Semiempirical Surface Research Articles

A Self-Starting Method for Obtaining Analytic Potential-Energy Surfaces from ab Initio Electronic Structure Calculations

Previous methods proposed for obtaining analytic potential-energy surfaces (PES) from ab initio electronic structure calculations are not self-starting. They generally require that the sampling of configuration space important in the reaction dynamics of the process being investigated be initiated by using chemical intuition or a previously developed semiempirical potential-energy surface. When the system under investigation contains four or more atoms undergoing three- and four-center reactions in addition to bond scission processes, obtaining a sufficiently converged initial sampling can be very difficult due to the extremely large volume of configuration space that is important in the reaction dynamics. It is shown that by combining direct dynamics (DD) with previously reported molecular dynamics (MD), novelty sampling (NS), and neural network (NN) methods, an analytical surface suitable for MD computations for large systems may be obtained. Application of the method to the investigation of N-O bond scission and cis-trans isomerization reactions of HONO followed by comparison of the resulting neural network potential-energy surface to one obtained by using a semiempirical potential to initiate the sampling shows that the two potential surfaces are the same within the fitting accuracy of the surfaces. It is concluded that the combination of direct dynamics, molecular dynamics, novelty sampling, and neural network fitting provides a self-starting, robust, and accurate DD/MD/NS/NN method for the execution of first-principles, ab initio, molecular dynamics studies in systems containing four or more atoms which are undergoing simultaneous two-, three-, and four-center reactions.

Diffusion on Metal Surfaces: Formalism and Application to CO Diffusion

An analytical approach searching for the saddle point on a semiempirical adiabatic potential surface (SP-SE-APES) is proposed in order to study diffusion of adsorbed molecules on a metal surface. Three reaction coordinates are used to construct this potential surface: the perpendicular adsorption height z, the distance x between adsorption sites (top, bridge, or hollow), and coordinate θ describing phonon vibrations of the metal atoms. Harmonic approximation is used to describe the potential energy along the θ coordinate, and anharmonic Morse functions are used to describe the potential energies along z and x coordinates. The theory is applied to study CO diffusion on five transition metals (rhodium, ruthenium, palladium, iridium, and platinum), and the activation energies and the prefactors are calculated for various hopping mechanisms. The one-step top−bridge−top path with the bridge state as a transition state, which was generally assumed to be the dominant mechanism of CO diffusion on these metals, is shown not to be the most likely path. Results of calculation of the diffusion coefficients are compared with experimental data.

On the number of significant mode-mode anharmonic couplings in vibrational calculations: Correlation-corrected vibrational self-consistent field treatment of di-, tri-, and tetrapeptides

A computational study is made of the number of important anharmonic mode-mode couplings in the context of vibrational calculations for di-, tri-, and tetrapeptides. The method employed is the correlation-corrected vibrational self-consistent field (CC-VSCF) algorithm, which includes correlation effects between different vibrational modes. It is found that results of good accuracy can be obtained in calculations that include only N log N mode-mode coupling terms, where N is the number of modes. This simplification significantly accelerates CC-VSCF calculations for large molecules. A criterion based on the characteristics of the normal-mode displacements is employed to predict a priori unimportant coupling terms. The criterion is tested statistically using Spearman's rank correlation coefficient. The results are illustrated by calculations for several di-, tri-, and tetrapeptides using semiempirical PM3 potential surfaces. These results are analyzed and a statistical model for error estimation is given. The decrease in the number of included coupling from N(2) to N log N opens possibilities of anharmonic vibrational calculations for large peptides.

A Semiempirical Microscale Model of the Surface Energy Balance and Its Application to Two Urban Rooftops

Abstract Individual structures within the same local-scale (102–104 m) environment may experience different microscale (<103 m) climates. Urban microclimate variations are often a result of site-specific features, including spatial and material characteristics of surfaces and surrounding structures. A semiempirical surface energy balance model is presented that incorporates radiative and meteorological measurements to statistically parameterize energy fluxes that are not measured directly, including sensible heat transport, storage heat flux through conduction, and evaporation (assumed to be negligible under dry conditions). Two Chicago rooftops were chosen for detailed study. The City Hall site was located in an intensely developed urban area characterized by close-set high-rise buildings. The University rooftop was in a highly developed area characterized by three- to seven-story buildings of stone, concrete, and brick construction. Two identical sets of instruments recorded measurements contemporaneously from these rooftops during summer 2005, and results from the week of 29 July to 5 August are presented here. The model explains 83.7% and 96% of the variance for the City Hall and University sites, respectively. Results apply to a surface area of approximately 1260 m2, at length scales similar to the dimensions of built structures and other urban elements. A site intercomparison revealed variations in surface energy balance components caused by site-specific features and demonstrated the relevance of the model to urban applications.

Vibrational inelastic and charge transfer processes in H++H2 system: An ab initio study

State-resolved differential cross sections, total and integral cross sections, average vibrational energy transfer, and the relative probabilities are computed for the H(+)+H2 system using the newly obtained ab initio potential energy surfaces at the full CI/cc-pVQZ level of accuracy which allow for both the direct vibrational inelastic and the charge transfer processes. The quantum dynamics is treated within the vibrational close-coupling infinite-order-sudden approximation approach using the two ab initio quasidiabatic potential energy surfaces. The computed collision attributes for both the processes are compared with the available state-to-state scattering experiments at E(c.m.)=20 eV. The results are in overall good agreement with most of the observed scattering features such as rainbow positions, integral cross sections, and relative vibrational energy transfers. A comparison with the earlier theoretical study carried out on the semiempirical surfaces (diatomics in molecules) is also made to illustrate the reliability of the potential energy surfaces used in the present work.

New Analytical Potential Energy Surface for the F(2P) + CH4 Hydrogen Abstraction Reaction: Kinetics and Dynamics

A new potential energy surface for the gas-phase F(2P)+CH4 reaction and its deuterated analogues is reported, and its kinetics and dynamics are studied exhaustively. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and it is calibrated to reproduce the topology of the reaction and the experimental thermal rate constants. For the kinetics, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. The theoretical results reproduce the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. Two sets of kinetic isotope effects were calculated, and they show good agreement with the sparse experimental data. The coupling between the reaction coordinate and the vibrational modes shows qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. The dynamics study was performed using quasi-classical trajectory calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found that the FH(nu',j') rovibrational distributions agree with experiment. Second, the excitation function presents an oscillatory pattern, reminiscent of a reactive resonance. Third, the state specific scattering distributions present reasonable agreement with experiment, and as the FH(nu') vibrational state increases the scattering angle becomes more forward. These kinetics and dynamics results seem to indicate that a single, adiabatic potential energy surface is adequate to describe this reaction, and the reasonable agreement with experiment (always qualitative and sometimes quantitative) lends confidence to the new surface.

Doppler Spectra From a Two-Dimensional Ocean Surface at L-Band

An approximate time-harmonic three-dimensional electromagnetic boundary-integral method, the small-slope integral equation, is combined with a series expansion of the Creamer surface representation at second order with respect to the height, denoted by Creamer (2). The resulting model provides at low numerical cost simulations of the nonlinear ocean surface Doppler spectrum at L-band. As a result of approximations, the model is designed for a low-wind speed, typically up to 5 m/s. It is shown that applying directly a second-order model such as Creamer (2) to a semiempirical sea surface spectrum induces an unrealistic magnification of small-scale roughness that is involved in the scattering process at microwave frequencies. This paper thus proposes an undressed version of the Pierson-Moskowitz spectrum that corrects this artifact. Full-polarized Doppler simulations at L-band and 70deg incidence are presented. Effects of the surface nonlinearities are outlined, and the simulated Doppler spectra show correct variations with respect to wind speed and direction

Acceleration of Correlation-corrected Vibrational Self-consistent Field Calculation Times for Large Polyatomic Molecules

Acceleration of the correlation-corrected Vibrational self-consistent field (CC-VSCF) method for anharmonic calculations of vibrational states of polyatomic molecules is described. The acceleration assumes pairwise additive interactions between different normal modes, and employs orthogonality of the single-mode vibrational wavefunctions. This greatly reduces the effort in computing correlation effects between different vibrational modes, which is treated by second order perturbation theory in CC-VSCF. The acceleration can improve the scaling of the overall computational effort from N6 to N4, where N is the number of vibrational modes. Sample calculation times, using semi-empirical potential surfaces (PM3), are given for a series of glycine peptides. Large computational acceleration, and significant reduction of the scaling of the effort with system size, is found and discussed.

A semiempirical surface scattering model for quantum corrected full-band Monte-Carlo simulation of biaxially strained Si/SiGe NMOSFETs

A new semiempirical surface scattering model for electrons in strained Si devices including a quantum correction has been developed and implemented into our FBMC simulator. The strain is assumed to be consistent with pseudomorphic growth on a relaxed SiGe buffer. By introducing a few additional terms into the physical scattering rates which depend on the Ge-content in the SiGe buffer, the new surface scattering model can excellently reproduce low-field inversion layer mobility measurements for a wide range of Ge-content (0–30%) and substrate doping levels (10 16–5.5 × 10 18 cm −3). As a device example, an NMOSFET with 23 nm gate length with and without a strained Si channel has been simulated by the new FBMC model.

A semiempirical surface backscattering model for bare soil surfaces based on a generalized power law spectrum approach

An adequate characterization of surface roughness is crucial to obtain reliable backscatter simulation results from existing analytical backscattering models. The surface roughness is typically characterized using root mean square height, autocorrelation length, and shape of the autocorrelation function. For the solution of inverse problems it is of interest to reduce the number of unknown surface parameters. Simplified backscattering models are required in this context. The paper introduces a new semiempirical backscattering model in C-band for rough dielectric surfaces which is based on the integral equation model. It is shown that a surface roughness description can be reduced using a single surface roughness parameter. To account for the high variability of autocorrelation function types, the proposed model is based on a generalized power law spectrum approach which mediates between Gaussian and exponential correlated surfaces. The approach is validated against analytical backscatter simulations and laboratory-measured microwave signatures, and the surface parameter retrieval capabilities of the suggested model are investigated.

H + CD4 Abstraction Reaction Dynamics: Product Energy Partitioning

This paper presents experimental and theoretical studies of the product energy partitioning associated with the H + CD4 (nu = 0) --> HD + CD3 reaction for the collision energy range 0.5-3.0 eV. The theoretical results are based on quasiclassical trajectories from (1) first principles direct dynamics calculations (B3LYP/6-31G), (2) an empirical surface developed by Espinosa-García [J. Chem. Phys. 2002, 116, 10664] (EG), and (3) two semiempirical surfaces (MSINDO and reparametrized MSINDO). We find that most of the energy appears in product translation at energies just above the reactive threshold; however, HD vibration and rotation become quite important at energies above 1 eV, each accounting for over 20% of the available energy above 1.5 eV, according to the B3LYP calculations. The barrier on the B3LYP surface, though being later than that on EG, predicts significantly higher HD vibrational excitation than EG. This deviation is contradictory to what would be expected on the basis of the Polanyi rules and derives from modest differences in the potential energy surfaces. The CD3 internal energy is generally quite low, and we present detailed rotational state distributions which show that the CD3 rotational distribution is largely independent of collision energy in the 0.75-1.95 eV range. The most populated rotational levels are N = 5 and 6 on B3LYP, with most of that excitation being associated with motion about the C2 axes, rather than C3 axis, of the CD3 product, in good agreement with the experimental results. Through our extensive studies in this and previous work concerning the scattering dynamics, we conclude that B3LYP/6-31G provides the best available description of the overall dynamics for the title reaction at relatively high collision energies.

A semi-empirical approach for surface soil water content estimation from radar data without a-priori information on surface roughness

In this study, the spatial distribution of soil water content in an agricultural area of 30 km 2 in Southern Italy has been estimated by using high-resolution space-borne Synthetic Aperture Radar data. Multi-polarised SAR images acquired during the SIR-C mission in April 1994 have been analysed by using the semi-empirical surface backscattering model derived by Oh, Y., Sarabandi K., Ulaby F.T., 1992. An empirical model and an inversion technique for radar scattering from bare soil surface. IEEE Trans. Geosci. Remote Sensing, 30(2), 370381. A site-specific calibration procedure of the cited model has been proposed to derive soil dielectric constant values without a-priori information on the surface roughness by using ground measurements on a regular grid in two bare-soil fields. The calibrated model applied to L-band data reproduced quite satisfactorily the spatial variability of the soil dielectric constant in the two fields. Diversely, C-band data gave poor results. Successively, the calibrated Oh's model was applied to estimate the soil dielectric constant in bare soil and low vegetation fields of the entire irrigation district, where the output of a distributed simulation model of soil water balance were available. From the comparison between the Oh's backscattering model and the soil water balance model, it was confirmed that, under bare soil conditions, the values of soil water content near the soil surface estimated from SIR-C L-band data differ by ±20% from the simulated ones. Furthermore, as expected, the presence of a fractional vegetation cover, even if small, reduced the sensitivity of radar backscattering to soil moisture. The results of this study confirmed that L-band SAR data represent a minimum requirement for possible assimilation schemes in regional hydrological modelling.

Different chemical dynamics for different conformers of biological molecules: Photoionization of glycine

Single-photon ionization dynamics of two conformers of glycine is studied by classical trajectory simulations using the semiempirical PM3 potential surface in "on the fly" calculations. Initial conditions for the trajectories are weighted according to the Wigner distribution function computed for the initial vibrational ground state. Vertical ionization in the spirit of the classical Franck-Condon principle is assumed. The dynamics of the two conformers are compared during the first 10 ps. The comparison shows very different dynamical behavior for the two conformers. In particular, the chemical fragmentation pathways differ in part. Also, one of the conformers gives much higher rates of conformational transitions, while the other conformer gives larger chemical fragmentation yields. The example shows significantly different chemical dynamics for two conformers close in energy and separated by a low barrier.

An algorithm for semi-empirical design of nucleation rate surface

During the last half of century, Classical Nucleation Theory (CNT) has been developed and there have been advances in the molecular theory of nucleation. Most of these efforts have been directed towards small molecule system modeling using intermolecular potentials. Summarizing the nucleation theory, it can be concluded that the current theory is far from complete. Agreement is generally not obtained between experimental and theoretical results. In practical applications, parametric theories can be used for the systems of interest. However, experimental measurements are still the best source of information on nucleation. Experiments are labor intensive and costly, and thus, it is useful to extend the value of limited experimental measurements to a broader range of nucleation conditions. The available nucleation parameters represent only small regions of possible nucleation conditions over the range from the critical temperature to absolute zero. Thus, it is useful to develop better tools to use the data to estimate semi-empirical nucleation rate surfaces. Following our published approach, the nucleation rate surface for any system can be constructed over its phase diagram. This concept involves using the phase equilibrium diagram to establish lines of zero nucleation rates. Nucleation rate surfaces arise from equilibrium lines and their extensions that are representing unstable equilibria. Only limited experimental data is available for use in normalizing the slopes of the linearized nucleation rate surfaces. The nucleation rate surface is described in terms of steady-state nucleation rates. To design the surfaces of nucleation rates, several assumptions are presented. In the present study, an algorithm for the semi-empirical design of nucleation rate surfaces is introduced. The topology of the nucleation rate surface for a unary system using the example of water vapor nucleation is created semi-empirically. The nucleation of two concurrent (stable and unstable) phase states of critical embryos is considered in the context of multi-surface nucleation rates. Only one phase transition (melting) in the condensed state of water is considered for simplicity. The nucleation rate surface is constructed numerically using the available experimental results for vapor nucleation and phase diagram for water. The nucleation rate for water vapor is developed for the full temperature interval, i.e. from critical point to absolute zero. The results help to suggest a new direction for experimental nucleation research.

Potential Energy Surface for the F(2P3/2,2P1/2) + CH4 Hydrogen Abstraction Reaction. Kinetics and Dynamics Study

A modified and recalibrated potential energy surface (PES) is reported for the gas-phase F(2P(3/2),2P(1/2)) + CH4 reaction and its deuterated analogue. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms and is calibrated with respect to the updated experimental and theoretical stationary point properties and experimental thermal rate constants. To take into account the two spin-orbit electronic states of the fluorine atom, two versions of the surface were constructed, the PES-SO and PES-NOSO surfaces, which differ in the choice of the zero reference level of the reactants. On both surfaces, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. While the PES-SO surface overestimates the experimental rate constants, the PES-NOSO surface shows a better agreement, reproducing the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. The kinetic isotope effects calculated showed good agreement with the sparse experimental data at 283 and 298 K. Finally, on the PES-NOSO surface, other dynamical features, such as the coupling between the reaction coordinate and the vibrational modes, were analyzed. It was found qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. These kinetics and dynamics results seem to indicate that a single, adiabatic PES is adequate to describe this reaction.

Photoionization Dynamics of Glycine: The First 10 Picoseconds

Single photon ionization dynamics of glycine is studied by classical trajectory simulations using the semiempirical PM3 potential surface in “on the fly” calculations. The glycine conformer is assumed to be in the vibrational ground state prior to ionization. Initial conditions for the trajectories are weighted according to the Wigner distribution function computed for that state. Vertical ionization in the spirit of the classical Franck−Condon principle is assumed. The main findings are as follows: (1) The photoionization triggers a fast internal rotation about the C−C bond, with the NH2 group rotating in one direction, and the COOH group rotating in the opposite direction. For the trajectories where the fast rotation occurs, it persists till the end of the simulation (10 ps). The yield for this process is about 6%, suggesting it may be experimentally observable. (2) For many of the trajectories, the photoproduced glycine ion exhibits “hops” between two conformer structures. The rates computed from the dynamics for these conformational transitions differ considerably from RRK predictions. (3) Different behavior of vibrational energy flow is found for different types of modes. There is no significant approach to statistical distribution of the energy throughout the first 10 picoseconds. (4) The preferred dissociation channel is the C−C bond cleavage. Indeed, fragmentation is observed for a few trajectories, one of them shows H atom hopping from the amino group to the carbonyl group prior to dissociation. Another trajectory shows only this hydrogen transfer and the transfer back. Possible experimental implications of some of the findings are briefly discussed.

Approaches to surface complexation modeling of Uranium(VI) adsorption on aquifer sediments

Uranium(VI) adsorption onto aquifer sediments was studied in batch experiments as a function of pH and U(VI) and dissolved carbonate concentrations in artificial groundwater solutions. The sediments were collected from an alluvial aquifer at a location upgradient of contamination from a former uranium mill operation at Naturita, Colorado (USA). The ranges of aqueous chemical conditions used in the U(VI) adsorption experiments (pH 6.9 to 7.9; U(VI) concentration 2.5 · 10 −8 to 1 · 10 −5 M; partial pressure of carbon dioxide gas 0.05 to 6.8%) were based on the spatial variation in chemical conditions observed in 1999–2000 in the Naturita alluvial aquifer. The major minerals in the sediments were quartz, feldspars, and calcite, with minor amounts of magnetite and clay minerals. Quartz grains commonly exhibited coatings that were greater than 10 nm in thickness and composed of an illite-smectite clay with occluded ferrihydrite and goethite nanoparticles. Chemical extractions of quartz grains removed from the sediments were used to estimate the masses of iron and aluminum present in the coatings. Various surface complexation modeling approaches were compared in terms of the ability to describe the U(VI) experimental data and the data requirements for model application to the sediments. Published models for U(VI) adsorption on reference minerals were applied to predict U(VI) adsorption based on assumptions about the sediment surface composition and physical properties (e.g., surface area and electrical double layer). Predictions from these models were highly variable, with results overpredicting or underpredicting the experimental data, depending on the assumptions used to apply the model. Although the models for reference minerals are supported by detailed experimental studies (and in ideal cases, surface spectroscopy), the results suggest that errors are caused in applying the models directly to the sediments by uncertain knowledge of: 1) the proportion and types of surface functional groups available for adsorption in the surface coatings; 2) the electric field at the mineral-water interface; and 3) surface reactions of major ions in the aqueous phase, such as Ca 2+, Mg 2+, HCO 3 −, SO 4 2−, H 4SiO 4, and organic acids. In contrast, a semi-empirical surface complexation modeling approach can be used to describe the U(VI) experimental data more precisely as a function of aqueous chemical conditions. This approach is useful as a tool to describe the variation in U(VI) retardation as a function of chemical conditions in field-scale reactive transport simulations, and the approach can be used at other field sites. However, the semi-empirical approach is limited by the site-specific nature of the model parameters.

Predicting Adsorption Coefficients at Air−Water Interfaces Using Universal Solvation and Surface Area Models

Vapor-phase molecules are adsorbed at air−water interfaces to a much greater extent than can be explained by air−water partition coefficients, indicating that interface adsorption can play an important role, and this can be very important for environmental phenomena. On the basis of a statistical thermodynamic analysis, we separate the observable free energy of adsorption into a dimensionality change and a coupling part so that the modeling effort is correctly focused on the coupling part. On the basis of this analysis, we present two kinds of models for predicting partitioning between the vapor phase and the macroscopic surface of liquid water. The first model, called SM5.0R-Surf, involves atomic surface tensions developed previously for bulk solvation in organic liquids and a set of four solvent descriptors that characterize the properties of the water layer at the air−water interface. The latter descriptors are treated as parameters that are determined empirically by optimization for a set of 85 solutes for which the air−water surface adsorption coefficients (Ki/a) are known experimentally. The resulting descriptors indicate that interfacial water has increased hydrogen-bond acidity and increased hydrogen-bond basicity as compared to bulk water. A second kind of model involves an empirical correlation of the interfacial-water partition coefficient Ki/w with the calculated van der Waals surface area, and this kind of model can be based either on experimental data, yielding the semiempirical surface area (SESA) model, or on theoretical data, yielding the semitheoretical surface area (STSA) model. The SM5.0R-Surf and STSA models should be especially useful for environmental modeling because neither model requires any experimental data about the solute, other than its molecular structure. As an example, we use the above models to calculate air−water adsorption coefficients for 24 different pesticides, chlorinated arenes, and polyaromatic hydrocarbons (PAHs). We also show that several models in the literature can be used successfully even if we substitute calculated instead of experimental data for the solute parameters that they originally required. In related work reported here, the SM5.0R parametrization for predicting free energies of solvation in organic solvents is extended to include solutes containing phosphorus. This extension is based on the experimental free energies of 13 solutes in 9 organic solvents (37 data points). The SM5.0R model extended in this way and the new SM5.0R-Surf model can therefore be used to predict the free energy of solvation at air−water interfaces and in bulk organic liquids for any solute composed of H, C, N, O, F, S, Cl, Br, I, and/or P, whereas the STSA model does not contain parameters that depend on atomic number and can, in principle, be used for any molecule.

Chip-Scale Modeling of Electroplated Copper Surface Profiles

In this paper, a methodology for the characterization and modeling of pattern-dependent problems in copper interconnect topography is presented. For the electroplating process, the methodology consists of test structure and mask design to examine feature scale copper step height and the height of copper array regions as a function of underlying layout parameters. Semiempirical response surface models are then generated with model parameters extracted from conventional and superfill plating processes. Once the models are calibrated, layout parameters including pattern density, linewidth distributions, and line length are extracted for each cell in a μm discretization of any random chip layout. Then, a chip-scale prediction is achieved by simulating generalized average heights for each grid cell across the entire chip. The prediction result shows root mean square errors of less than 1000 Å for array height and around 500 Å for step height. This methodology provides the first known chip-scale prediction of electroplated topography. © 2004 The Electrochemical Society. All rights reserved.

New developments in PEST shape/property hybrid descriptors.

Recent investigations have shown that the inclusion of hybrid shape/property descriptors together with 2D topological descriptors increases the predictive capability of QSAR and QSPR models. Property-Encoded Surface Translator (PEST) descriptors may be computed using ab initio or semi-empirical electron density surfaces and/or electronic properties, as well as atomic fragment-based TAE/RECON property-encoded surface reconstructions. The RECON and PEST algorithms also include rapid fragment-based wavelet coefficient descriptor (WCD) computation. These descriptors enable a compact encoding of chemical information. We also briefly discuss the use of the RECON/PEST methodology in a virtual high-throughput mode, as well as the use of TAE properties for molecular surface autocorrelation analysis.