Showing papers on "Lattice energy published in 2008"
TL;DR: The range of factors that can determine which of the thermodynamically feasible crystal structures are observed polymorphs, shows the many further challenges in developing crystal energy landscapes as a tool for control of the organic solid state.
Abstract: Many organic molecules are emerging as having many crystalline forms, including polymorphs and solvates, as more techniques are being used to generate and characterise the organic solid state. The fundamental scientific and industrial interest in controlling crystallisation is inspiring the development of computational methods of predicting which crystal structures are thermodynamically feasible. Sometimes, computing this crystal energy landscape will reveal that a molecule has one way of packing with itself that is sufficiently more favourable than any other so only this crystal structure will be observed. More frequently, there will be many energy minima that are energetically feasible, showing approximately equi-energetic compromises between the various intermolecular interactions allowed by the conformational flexibility. Such cases generally lead to multiple solid forms. At the moment, we usually calculate the lattice energy landscape, an approximation to the real crystal energy landscape at 0 K. Despite its limitations, many studies show that this is a valuable complement to solid form screening, which helps in discovering new structures as well as rationalising the solid forms that are found in experimental searches. The range of factors that can determine which of the thermodynamically feasible crystal structures are observed polymorphs, shows the many further challenges in developing crystal energy landscapes as a tool for control of the organic solid state.
154 citations
TL;DR: The results underscore the necessity of using highly correlated electronic structure methods to determine thermodynamic properties within chemical accuracy.
Abstract: We provide a first-principles methodology to obtain converged results for the lattice energy of crystals of small, neutral organic molecules. In particular, we determine the lattice energy of crystalline benzene using an additive system based on the individual interaction energies of benzene dimers. Enthalpy corrections are estimated so that the lattice energy can be directly compared to the experimentally determined sublimation energy. Our best estimate of the sublimation energy is 49.4 kJ mol(-1), just over the typical experimentally reported values of 43-47 kJ mol(-1). Our results underscore the necessity of using highly correlated electronic structure methods to determine thermodynamic properties within chemical accuracy. The first coordination sphere contributes about 90 % of the total lattice energy, and the second coordination sphere contributes the remaining 10 %. Three-body interactions are determined to be negligible.
101 citations
TL;DR: The calculated lattice energies, along with hydrogen-bond patterns, of crystal structures predicted with the three stoichiometries are presented and analysed to provide a rationalisation for the stoichiometry observed.
Abstract: We report on the crystal structure of urea (U) with acetic acid (A), its physical stability and its predictability using computational methods. The crystal structure of urea:acetic acid (U:A) shows hydrogen-bond ribbons and a 1:2 stoichiometry. Crystal structure prediction calculations are presented for two sets of U:A stoichiometries: 1:1 and 1:2. A 1:3 stoichiometry is also partially explored by means of a synthon approach. The calculated lattice energies, along with hydrogen-bond patterns, of crystal structures predicted with the three stoichiometries are presented and analysed to provide a rationalisation for the stoichiometry observed. Exploring stoichiometric diversity using computational methods provides a tool for the rationalisation of stoichiometry preferences in crystalline multicomponent systems and a first step towards their prediction.
91 citations
TL;DR: It is found that introducing a simple model for the bulk crystalline environment when calculating the molecular energy and electron density distribution leads to important changes in relative total crystal energies and correctly distinguishes the observed crystal structures from the set of computer-generated possibilities.
Abstract: The computer-generation of the crystal structures of the α-amino acid valine is used as a challenging test of lattice energy modeling methods for crystal structure prediction of flexible polar organic molecules and, specifically, to examine the importance of molecular polarization on calculated relative energies. Total calculated crystal energies, which combine atom-atom model potential calculations of intermolecular interactions with density functional theory intramolecular energies, do not effectively distinguish the real (known) crystal structures from the rest of the low energy computer-generated alternatives when the molecular electrostatic models are derived from isolated molecule calculations. However, we find that introducing a simple model for the bulk crystalline environment when calculating the molecular energy and electron density distribution leads to important changes in relative total crystal energies and correctly distinguishes the observed crystal structures from the set of computer-gener...
79 citations
TL;DR: Two methods for estimating the induction energy in organic molecular crystals by approximating the charge density polarization in the crystalline state are compared and it is shown that the inductionEnergy can be an important discriminator in the relative lattice energies of structures with substantially different hydrogen-bonding motifs.
Abstract: We compare two methods for estimating the induction energy in organic molecular crystals by approximating the charge density polarization in the crystalline state. The first is a distributed atomic polarizability model combined with distributed multipole moments, derived from ab initio monomer properties. The second uses an ab initio calculation of the molecular charge density in a point-charge field. Various parameters of the models, such as the rank of polarizability model, effect of self-consistent iterations, and damping, are investigated. The methods are applied to a range of observed and predicted crystal structures of three particularly challenging molecules, namely oxalyl dihydrazide, 3-azabicyclo[3,3,1]nonane-2,4-dione, and carbamazepine, as well as demonstrating the importance of induction in the naphthalene crystal. The two models agree well considering the different approximations made, and it is shown that the induction energy can be an important discriminator in the relative lattice energies of structures with substantially different hydrogen-bonding motifs.
56 citations
TL;DR: In this paper, the molecular packing motifs occurring in the crystal structures of pentacene and its two oxo-derivatives (6,13-pentacenequinone and 5,7,12,14 -pentacenetetrone) have been analyzed.
Abstract: The molecular packing motifs occurring in the crystal structures of pentacene and its two oxo-derivatives (6,13-pentacenequinone and 5,7,12,14-pentacenetetrone) have been analyzed. Both oxygen containing species exhibit an almost coplanar stacking while pentacene adopts a face-on-edge herringbone packing. The different packing motifs are well explained by quantum chemical ab initio calculations of the electronic structure of the molecular entities exhibiting a pronounced charge localization at the oxygen atoms of both oxo-derivatives which causes an additional electrostatic O-π interaction favoring a planar stacking. Moreover, the polarizability of the π-system is reduced and the molecular quadrupole moment is altered, both resulting in a decrease of the lattice energy of the oxo-species as evidenced by the sublimation energy obtained for all three species from thermal desorption measurements. This emphasizes the importance of the balance between electrostatic and van der Waals interactions for the packin...
53 citations
TL;DR: 1W appeared soluble in common solvents; this finding opens the way to the study of the chemistry of 1 and investigation of its electrooptical properties.
Abstract: Sublimation of sulflower, octathio[8]circulene C16S8 (1), on heating under high vacuum (∼10−5 Torr) leads to successive formation of two modifications: a white film (1W) and a red polycrystalline solid (1R). When kept at room temperature for several weeks, 1W spontaneously turns pink, reflecting the monotropic phase transition 1W → 1R. The accurate molecular and crystal structure of 1R has been studied using low-temperature (100 K) high-resolution single crystal X-ray analysis. The C16S8 molecule in crystal is strictly planar with nearly equalized bonds of each type (C−C, C−S, and C═C). The point symmetry group of the free molecule is D8h, and the crystal space group is P21/n. These data allowed group-theoretical analysis of vibrational normal modes to be accomplished. Investigation of the charge density distribution of 1R including Bader’s AIM approach has revealed rather strong intermolecular S···S, S···C, and C···C interactions of charge transfer and π-stacking types with overall lattice energy of 28.5...
50 citations
TL;DR: In this paper, the contribution of differences in zero-point energy and vibrational enthalpy to the relative stability of polymorphic crystal forms is investigated, and the specific case investigated is that of α- and γ-glycine.
Abstract: The relative stability of polymorphic crystal forms is a challenging conceptual problem of considerable technical interest. Current estimates of relative polymorph energies concentrate on lattice energy. In this work the contribution of differences in zero-point energy and vibrational enthalpy to the enthalpy difference for polymorphs is investigated. The specific case investigated is that of α- and γ-glycine, for which the experimental enthalpy difference is known. Periodic lattice density functional theory (DFT) computations are used to provide the vibrational spectrum at the Γ-point. It is confirmed that these methods provide reasonable descriptions of the inelastic neutron scattering spectra of these two crystals. It is found that the difference in the zero-point energy is about 1.9 kJ/mol and that the vibrational thermal population difference is 0.9 kJ/mol in the opposite sense. The overall vibrational contributions to the enthalpy difference are much larger than the observed value of ca. 0.3 kJ/mol....
49 citations
TL;DR: In this paper, the authors used density-functional theory to calculate the thermal equation of state of platinum up to 550 GPa and 5000 K. The static lattice energy was computed by using the linearized augmented plane-wave method with local density approximation (LDA), Perdew-Burke-Ernzerhof, and the recently proposed Wu-Cohen functional.
Abstract: Platinum is widely used as a pressure calibration standard. However, the established thermal equation of state (EOS) has uncertainties especially in the high $P\text{\ensuremath{-}}T$ range. We use density-functional theory to calculate the thermal equation of state of platinum up to 550 GPa and 5000 K. The static lattice energy was computed by using the linearized augmented plane-wave method with local-density approximation (LDA), Perdew-Burke-Ernzerhof, and the recently proposed Wu-Cohen functional. The electronic thermal free energy was evaluated using the Mermin functional. The vibrational part was computed within the quasiharmonic approximation using density-functional perturbation theory and pseudopotentials. Special attention was paid to the influence of the electronic temperature on the phonon frequencies. We find that, in overall, LDA results agree best with the experiments. Based on the density-functional theory calculations and the established experimental data, we develop a consistent thermal EOS of platinum as a reference for pressure calibration.
44 citations
TL;DR: In this paper, a search of the Cambridge Structural Database for crystal structures of 5-substituted uracils was performed and the lattice energies were calculated for the known crystal structures and compared with the computed lattice energy landscape for each molecule (except R = Br and I).
Abstract: A search of the Cambridge Structural Database for crystal structures of 5-substituted uracils shows that, although there is a recurrent motif with symmetric hydrogen bonding and interdigitation of the 5-substituent R, a range of other hydrogen bonded ribbons, sheets and three-dimensional motifs are possible. In order to try and rationalize this, we have performed a combination of experimental studies and computational searches for low energy structures for the 12 simple 5-substituted uracils with R = H, CH3, CH2CH3, CHCH2, CN, OH, NH2, NO2, F, Cl, Br and I. Crystallization experiments on these compounds yielded the first single crystal X-ray determinations of 5-ethyluracil and 5-cyanouracil, as well as low temperature redeterminations of the disordered structures of 5-chlorouracil and 5-bromouracil. The lattice energies were calculated for the known crystal structures and compared with the computed lattice energy landscape for each molecule (except R = Br and I). Although the symmetric ribbon motif often dominates the computed crystal energy landscape, all of the molecules show a variety of different hydrogen bonding structures within a small energy range (5 kJ mol−1) of the global minimum and exhibit quite a diverse range of energetically competitive motifs. Thus, the range of crystallization outcomes, from polymorphism and other multiple forms, to the difficulty in growing single crystals (R = CHCH2 and NH2) probably reflects the sensitivity of the various hydrogen bonding motifs to the substituent and limited range of crystallization conditions that can be applied.
43 citations
Journal Article•
TL;DR: In this paper, the authors used density-functional theory to calculate the thermal equation of state of platinum up to 550 GPa and 5000 K. The static lattice energy was computed by using the linearized augmented plane-wave method with local-density approximation LDA, Perdew-BurkeErnzerhof, and the recently proposed Wu-Cohen functional.
Abstract: Platinum is widely used as a pressure calibration standard. However, the established thermal equation of state EOS has uncertainties especially in the high P-T range. We use density-functional theory to calculate the thermal equation of state of platinum up to 550 GPa and 5000 K. The static lattice energy was computed by using the linearized augmented plane-wave method with local-density approximation LDA, Perdew-BurkeErnzerhof, and the recently proposed Wu-Cohen functional. The electronic thermal free energy was evaluated using the Mermin functional. The vibrational part was computed within the quasiharmonic approximation using density-functional perturbation theory and pseudopotentials. Special attention was paid to the influence of the electronic temperature on the phonon frequencies. We find that, in overall, LDA results agree best with the experiments. Based on the density-functional theory calculations and the established experimental data, we develop a consistent thermal EOS of platinum as a reference for pressure calibration.
TL;DR: In this paper, a computational strategy for the evaluation of the crystal lattice constants and cohesive energy of the weakly bound molecular solids is proposed, based on the high level ab initio coupled-cluster determination of the pairwise additive contribution to the interaction energy.
Abstract: A computational strategy for the evaluation of the crystal lattice constants and cohesive energy of the weakly bound molecular solids is proposed. The strategy is based on the high level ab initio coupled-cluster determination of the pairwise additive contribution to the interaction energy. The zero-point-energy correction and nonadditive contributions to the interaction energy are treated using density functional methods. The experimental crystal lattice constants of the solid benzene are reproduced, and the value of 480 meV/molecule is calculated for its cohesive energy.
TL;DR: In this paper, a novel solid-state metathesis (SSM) approach represented by the reaction (LCl3+Na3VO4→ LVO4+NaCl) (L = Y, La) is proposed to synthesize technologically important rare-earth phosphates and vanadates.
TL;DR: A new approach to split specific and nonspecific energetic terms in the crystal lattice was developed and position of substituents essentially affects the mechanism of partitioning in buffer pH 2.0, however, at pH 7.4, the mechanism is independent of the position of the substituent.
TL;DR: The simulations suggest that the NaEuMo2O8 end-member is nearly fully ordered and has I4[combining macron] symmetry, and the calculated subsolidus temperature-composition phase diagram is dominated by three miscibility gaps.
Abstract: Thermodynamic mixing properties and subsolidus phase relations of Ca2Mo2O8–NaEuMo2O8 powellites were modelled in the temperature range of 423–1773 K with static lattice energy calculations based on empirically constrained interatomic potentials. Relaxed static lattice energies (SLE) of a large set of randomly varied structures in a 4 × 4 × 2 supercell of I41/a powellite (a = 5.226 A, c = 11.433 A) containing 128 exchangeable (Ca, Na and Eu) atoms were calculated using the general utility lattice program (GULP). These energies were cluster expanded in the basis set of 69 pair-wise effective interactions and three configuration-independent parameters. Temperature-dependent enthalpies of mixing were calculated using the Monte Carlo method. Free energies of mixing were obtained by thermodynamic integration of the Monte Carlo results. The simulations suggest that the NaEuMo2O8 end-member is nearly fully ordered and has I symmetry. The calculated subsolidus temperature-composition phase diagram is dominated by three miscibility gaps which are separated by narrow fields of stability of two ordered phases with the compositions of x = 4/9 and x = 2/3, where x is the mole fraction of the NaEuMo2O8 end-member.
TL;DR: In this article, atomic scale local density functional simulations and configurational averaging are used to predict the energies and lattice parameters associated with mixed calcium/strontium fluorapatites, CaxSr10−x(PO4)6F2.
TL;DR: The vibrational spectrum of a computer model of crystalline RDX was studied using a 216-molecule periodic supercell, allowing for intra- and intermolecular degrees of freedom using the force field, with particular attention to correlations between nitro rotations and lattice modes.
Abstract: The vibrational spectrum of a computer model of crystalline RDX was studied using a 216-molecule periodic supercell, allowing for intra- and intermolecular degrees of freedom using the force field by Boyd et al. [J. Chem. Phys. 124, 104508 (2006)]. The normal modes were analyzed with regard to their activity involving molecule center-of-mass translations and rotations, as well as 15 intramolecular degrees of freedom, including bond stretches, bend and dihedral angle variations, and out-of-plane motions of the nitro groups. We correlate center-of-mass motions with the occupation of internal degrees of freedom for all of the normal modes in the model with particular attention to correlations between nitro rotations and lattice modes. Transfer of lattice energy to internal degrees of freedom can occur through doorway modes and is significant for the initiation of detonation. Several clusters of potential doorway modes are found which involve significant lattice motion as well as nitro rotations. Such groups ...
TL;DR: A time-convolutionless master equation is established for describing the amide-I vibrational energy flow in a lattice of H-bonded peptide units and it has been shown that the energy relaxation occurs over a very short time scale which prevents any significant delocalization of the polaron.
Abstract: A time-convolutionless master equation is established for describing the amide-I vibrational energy flow in a lattice of H-bonded peptide units. The dynamics is addressed within the small polaron formalism to account for the strong coupling between the amide-I vibron and the phonons describing the H-bond vibrations. Therefore, special attention is paid to characterize the influence of the amide-I relaxation on the polaron transport properties. This relaxation is modeled by assuming that each amide-I mode interacts with a bath of intramolecular normal modes whose displacements are strongly localized on the C=O groups. It has been shown that the energy relaxation occurs over a very short time scale which prevents any significant delocalization of the polaron. At biological temperature, the polaron explores a finite region around the excited site whose size is about one or two lattice parameters. However, two regimes occur depending on whether the vibron-phonon coupling is weak or strong. For a weak coupling, the energy propagates coherently along the lattice until the polaron disappears. By contrast, for a strong coupling, a diffusive regime occurs so that the polaron explores a finite size region incoherently. In both cases, the finite polaron lifetime favors the localization of the vibron density whose amplitude decreases exponentially.
TL;DR: In this paper, the lattice energy of AIIIBV and AIIBVI semiconductors with the product of ionic charges (Z1Z2) and nearest-neighbor distance d (A) was analyzed.
Abstract: In this paper we present an expression relating the lattice energy (U in kcal/mol) for the AIIIBV and AIIBVI semiconductors with the product of ionic charges (Z1Z2) and nearest-neighbor distance d (A). The lattice energy of these compounds exhibit a linear relationship when plotted on a log–log scale against the nearest-neighbor distance d (A), but fall on different straight lines according to the ionic charge product of the compounds. A fairly good agreement has been found between the observed and calculated values of the lattice energy for AIIIBV and AIIBVI semiconductors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this article, an ionic liquid (IL) BMIZn3Cl7 has been synthesized by directly mixing zinc chloride and 1-butyl-3-methylimidazolium chloride (BMIC) with molar ratio 3/1 under dry argon atmosphere according to the procedure of Lecocq and Abbott.
TL;DR: In this article, a solid-state metathesis approach for the synthesis of hydrated MnV 2 O 6 · x H 2 O (x = 2, 4) materials driven by mechanochemical activation energy has been demonstrated.
01 Jan 2008
TL;DR: Several organic conformational polymorph clusters that highlight this principle are discussed in this article, where the two contributions may be additive or cancel one another, i.e., a metastable conformer and crystal lattice energy effects compensate each other.
Abstract: In crystal structures of flexible molecules the total energy is a summation of the molecular conformer and crystal lattice energy contribution. These two energy factors are of comparable magnitude in organic solids because bond torsions and intermolecular interactions have similar energies, worth a few kcal mol-1. The two contributions may be additive or cancel one another. Polymorphism is likely in molecular systems wherein molecular conformer and crystal lattice energy effects compensate each other, i.e. a metastable conformer resides in a stable packing arrangement or a stable rotamer is assembled in a metastable crystal environment. Consequently, conformational polymorph energy differences occur in a small window of <3 kcal mol-1. Several organic conformational polymorph clusters that highlight this principle are discussed in this chapter
01 Sep 2008
TL;DR: The bond energy between the interlayer cation and the layer appears to follow the same order and to depend both on the nature of the most electropositive elements of the layer and on their location.
Abstract: The surface energy of some clays belonging to the smectite group has been calculated starting from crystal structures and combining a partial charge model with the computation of the lattice energy. The dioctahedral smectites studied here include montmorillonite; beidellites; and nontronite. One of the differences between these clays is the location of the substitution in the octahedral sheet or in the tetrahedral one. Another is the possibility of vacancies in cis- or trans-octahedral positions. These locations and vacancies have an effect on the distortion of the crystal framework and therefore on the surface energy. Calculated surface energies of the solid samples increase in the order beidellites > montmorillonite > nontronite. The bond energy between the interlayer cation and the layer appears to follow the same order and to depend both on the nature of the most electropositive elements of the layer and on their location. The trends obtained provide elements for an analysis of data related to interlayer enlargement.
TL;DR: In this article, a solution-based approach for the metathetic synthesis of hydrated MnV2O6 driven by microwave energy has been successfully demonstrated, which is confirmed by the presence of high lattice energy byproduct NaCl as per the reaction.
TL;DR: A non-empirical fully ionic description, with the anion wavefunctions in their compressed but still spherically symmetrical states optimal for the crystal, is presented for the cohesive energetics of two cubic phases of three solid iodides, KI, RbI and CsI as mentioned in this paper.
Abstract: A non-empirical fully ionic description, with the anion wavefunctions in their compressed but still spherically symmetrical states optimal for the crystal, is presented for the cohesive energetics of two cubic phases of three solid iodides, KI, RbI and CsI The non-correlated part of the energy is computed using the RELCRION program which takes full account of relativistic effects Both the dispersive attractions and energies arising from electron correlations of short range are computed For each polymorph stable under ambient conditions, the rock-salt (B1) phases of KI and RbI and the eightfold coordinated (B2) phase of CsI, the cohesion is slightly underestimated The lattice energy deficits of around 22 kJ mol−1 for KI and RbI are reduced to around 13 kJ mol−1 for CsI, with overestimations of some 02 au in the equilibrium cation–anion separations R decreasing as the metal becomes more electropositive The prediction that the B2 phase of CsI is more stable (by 6 kJ mol−1) than the B1 polymorph agrees with experiment For both KI and RbI, the zinc-blende polymorph is predicted to lie some 37 kJ mol−1 in energy above the B1 polymorph An additional potential, plausibly ascribed to slight covalency, correcting these underestimations is derived semi-empirically
TL;DR: In this article, a novel solid-state metathesis (SSM) approach represented by the reaction (LCl3+Na3VO4→ LVO4+NaCl) (L = Y, La) is proposed to synthesize technologically important rare-earth phosphates and vanadates.
Abstract: A novel solid-state metathesis (SSM) approach represented by the reaction (LCl3 + Na3VO4 → LVO4 + NaCl) (L = Y, La) is proposed to synthesize technologically important rare-earth phosphates and vanadates. The SSM reaction, driven in the forward direction is facilitated by the formation of high lattice energy by-product like NaCl. The structural, thermal, optical, and chemical properties of synthesized powders are determined by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and diffused reflectance (DR) spectra in the UV–vis range. The direct band gap of the synthesized materials was found out to be ∼3.5 eV for LaVO4, YVO4, YPO4 and ∼2.6 eV for LaPO4.
TL;DR: In this paper, a graph-based method based on graph theory was proposed for interpolation calculation of the lattice energy U of ionic crystals, based on revealing matrix correlation between the ionic radii and U values for MX compounds, where M is a metal and X is halogen, hydrogen, or chalcogen.
Abstract: A new method based on graph theory was suggested for interpolation calculation of the lattice energy U of ionic crystals. The method is based on revealing matrix correlation between the ionic radii and U values for MX compounds, where M is a metal and X is halogen, hydrogen, or chalcogen. A new formula was obtained for calculating the lattice energy solely from the ionic radii, without introduction of abitrary factors. The mean error of determining U for alkali metal halides is 0.49%. The lattice energies were calculated for a large group of inorganic substances. The accuracy of the interpolation calculation of the lattice energy of ionic crystals depends on the degree of ionicity of the bond: With an increase in the covalent contribution, the error increases.
TL;DR: In this paper, the effect of x on c-lattice parameters, activation energy, and the magnetic power constant of activation energy were investigated. But the results of the experiments were limited to the case of x = 0.
Journal Article•
TL;DR: In this paper, a fast and reliable Monte Carlo simulated annealing process (MC-SA) was used to search for probable crystal packing alternatives on lattice energy hyper-surface; optionally, these potential structures were clustered into unique groups based on packing similarity.
Abstract: The crystal structure prediction of 5′-GMP was performed using Polymorph in Cerius~2.A fast and reliable Monte Carlo simulated annealing process(MC-SA) was used to search for probable crystal packing alternatives on lattice energy hyper-surface;Optionally,these potential structures were clustered into unique groups based on packing similarity.The geometry of each unique structure was optimized with respect to all degrees of freedom or with rigid body constraints where the relative distance between a group of atoms are fixed;the optimized structures were clustered again to remove duplicates;the final structures were ranked according to lattice energy.Finally,the most likely stable space group of 5′-GMP was found to be P2_12_12_1.
27 Mar 2008
TL;DR: In this article, the effects of various treatments of the intermolecular potential, inclusion of intramolecular flexibility, and simulation size dependence of the results were considered, and the density, lattice energy, and lattice parameters are made over a wide domain of pressures; thereby allowing for predictions of the bulk and linear coefficients of isothermal expansion of the crystals.
Abstract: Atomistic Monte Carlo simulations in the NpT ensemble are used to calculate the physical properties of crystalline RDX, β-HMX, and TATB. Among the issues being considered are the effects of various treatments of the intermolecular potential, inclusion of intramolecular flexibility, and simulation size dependence of the results. Calculations of the density, lattice energy, and lattice parameters are made over a wide domain of pressures; thereby allowing for predictions of the bulk and linear coefficients of isothermal expansion of the crystals. Comparison with experiment is made where possible.