Showing papers in "The Journal of Physical Chemistry in 1990"

DREIDING: A generic force field for molecular simulations

Abstract: We report the parameters for a new generic force field, DREIDING, that we find useful for predicting structures and dynamics of organic, biological, and main-group inorganic molecules. The philosophy in DREIDING is to use general force constants and geometry parameters based on simple hybridization considerations rather than individual force constants and geometric parameters that depend on the particular combination of atoms involved in the bond, angle, or torsion terms. Thus all bond distances are derived from atomic radii, and there is only one force constant each for bonds, angles, and inversions and only six different values for torsional barriers. Parameters are defined for all possible combinations of atoms and new atoms can be added to the force field rather simply. This paper reports the parameters for the "nonmetallic" main-group elements (B, C, N, 0, F columns for the C, Si, Ge, and Sn rows) plus H and a few metals (Na, Ca, Zn, Fe). The accuracy of the DREIDING force field is tested by comparing with (i) 76 accurately determined crystal structures of organic compounds involving H, C, N, 0, F, P, S, CI, and Br, (ii) rotational barriers of a number of molecules, and (iii) relative conformational energies and barriers of a number of molecules. We find excellent results for these systems.

Reaction Path Following in Mass-Weighted Internal Coordinates

Abstract: Our previous algorithm for following reaction paths downhill (J. Chem. Phys. 1989, 90, 2154), has been extended to use mass-weighted internal coordinates. Points on the reaction path are round by constrained optimizations involving the internal degrees or freedom or the molecule. The points are optimized so that the segment or the reaction path between any two adjacent points is described by an arc or a circle in mass-weighted internal coordinates, and so that the gradients (in mass-weighted internals) at the end points or the arc are tangent to the path. The algorithm has the correct tangent vector and curvature vectors in the limit or small step size but requires only the transition vector and the energy gradients; the resulting path is continuous, differentiable, and piecewise quadratic

Second-order perturbation theory with a CASSCF reference function

Abstract: Second-order perturbation theory based on a CASSCF reference state is derived and implemented. The first-order wave function includes the full space of interacting states. Expressions for the contributions to the second-order energy are obtained in terms of up to four-particle density matrices for the CASSCF reference state. The zeroth-order Hamiltonian reduces to the MOller-Plesset Hamiltonian for a closed-shell reference state. The limit of the implementation is given by the number of active orbitals, which determines the size of the density matrices. It is presently around 13 orbitals. The method is illustrated in a series of calculations on H 2 , H 2 O, CH 2 , and F - , and the results are compared with corresponding full CI results

Microcalorimetric detection of lower critical solution temperatures in aqueous polymer solutions

Abstract: A sensitive solution microcalorimeter was used to study lower critical solution temperatures (LCSTs) of aqueous polymer solutions. Endotherms with enthalpies on the order of the strength of hydrogen bonds were observed at temperatures concurring with LCSTs detected by classical cloud-point measurements for solutions of poly(N-isopropylacrylamide) (PNIPAAM), poly(vinyl methyl ether) (PVME), poly(propylene glycol) (PPG), and hydroxypropylcelluloe (HPC). The LCST of PNIPAAM was found to be dependent on the chain length, generally increasing with decreasing M-bar_n in the range M-bar_n = 1.6 X 10^5 to 5.4 X 10^3. Treatment of the calorimetric endotherm according to a two-state transiton model afforded cooperative unit sizes of the order of the chain length for PNIPAAM, PVME, and PPG. The apparent cooperative unit for the demixing transition of HPC was found to be larger than a single chain, consistent with previous observations of aggregation of HPC below the LCST. Depression of the LCST by added salts was observed.

Efficient production of C60 (buckminsterfullerene), C60H36, and the solvated buckide ion

Abstract: Buckminsterfullerene, C{sub 60}, was prepared in gram quantities by contact-arc vaporization of a graphite rod in a 100-Torr atmosphere of helium, followed by extraction of the resultant graphite soot with toluene. The dominance of C{sub 60} in this extract was verified by mass, FTIR, and NMR spectroscopy. The molecule was successfully hydrogenated to C{sub 60}H{sub 36} via a Birch reduction and dehydrogenated back to bare C{sub 60} by treatment with DDQ reagent. Cyclic voltammetry of C{sub 60} in methylene chloride revealed highly reversible formation of at least two stable anionic forms of C{sub 60} in solution. A broad new class of these fulleride and fulleronium ions is anticipated, both with the internal cavity empty and with any one of a large number of elements inside, thus providing a means of fine-tuning the chemical, optical, and redox properties.

Ab initio vibrational Raman and Raman optical activity spectra

Abstract: Ab initio methods for vibrational Raman optical activity (VROA) spectral predictions are presented. Further developments that are desired for routine VROA predictions are pointed out. To assess the ab initio predictive capabilities, both Raman and VROA are considered. For evaluation of Raman predictions, calculations on H{sub 2}, D{sub 2}, HF, F{sub 2}, CO, CO{sub 2}, CH{sub 4}, CH{sub 3}D, CH{sub 2}D{sub 2}, CHD{sub 3}, CD{sub 4}, H{sub 2}O{sub 2}, D{sub 2}O{sub 2}, oxirane, thiirane, and methylthiirane are compared with the corresponding experimental observations. The VROA predictions for H{sub 2}O{sub 2}, D{sub 2}O{sub 2}, and trans-2,3-dideuteriothiirane are compared, wherever appropriate, with the simple two-group model predictions.

Characterization of the soluble all-carbon molecules C60 and C70

Abstract: The authors report on the further physical and chemical characterization of the new forms of molecular carbon, C{sub 60} and C{sub 70}. Results demonstrate a high yield of production (14%) under optimized conditions and reveal only C{sub 60} and C{sub 70} in measurable quantity, in an 85:15 ratio. These two new molecular forms of carbon can be completely separated in analytical amounts by column chromatography on alumina. Comparison among mass spectra obtained by the electron impact, laser desorption, and fast atom bombardment (FAB) methods allows a clear assessment of the composition of the mixed and pure samples, and of the fragmentation and double ionization patterns of the molecules. In addition, spectroscopic analyses are reported for the crude mixture by {sup 13}C NMR and by IR spectroscopy in KBr pellet, and for pure C{sub 60} and C{sub 70} in solution by UV-vis spectroscopy.

Vectorial electron injection into transparent semiconductor membranes and electric field effects on the dynamics of light-induced charge separation

Abstract: Transparent titanium dioxide membranes (thickness 2.7 {mu}m) were prepared by sintering of 8-nm colloidal anatase particles on a conducting glass support. The dynamics of charge recombination following electron injection from the excited state of RuL{sub 3} (L = 2,2{prime}-bipyridine-4,4{prime}-dicarboxylic acid) into the conduction band of the semiconductor were examined under potentiostatic control of the electric field within the space charge layer of the membrane. Biasing the Fermi level of the TiO{sub 2} positive of the flat-band potential sharply reduced the recombination rate, a 1,000-fold decrease being associated with a potential change of only 300 mV. Photoelectrochemical experiments performed with the same RuL{sub 3}-loaded membrane in NaI-containing water show the onset of anodic photocurrent to occur in the same potential domain. Forward biasing of the membrane potential impairs photosensitized charge injection turning on the photoluminescence of the adsorbed sensitizer.

The restricted active space self-consistent-field method, implemented with a split graph unitary group approach

Abstract: An MCSCF method based on a restricted active space (RAS) type wave function has been implemented. The RAS concept is an extension of the complete active space (CAS) formalism, where the active orbitals are partitioned into three subspaces: RAS1, which contains up to a given maximum number of hole; RAS2, where all possible distributions of electrons are allowed; and RAS3, which contains up to a given maximum number of electrons. A typical example of a RAS wave function is all single, double, etc. excitations with a CAS reference space. Spin-adapted configurations are used as the basis for the MC expansion

Effects of common inorganic anions on rates of photocatalytic oxidation of organic carbon over illuminated titanium dioxide

Abstract: Effects of pH and common inorganic anions on the rates of photocatalytic oxidation of salicylic acid, aniline, and ethanol to CO{sub 2} over glass-supported near-UV illuminated TiO{sub 2} have been investigated by using conductivity detection of evolved CO{sub 2}. In the presence of chlorides and the absence of organics, lowering the pH of the solution below 3.6 caused an increase in conductivity with increasing circulation time. At pH 4.0 {plus minus} 0.2, the rate of oxidation decreased with increasing concentration of chlorides. Perchlorates and nitrates have very little effect. Sulfates or phosphates even at millimolar concentrations are rapidly adsorbed by the catalyst and reduce the rates of oxidation by 20-70%. The catalyst can be readily regenerated by washing with 0.1 M NaHCO{sub 3} solution. The data are considered in terms of kinetic salt effects, surface active sites having discrete adsorption properties, and Langmuir-Hinshelwood kinetic models. The implications for water purification processes based on photocatalytic oxidation using titanium dioxide are discussed.

Calculating total electrostatic energies with the nonlinear Poisson-Boltzmann equation

Abstract: The Poisson-Boltzmann (PB) equation is enjoying a resurgence in popularity and usefulness in biophysics and biochemistry due to numerical advances which allow the equation to be rapidly solved for arbitrary geometries and nonuniform dielectrics. The great simplification of PB models is to use the mean electrostatic potential to give an estimate of the potential of mean force (PMF) governing the distribution of the mobile ions in the solvent. This approximation enables both the mean potential and mean ion distribution to be obtained directly from solutions to the PB equation without performing complex statistical mechanical integrations. The nonlinear form of the PB equation has greater accuracy and range of validity than the linear form, but the approximation of the PMF by the mean potential creates theoretical difficulties in defining the total electrostatic energy for the former. In this paper we use the calculus of variations to provide a unique definition of the total energy and to obtain expressions for the total electrostatic free energy for various forms of the PB equation. These expressions involve energy density integrals over the volume of the system. Various equivalent expressions for the total energy are given and the physical meaning of the different terms that appear is discussed. Numerical calculations are carried out to demonstrate the feasibility of our approach and to assess the magnitude of the various terms that arise in the theory. Both the more familiar charging integral and the energy density integral methods can be applied to the PB equation with equal accuracy, but the latter is much more efficient computationally. The energy density integral involves the integral of the excess osmotic pressure of the ion atmosphere. The various forms of the PB equation which have been most widely discussed to date because of the availability of analytical solutions are shown to be special cases where the osmotic term is absent.

Relative partition coefficients for organic solutes from fluid simulations

Abstract: A procedure is noted for obtaining the difference in partition coefficients (log P) for two solutes between two solvents. Fluid simulations are required in which one solute is mutated to the other in both solvents, and the changes in free energies of solvation are computed. The method is illustrated for eight pairs of organic solutes partitioning between water and chloroform. Monte Carlo statistical mechanics simulations are used with statistical perturbation theory to calculate the requisite free energy changes

Microcalorimetric study of the acidity and basicity of metal oxide surfaces

Abstract: The adsorption of probe molecules such as NH{sub 3} and CO{sub 2} was studied on about 20 simple metallic oxides by use of microcalorimetry in order to determine the number and character of basic and acidic surface sites. Microcalorimetry allowed a simultaneous determination of the strength and energy distribution of the adsorption sites. The adsorption isotherms, the differential heats, the integral heats, and the energetic histograms are given for NH{sub 3} and CO{sub 2} adsorptions. Relationships between the average adsorption heats of NH{sub 3} and CO{sub 2} have been found as a function of the ionic character percentage and of the charge/radius ratio.

Photophysical and photochemical aspects of coupled semiconductors: charge-transfer processes in colloidal cadmium sulfide-titania and cadmium sulfide-silver(I) iodide systems

Abstract: The mechanistic and kinetic details of the charge injection from excited CdS into a large bandgap semiconductor such as AgI and TiO{sub 2} have been investigated by coupling the two semiconductor systems in the colloidal form. The interaction between the two colloids led to the quenching of CdS emission. The rate constants for the charge injection from excited CdS into the conduction band of AgI and TiO{sub 2} colloids were determined to be 2.2 {times} 10{sup 7} and >5 {times} 10{sup 10} s{sup {minus}1}, respectively. Transmission electron microscopic analysis indicated the possibility of several CdS colloidal particles interacting with a single particle of TiO{sub 2} and participating in the charge injection process. Primary photochemical events in the CdS-TiO{sub 2} system were investigated by picosecond laser flash photolysis. The charge injected into the TiO{sub 2} colloid and trapped at the Ti{sup 4+} site was characterized from its broad absorption in the region of 500-760 nm. The extended lifetime of these trapped charge carriers indicated an improved charge separation in the coupled semiconductor system.

Laser raman spectroscopy of supported vanadium oxide catalysts

Abstract: Supported vanadia catalysts were studied by Raman spectroscopy to establish the effects of vanadia loading and support composition on the structure of the dispersed vanadia species. Three types of vanadia species were found to be present on SiO{sub 2}, TiO{sub 2}, and Al{sub 2}O{sub 3} supports: monomeric vanadyls, one- and two-dimensional vanadate chains, and crystallites of V{sub 2}O{sub 5}. Monomeric vanadyls are characterized by a narrow Raman band in the region of 1,026-1,042 cm{sup {minus}1}, the position being sensitive to the identity of the support. The terminal V{double bond}O groups of polyvanadate chains give rise to a broad feature in the region of 800-1,000 cm{sup {minus}1} which, in the case of TiO{sub 2}-supported V{sub 2}O{sub 5}, shifts to higher frequency as the V{sub 2}O{sub 5} loading increases. Crystalline V{sub 2}O{sub 5} was detected at higher loadings on all three supports. Evidence from Raman spectroscopy and O{sub 2} chemisorption indicates that the dispersion of vanadia increases in the order SiO{sub 2} < Al{sub 2}O{sub 3} {approximately} TiO{sub 2}.

Influence of the preparation methods of titanium dioxide on the photocatalytic degradation of phenol in aqueous dispersion

Abstract: Several commercial and homemade TiO{sub 2} samples were used as photocatalysts to prove that the physicochemical features as determined by the origin and preparation methods affect the photocatalytic behavior, in addition to the semiconducting properties. The photodegradation of aqueous phenol solution in TiO{sub 2} dispersion, carried out in a batch reactor, was used as a test reaction. The results show a great variability of photocatalytic behavior, and in particular it has been observed that the rutile phase is active or inactive according to the preparation conditions.

Single-chain transition of poly(N-isopropylacrylamide) in water

Abstract: The temperature dependence of the radius of gyration and the hydrodynamic radius or poly(N-isopropylacrylamides) with various molecular weights (1.63×10 6 -2.52×10 7 ) in water was obtained in the relatively good through poor regions by use of static and dynamic light scattering techniques. The dimensions of the polymer show sharp decreases in the poor solvent (collapsed) region. The observed transition in polymer dimensions is analyzed in terms of a coil-globule transition, which is one of the most important problems of polymer solution physics. Reasonable agreement with the theoretical predictions is obtained. This water-soluble polymer system shows a fairly narrow crossover region compared with the polystyrene systems

Infrared spectra of the solvated hydronium ion: Vibrational predissociation spectroscopy of mass-selected H3O+.cntdot.(H2O)n.cntdot.(H2)m

Abstract: Infrared spectra of the mass-selected clusters H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}{sm bullet}(H{sub 2}) and H{sub 3}O{sup +}{sm bullet}(H{sub 2}){sub n} (n = 1,2, and 3) were observed by vibrational predissociation spectroscopy The clusters were mass-selected and then trapped in a radio frequency ion trap Cluster dissociation by loss of H{sub 2} followed excitation of OH or H{sub 2} stretches Spectra were recorded by detecting fragment ions as a function of laser frequency From spectra of H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}{sm bullet}(H{sub 2}), the authors were able to determine the spectrum of the hydrated hydronium ion H{sub 3}O{sup +}{sm bullet}(H{sub 2}O){sub n}, because the H{sub 2} formed weak complexes with the hydrates Spectra in the OH stretching region (3,000-4,000 cm{sup {minus}1}) were observed at a resolution of 13 cm{sup {minus}1} for clusters n = 1,2, and 3 The structure of the clusters and the perturbing effect of the H{sub 2} were inferred from a comparison with recent unpublished ab initio vibrational frequencies calculated by Remington and Schaefer

Test of surface selection rules for surface-enhanced Raman scattering: the orientation of adsorbed benzene and monosubstituted benzenes on gold

Abstract: Surface-enhanced Raman spectra for benzene, toluene, and benzonitrile adsorbed at gold-aqueous interfaces and on gold in vacuum at 20 K are analyzed in terms of previously proposed surface selection rules. The electrochemical adsorbate systems are of particular interest in this regard since independent information on the surface binding geometries have been obtained previously from alterations in the band frequency and band shape upon adsorption and from dipole-dipole coupling, thereby providing a bona fide test of the surface selection rules

Dynamics of electron-hole pair recombination in semiconductor clusters

Abstract: The kinetics of radiative electron-hole pair recombination in CdS and Cd{sub 3}As{sub 2} clusters (where the radius of the cluster is smaller than the de Broglie wavelength of photogenerated excitons) were studied with picosecond photon counting luminescence decay measurements over wide temperature and energy ranges. The decay profiles were quantitatively examined with several models. The decays are composed of two distinct time regimes, each with very different temperature and emission energy dependence. The first (fast) regime is attributed to an unusually efficient thermal repopulation mechanism. The second (slow) component is well described by a distributed kinetic model. The kinetic behavior of wide (CdS) and narrow (Cd{sub 3}As{sub 2}) band gap materials was remarkably similar when composed of clusters in the quantum confined regime.

Quantitative evaluation of molecular orientation in thin Langmuir-Blodgett films by FT-IR transmission and reflection-absorption spectroscopy

Abstract: By a comparison of infrared reflection-absorption (RA) and transmission intensities, a method for the quantitative evaluation of molecular orientation in thin Langmuir-Blodgett (LB) films has been developed. The enhancement factors for the RA to transmission absorption intensities of hypothetical isotropic films were theoretically calculated by using Hansen's optical formulas for thin multilayer films. By combination of these values with the experimentally determined intensity ratios of the RA to transmission spectra for uniaxially oriented LB films, the orientation angles of the transition moments of major infrared bands were evaluated. Application of this method to the 7-monolayer LB film of cadmium stearate deposited on AG (for RA measurements) and on ZnSe (for transmission measurements) substrates gave reasonable tilt angles for the molecular chain as compared to those reported by other investigators and to the data from the X-ray analysis. This method will be discussed in detail and the various factors which influence the accuracy of the orientation analysis will also be covered.

Comparison of the quadratic configuration interaction and coupled cluster approaches to electron correlation including the effect of triple excitations

Abstract: The recently proposed quadratic configuration interaction (QCI) method is compared with the more rigorous coupled cluster (CC) approach for a variety of chemical systems Some of these systems are well represented by a single-determinant reference function and others are not The finite order singles and doubles correlation energy, the perturbational triples correlation energy, and a recently devised diagnostic for estimating the importance of multireference effects are considered The spectroscopic constants of CuH, the equilibrium structure of cis-(NO)2 and the binding energies of Be3, Be4, Mg3, and Mg4 were calculated using both approaches The diagnostic for estimating multireference character clearly demonstrates that the QCI method becomes less satisfactory than the CC approach as non-dynamical correlation becomes more important, in agreement with a perturbational analysis of the two methods and the numerical estimates of the triple excitation energies they yield The results for CuH show that the differences between the two methods become more apparent as the chemical systems under investigation becomes more multireference in nature and the QCI results consequently become less reliable Nonetheless, when the system of interest is dominated by a single reference determinant both QCI and CC give very similar results

Fifth order Moeller-Plesset perturbation theory: comparison of existing correlation methods and implementation of new methods correct to fifth order

Abstract: Full fifth-order MOller-Plesset perturbation theory of electron correlation is presented in algebraic form and used to compare the behavior of other approximate methods that are size-consistent and exact for two electrons. Considering only single and double substitutions, quadratic configuration interaction (QCISD), coupled cluster (CCSD), and Brueckner doubles (BD) theories are shown to deviate from each other in fifth order. The BD method contains the most parts of the fifth-order energy in a correct manner. The corresponding methods with noniterative triples corrections QCISD(T), CCSD(T), and BD(T) are also analyzed. These methods are all correct in those parts of the fifth-order energy that are linear in the higher (triple, quadruple) substitutions

Consistent porphyrin force field. 1. Normal-mode analysis for nickel porphine and nickel tetraphenylporphine from resonance Raman and infrared spectra and isotope shifts

Abstract: Resonance Raman spectra with variable-wavelength excitation are reported for Ni{sup II} porphine (NiP) and for the pyrrole-d{sub 8}, meso-d{sub 4}, and (pyrrole + meso)-d{sub 12} isotopomers, as well as for Ni{sup II} meso-tetraphenylporphine (NiTPP) and its pyrrole-{sup 15}N{sub 4}, pyrrole-d{sub 8}, {sup 13}C{sub 4}-meso, and phenyl-d{sub 20} isotopomers. All the Raman-active in-plane modes have been identified and are assigned to local coordinates which take into account the phasing of adjacent bond stretches within the pyrrole rings and at the methine bridges. The IR spectra of NiP and its isotopomers are also assigned. For most of the local coordinates good frequency agreement is seen for the different symmetry blocks, showing that longer range phasings have minor effects. These in-plane mode assignments are supported by normal-coordinate calculations with a physically reasonable valence force field, which is nearly the same for NiP and NiTPP. The principal force constants are in good accord with bond length relationships selected on the basis of scaled ab initio calculations. The phenyl substituents of NiTPP lower the frequencies of the asymmetric methine bridge stretching modes {nu}{sub 10}(B{sub 1g}) and {nu}{sub 19}(A{sub 2g}) by {approximately}60 cm{sup {minus}1}; this shift is attributable partly to the loss of coupling with themore » C{sub m}H bending modes in NiP and partly to an electronic effect of the phenyl group. There are also near-resonant interactions in NiTPP between porphyrin and phenyl modes near 740 and 200 cm{sup {minus}1} resulting in strongly displaced modes. Otherwise the phenyl groups have little influence on the porphyrin skeletal mode frequencies. Several phenyl modes are subject to moderate RR enhancement, probably via intensity borrowing from nearby porphyrin modes.« less

Consistent porphyrin force field. 2. Nickel octaethylporphyrin skeletal and substituent mode assignments from nitrogen-15, meso-d4, and methylene-d16 Raman and infrared isotope shifts

Abstract: Resonance Raman spectra with variable-wavelength excitation are reported for nickel octaethylporphyrin and its isotopomers containing {sup 15}N, and {sup 2}H at the methine (meso-d{sub 4}) and methylene (methylene-d{sub 16}) carbon atoms. The {sup 15}N, meso-d{sub 4} double isotopomer is also examined. The infrared spectrum of the methylene-d{sub 16} isotopomer is reported, and the frequencies are combined with recently published infrared results for the other isotopomers. Essentially all of the porphyrin skeletal modes have been assigned and have been allocated to local coordinates which recognize the pyrrole rings as cooperative vibrational units. The assignments are supported by a normal-coordinate analysis with a valence force field involving standard ethyl force constants and porphyrin in-plane force constants which are transferred nearly intact from Ni porphine and Ni tetraphenylporphine. Many vibrational modes of the NiOEP ethyl substituents have also been located in the spectra and assigned. Bands assignable to ethyl C-C stretching and C-H bending modes are surprisingly strong in the resonance Raman spectra and suggest appreciable involvement of the ethyl groups in the porphyrin {pi}-{pi}{sup *} excited states. The conformations of the ethyl substituents have a marked influence on the low-frequency vibrational spectra.

Microemulsions — A qualitative thermodynamic approach

Abstract: Microemulsions, that is, stable colloidal dispersions of water and nonpolar solvents stabilized by amphiphiles, are of growing interest in research and industry. The phase behavior of the multicomponent mixture is essentially determined by the features of corresponding binary mixtures. The efficiency of an amphiphile in solubilizing the solvents reaches its maximum in the temperature interval in which the mixture separates into three coexisting liquid phases. The domain size of the dispersion is determined by the interfacial tension between the aqueous and the oil-rich phase in the presence of a saturated monolayer. Because the interfacial tension reaches its minimum in the three-phase interval and, furthermore, decreases with increasing amphiphilicity, the transition from weakly structured solutions to microemulsions is gradual. It is, therefore, suggested that microemulsions be defined as stable colloidal dispersions of domains sufficiently large for the dispersed solvent to exhibit the properties as, e.g., the dielectric number of a bulk phase.

Estimation of effective diameters for molecular fluids

Abstract: Effective diameters of atoms and molecules can typically be estimated within about 1% from equation-of state or compressibility data. These estimates correlate well with critical volumes, with molar refractivities, and with tabulated van der Waals volume increments. The correlations hold well even for markedly aspherical or polar molecules. Comparison of a simple perturbed hard-sphere equation of state (the Carnahan-Starling-van der Waals equation, denoted CS-vdW) with molecular dynamics simulations shows that surprisingly good values for both the Lennard-Jones radius and well depth (typically within 1% for {sigma}{sub LJ}, 10% for {epsilon}{sub LJ}) can be obtained by using pressure-density isotherms or equivalent data. Together with empirical correlations, this permits the LJ potential parameters to be estimated simply from the density (at atmospheric pressure) and the heat of vaporization or boiling point temperature. The temperature dependence of the effective diameter can be estimated from a simple model originally introduced by Boltzmann. More sophisticated perturbative theories of liquids predict a small additional density dependence of the effective diameter. Simple analytical expressions for these theoretical models are presented. In particular, the CS-vdW equation is found to predict reliable absolute densities (within 1%) for liquids at high pressures where direct measurements become difficult.