able to take into account both effects is proposed.     

The london approximation and the calculation of dispersion interactions as a sum of atom-atom terms

A London approximation utilizing atomic valence state ionization potentials and static polarizabilities yields C₆ results in good agreement with accurate values for a number of systems. Generally there is considerable improvement over the results obtained using the London approximation in conjunction with molecular parameters and reasons for this are discussed.     

Compact method for calculating intermolecular interactions

The development of efficient methods for calculating intermolecular interactions (which are responsible for the existence of stable molecular associates, solvation shells, etc.) is a pressing problem of quantum chemistry. We propose a new method for correct calculations of intermolecular interactions, which is based on the solution of SCF equations with fractional occupation numbers. Calculating intermolecular interactions by this method does not require the use of exchange potentials in an explicit form. The method is intended primarily to describe the charge transfer between interacting subsystems. The calculations by this method are compact since the dimensions of matrix problems remain unchanged in the course of the numerical procedure. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 401–405, May–June, 1995. Translated by I. Izvekova     

Electrostatic and dispersion interactions during protein adsorption on topographic nanostructures

Recently, biomaterials research has focused on developing functional implant surfaces with well-defined topographic nanostructures in order to influence protein adsorption and cellular behavior. To enhance our understanding of how proteins interact with such surfaces, we analyze the adsorption of lysozyme on an oppositely charged nanostructure using a computer simulation. We present an algorithm that combines simulated Brownian dynamics with numerical field calculation methods to predict the preferred adsorption sites for arbitrarily shaped substrates. Either proteins can be immobilized at their initial adsorption sites or surface diffusion can be considered. Interactions are analyzed on the basis of Derjaguin-Landau-Verway-Overbeek (DLVO) theory, including electrostatic and London dispersion forces, and numerical solutions are derived using the Poisson-Boltzmann and Hamaker equations. Our calculations show that for a grooved nanostructure (i.e., groove and plateau width 8 nm, height 4 nm), proteins first contact the substrate primarily near convex edges because of better geometric accessibility and increased electric field strengths. Subsequently, molecules migrate by surface diffusion into grooves and concave corners, where short-range dispersion interactions are maximized. In equilibrium, this mechanism leads to an increased surface protein concentration in the grooves, demonstrating that the total amount of protein per surface area can be increased if substrates have concave nanostructures.