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The 3:2 Charge Transfer (CT) co-crystal (Perylene)3(TCNQF1)2 is grown by the Physical Vapor Transport (PVT) method, and characterized structurally and spectroscopically. Infrared analysis of the charge sensitive modes reveals a low degree of charge transfer (less than 0.1) between donor and acceptor molecules. The crystal is isostructural to the other 3:2 CT crystals formed by Perylene with TCNQF2 and TCNQF4, whereas such stoichiometry and packing is not known for the CT crystals with non-fluorinated TCNQ. The analysis of the isostructural family of 3:2 Perylene–TCNQFx (x = 1,2,4) co-crystal put in evidence the role of weak F…HC bonding in stabilizing this type of structure, The work has been supported by the local research funds of Bologna and Parma Universities.

We show that the development of highly accurate density functional theory calculations coupled to low-frequency Raman spectroscopy constitutes a valid method for polymorph characterization alternative/complementary to X-ray. The method is applied here to the temperature-induced, first-order phase transition of coronene, known for a long time, but has remained structurally uncharacterized due to crystal breaking during the process. The astonishing fidelity of the Raman calculated spectra to the experiments allows us to unambiguously identify the low-temperature phase with the β-coronene polymorph, recently reported as new and obtained in the presence of a magnetic field. We also suggest that additional measurements are needed to confirm that a magnetic field can actually drive the growth of a β-polymorph surviving indefinitely at ambient temperature.

The photodimerization of anthracene derivatives constitutes a model system for intermolecular [4 + 4] cycloadditions. In this paper we deal with the elusive 9-anthracene–carboxylic case and study the crystal state of the head-to-tail dimer, obtained by the reaction of the monomer in various solvents both in its unary and solvated forms, by X-ray diffraction and confocal Raman microscopy in the lattice phonon region. A number of solvates have been identified, and their structures have been solved and here presented. The 9-anthracene–carboxylic acid dimer appears to be an exemplary case of a molecular crystal easily prone to host solvent molecules in the interstices of the framework generated by homomolecular hydrogen bonds. Alternatively, the hydrogen bonds between solvent molecules and the carboxylic group may establish supramolecular structures of closely packed architectures. Raman microscopy has also allowed us to investigate the short-lived dimer, which is produced in the crystal-to-crystal photoreaction triggered by the irradiation of the monomer single crystal.

Recently, structural studies in the thin-film phase of 6,6′-dibromoindigo, the main component of the Tyrian purple dye, revealed the presence of surface-induced structures. This finding is important, as the system is a good candidate for applications in organic electronics, and knowledge of the film structure is needed to explain the physical properties of the conducting layer. In this work, combining the results of low-frequency Raman spectroscopy, X-ray diffraction, and accurate density-functional theory (DFT) calculations of the vibrational properties of the proposed surface-induced structures, we suggest that a newly detected 6,6′-dibromoindigo bulk crystal form is structurally closely related to the surface-induced modifications. Thus, 6,6′-dibromoindigo behavior shows similarities and differences at the same time, with respect to other investigated systems., We thank CINECA Supercomputing Center for providing computer time through the ISCRA scheme (project C—SCSCDFT). T.S. acknowledges the H2020-MSCA-COFUND-2014 Programme (P-SPHERE, Grant agreement 665919). The authors thank Prof. Roland Resel for the fruitful discussions. This work has been funded by the PRIN project “From natural to artificial light-harvesting systems: unveiling fundamental processes toward a bio-inspired materials design” (HARVEST) protocol 201795SBA3.

Singlet exciton fission is a spin-allowed process in organic semiconductors by which one absorbed photon generates two triplet excitons. Theory predicts that singlet fission is mediated by intermolecular charge-transfer states in solid-state materials with appropriate singlet-triplet energy spacing, but direct evidence for the involvement of such states in the process has not been provided yet. Here, we report on the observation of subpicosecond singlet fission in mixed films of pentacene and perfluoropentacene. By combining transient spectroscopy measurements to nonadiabatic quantum-dynamics simulations, we show that direct excitation in the charge-transfer absorption band of the mixed films leads to the formation of triplet excitons, unambiguously proving that they act as intermediate states in the fission process.

International audience; Thin films of the natural semiconductor indigo have been grown on Si/SiO2 substrates by physical vapour deposition under ultra-high vacuum conditions, monitoring the influence of the deposition rate on structure, morphology and thickness. This was done to allow well-defined films to be made in a controlled way, suitable for the manufacture of organic devices and the investigation of charge transport phenomena. The films have been characterized by X-ray diffraction, atomic force microscopy, microRaman spectroscopy and transmission electron microscopy, finding that the growth at the solid interface of polymorph B, the only phase present in the films, is governed by the formation of strong multiple hydrogen-bonds between neighbouring molecules, which play a predominant role in the interface organization of the molecular assemblies.

The classic photodimerization reaction of 9-cyanoanthracene in the solid state has been studied by means of Raman microscopy, analyzing the time evolution both of the molecular and of the lattice phonon spectra from the reactant to the product. This has allowed us to verify that the formation of the dimer crystal structure always takes place with some delay with respect to the onset of the chemical reaction, as already observed in another crystal to crystal photoreaction.1 This evidence points to a phenomenon of reconstructive phase transition.2 The kinetic analysis of the spectroscopic data confirms the previous hypothesis of an autocatalytic mechanism for the process.

Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics.

Raman microscopy in the lattice phonon region coupled with X-ray diffraction have been used to study the polymorphism in crystals and microcrystals of the organic semiconductor 9,10-diphenylanthracene (DPA) obtained by various methods. While solution grown specimens all display the well-known monoclinic structure widely reported in the literature, by varying the growth conditions two more polymorphs have been obtained, either from the melt or by sublimation. By injecting water as a nonsolvent in a DPA solution, one of the two new polymorphs was predominantly obtained in the shape of microribbons. Lattice energy calculations allow us to assess the relative thermodynamic stability of the polymorphs and verify that the energies of the different phases are very sensitive to the details of the molecular geometry adopted in the solid state. The mobility channels of DPA polymorphs are shortly investigated.

We employ atomistic molecular dynamics simulations to predict the alignment and anchoring strength of a typical nematic liquid crystal, 4-n-penty1-4'-cyano biphenyl (5CB), on different forms of silica. In particular, we study a thin (similar to 20 nm) film of 5CB supported on surfaces of crystalline (cristobalite) and amorphous silica of different roughness. We find that the orientational order at the surface and the anchoring strength depend on the morphology of the silica surface and its roughness. Cristobalite yields a uniform planar orientation and increases the order at the surface with respect to the bulk whereas amorphous glass has a disordering effect. Despite the low order at the amorphous surfaces, a planar orientation is established with a persistence length into the film higher than the one obtained for cristobalite.

Within the reinvestigation of Ni–Sb carbonyl cluster chemistry, we report here the synthesis and characterization of the new [Ni19Sb4(CO)26]4– cluster and the synthesis, structure, magnetic characterization and electrical resistivity of the viologen salts of the previously known [Ni13Sb2(CO)24]n– (n = 2, 3) anionic species. The crystal structures of [NEt4]4[Ni19Sb4(CO)26], [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 and [EtV]3[Ni13Sb2(CO)24]·1.5THF (EtV = 1,1′-diethyl-4,4′-bipyridilium cation, DMF = N,N-dimethylformamide, THF = tetrahydrofuran) are described. Notably, the unit cell of [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 involves a mixture of two [Ni13Sb2(CO)24]3– trianions and one [Ni13Sb2(CO)24]2– dianion, as it also contains eight [EtV]+· radical monocations, which are assembled in infinite stacks. In contrast, the unit cell of the [EtV]3[Ni13Sb2(CO)24]·1.5THF salt contains four [Ni13Sb2(CO)24]3– trianions along with twelve [EtV]+· radical monocations, four of which are arranged into two pairs of isolated dimers, whereas the other two sets of four form two infinite stacks that extend over the whole crystal. The charges of the miscellaneous ions have been assigned on the basis of electroneutrality and spectroscopic evidence. More specifically, the infrared spectra of [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14, both in the solid state and in solution, clearly indicate the presence of a 2:1 mixture of [Ni13Sb2(CO)24]3– and [Ni13Sb2(CO)24]2– anions. Resistivity measurements performed on pellets of powdered samples indicate that the [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 salt substantially behaves as an insulator. A study of the magnetic behaviour of [EtV]8[Ni13Sb2(CO)24]3·4DMF·2C6H14 evidences pairing among the electrons of the EtV+· molecules, in agreement with DFT calculations, and the odd-electron clusters behave as paramagnetic centres of spin S = 1.

We present a Raman investigation of polymorphism in 1,1,4,4-tetraphenyl-butadiene (TPB), a well-known blue luminescent molecule, which retains its emissive properties in the solid state. The use of low-wavenumber (10–150 cm−1) Raman microscopy, very sensitive to the crystal packing, allows us to single and pick up four different polymorphs. X-ray analysis shows that the TPB molecules assume different conformations in the various polymorphs, a fact revealed also by the Raman spectra in the region of intra-molecular vibrations. Lattice dynamics calculations yield a preliminary assignment of the low-wavenumber Raman spectra and provide information of the relative stability of the phases. Copyright © 2013 John Wiley & Sons, Ltd.

We present a working example of phase recognition by lattice phonon Raman spectroscopy, a technique which has proved to be a powerful tool for identifying new polymorphs. The spectra show that a newly resolved triclinic structure of the organic semiconductor dibenzo-tetrathiafulvalene (DB-TTF) is one of its four polymorphic modifications, labeled the δ-phase, which was previously identified only from spectroscopic data. The now available XRD data allow for the computation of the minimum potential energy structure. Calculated density and energy of the triclinic polymorph are found to be very close to the previously identified β polymorph. These similarities account for the often encountered co-existence of the β and δ crystal phases.

A multiscale investigation of N,N-bis(n-octyl)-x:y, dicyanoperylene-3,4:9,10- bis(dicarboximide), PDI8-CN2, shows the same molecular arrangement in the bulk and in thin films sublimated on SiO2/Si wafers. Non-conventional powder diffraction methods and theoretical calculations concur to provide a coherent picture of the crystalline structure. X-ray diffraction (XRD) and atomic force microscopy (AFM) analyses of films of different thickness depos- ited at different substrate temperatures indicate the existence of two temper- ature-dependent deposition regimes: a low-temperature (room temperature) regime and a high-temperature (80­120 °C) one, each characterized by dif- ferent growth mechanisms. These mechanisms eventually result in different morphological and structural features of the films, which appear to be highly correlated with the trend of the electrical parameters that are measured in PDI8-CN2-based field-effect transistors.

Absorption, photoluminescence, and Raman scattering measurements have been performed on a fourfold alkoxy-substituted phthalocyanine in solution and in the discotic liquid crystal phase. As already known for this class of compounds, the features of the electronic spectra in the condensed state are determined by the spatial closeness and arrangement of the systems of the phthalocyanine cores, with the occurrence of collectively excited states and excimer-like emission. Quantitative information about the alignment of the molecules which compose the liquid crystal phase has been obtained from the analysis of polarized Raman spectra of homeotropically aligned samples.

We report a combined spectroscopic and structural study of polymorphism of 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) blue luminescent crystals. We have identified two polymorphs, which can be grown by crystallization of the vapor. The first, α-TPB, has the monoclinic noncentrosymmetric structure (P2 1, two molecules per unit cell), whose lattice parameters have already been reported in the literature. The second, β-TPB, is a new polymorph, monoclinic centrosymmetric (P21/c, four molecules per unit cell). The previously reported triclinic structure, γ-TPB, has not been identified in samples grown by the physical vapor transport method. The effects of the different molecular configuration and packing of α- and β-TPB on Raman scattering, IR, and UV-vis optical properties are discussed.

Car-Parrinello molecular dynamics (CPMD) calculations are presented for a Na (+)(Phe) complex in aqueous solution and for various stable Na (+)(Phe) complexes and Na (+)(H 2O) n clusters in the gas phase (with up to six water molecules). The CPMD results are compared to available experimental and ab initio reference data, to DFT results obtained with various combinations of density functionals and basis sets, and to previous classical mechanics MD simulations. The agreement with the reference data in the gas phase validates the CPMD method, showing that it is a valid approach for studying these systems and that it describes correctly the competing Na (+)-Phe and Na (+)-H 2O interactions. Analysis of MD trajectories reveals that the Na (+)(Phe) complex in aqueous solution maintains a stable configuration in which the Na (+) cation hovers above the phenyl ring, at an average distance of 3.85 A from the ring center, while remaining strongly bound to one of the carboxylic oxygens of Phe. Constrained MD simulations indicate that the free energy barrier opposing dissociation of the complex exceeds 5.5 kcal/mol. We thus confirm that "cation- pi" interactions between alcali cations and the pi ring, combined with other kinds of interactions, may allow aromatic amino acids to overcome the competition with water in binding a cation.

Using sexithiophene as a benchmark compound, we present a very effective strategy for searching the potential energy minima of a crystalline material, described in terms of rigid molecules with Coulombic and atom-atom interactions. The strategy involves uniform sampling of the many-body energy hypersurface, mechanical identification of all constraints deriving from the crystallographic symmetry, and a "sight-resight" method, originally introduced in wildlife ecology, for assessing the completeness of the search. Thousands of distinct potential energy minima, with a surprising variety of structural arrangements, are identified for sexithiophene. Despite the large number of competing minima, the system presents a small number of deep minima, with very different structures and not particularly congested in energy or density. The two deepest minima correspond to the structures of the two known experimental polymorphs, which are satisfactorily described.

International audience; For molecules with C-H bonds, a local mode description of the bonds is often adopted to interpret the C-H stretching overtone spectra, although a representation in terms of normal mode is also possible. Equivalence between the two alternative representations demands simple numerical equations between the normal mode anharmonicity constants xrs and the quartic anharmonic coupling constants Krstu , the so-called x-K relations. The relations appropriate to low symmetry molecules with two non-equivalent C-H bonds (ν1 , ν2), such as XHC=CHY, are derived in this paper. The consequences of these relations are illustrated with a detailed analysis of the C-H stretching overtone region, where the naive application of the relations obtained for more symmetric molecules could lead to a different interpretation of the overtones 2ν1 , 2ν2 and of the combination band ν1 + ν2 . The relations are tested with theoretical calculations for difluoro- and chlorofluoro-substituted ethenes, propadiene, and butatrienes.

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.

Raman phonon spectra of 9,10-dimethylanthracene crystals have been measured as a function of pressure in the range 0–10 GPa for both lattice and intramolecular modes. Quasi-harmonic lattice dynamics calculations have been used to analyze the effects of the vibrational degrees of freedom on the pressure dependence of the crystallographic structure and phonon wave numbers. We find that the coupling between lattice and intramolecular vibrations contributes significantly to the pressure dependence of the phonon spectra, which is accurately reproduced by the calculations. Vibrational effects on the structure, instead, are less significant.

Raman phonon spectra of tetrathiafulvalene (TTF) crystals in their monoclinic (α-TTF) and triclinic (β-TTF) polymorphic modifications have been recorded as a function of temperature in the 30 to 130 cm−1 range. Along with other model potentials, the set of parameters already adopted for the study of ionic and neutral forms of bis-ethylen-dithio-tetrathiafulvalene (BEDT-TTF) has been used to compute, within a quasi-harmonic lattice dynamics framework, the crystallographic structures and the phonon frequencies of the two polymorphs as a function of temperature with good results. Introduction of the internal vibrational degrees of freedom of the TTF molecules in the crystals improves the computational approach and proves to be a valuable tool to judge the importance of the coupling between lattice phonons and intramolecular vibrations.

The crystal structure and lattice phonons of the $(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{I}}_{3}$ superconducting $\ensuremath{\beta}$ phase (where BEDT-TTF is bis-ethylen-dithio-tetrathiafulvalene) are computed and analyzed by the quasiharmonic lattice dynamics (QHLD) method. The empirical atom-atom potential is that successfully employed for neutral BEDT-TTF and for nonsuperconducting $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}{\mathrm{I}}_{3}.$ The rigid molecule approximation has to be relaxed by allowing mixing between lattice and low-frequency intramolecular vibrations. Such a mixing is found to be essential to account for the specific heat measurements, and also yields good agreement with the observed Raman and infrared frequencies. The crystal structure and its temperature and pressure dependence are also properly reproduced, though the effect of the mixing is less important in this case. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The charge transfer integrals are evaluated by the extended H\"uckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function ${\ensuremath{\alpha}}^{2}(\ensuremath{\omega})F(\ensuremath{\omega}),$ which compares well with that experimentally obtained from point contact spectroscopy. The corresponding dimensionless coupling constant $\ensuremath{\lambda}$ is found to be $\ensuremath{\sim}0.4.$

An effective one-dimensional Hamiltonian has been determined for the NH 2 inversion motion in aniline. The anharmonic potential is represented by a quartic polynomial, fitted to the experimental vibrational transitions in the electronic states S 0 and S 1 . In the S 0 state the barrier height is about 580 cm −1 and the equilibrium angle between the NH 2 and the ring planes is 44°, while in the S 1 state the potential well is extremely flat without a well-defined equilibrium angle. These results are compared with those obtained by ab initio methods. The long-standing discrepancy between the rotational and vibrational results is partially clarified.

Using a model potential function we have performed a molecular dynamics simulation of several static and dynamical properties of glycerol in the crystal, glass and liquid phases. Comparison with available experimental data shows an excellent agreeent and proves the validity of the potential model used. For the calculation of the molar specific heat of the liquid and of the glass we have developed a theoretical approach which takes into account the contributions of the conformational structure energy and of the vibrational energy computed using the Bose–Einstein statistics.

We present a quasi harmonic lattice dynamics calculation for the crystal structure and the phonon frequencies of orthorhombic and monoclinic benzene as a function of pressure and temperature. We employ a site-site potential model including Coulombic interactions, which were found necessary to obtain the correct low pressure phase. The calculations agree with the measurements in both phases and reproduce very well the experimental phase diagram. When applicable, quasi harmonic lattice dynamics is an accurate and extremely efficient method, which promises to outperform molecular dynamics and Monte Carlo simulations.

We choose naphthalene as a model system and perform a quasi-harmonic lattice dynamics calculation. The entropic and zero-point effects due to the lattice phonons are investigated and some commonly used potential models are tested. We find that quite reliable predictions of the crystallographic structure and the phonon frequencies of both temperature and pressure are feasible.

The photochemical reaction of 9,10-dinitroanthracene (DNO(2)A) to anthraquinone (AQ) + 2NO has been studied by means of lattice phonon Raman spectroscopy in the spectral region 10-150 cm(-1). In fact, crystal-to-crystal transformations are best revealed by following changes in the lattice modes, as even small modifications in the crystal structure lead to dramatic changes in symmetry and selection rules of vibrational modes. While analysis of the lattice modes allowed for the study of the physical changes, the chemical transformation was monitored by measuring the intramolecular Raman-active modes of both reactant and product. On the basis of the experimental data it has been possible, at a microscopic level, to infer crucial information on the reaction mechanism by simultaneously detecting molecular (vibrational modes) and crystal structure (lattice phonons) modifications during the reaction. At a macroscopic level we have detected an intriguing relationship between incident photons and mechanical strain, which manifests itself as a striking bending and unfolding of the specimens under irradiation. To clarify the mechanisms underlying the relationship between incoming light and molecular environment, we have extended the study to high pressure up to 2 GPa. It has been found that above 1 GPa the photoreaction becomes inhibited. The solid-state transformation has also been theoretically modeled, thus identifying the reaction pathway along which the DNO(2)A crystal lattice deforms to finally become the crystal lattice of the AQ product.

We have performed a molecular dynamics calculation of the vibrational spectrum of glassy silica. The simulated spectra compare favourably with the neutron scattering experiments, in both frequency and momentum space. A detailed analysis of one-body and two-body correlation functions yields an assignment of the spectrum which exhibits a high degree of internal consistency and agrees with the current understanding of the experimental data.

The paper reports a complete analysis of the phonon structure of crystalline picene, a recently announced organic semiconductor. Both lattice and intramolecular vibrations are investigated. An exhaustive assignment of lattice phonons is obtained through polarized Raman spectra assisted by lattice dynamics calculations based on a well tested atom–atom potential model. Raman, infrared spectra and density functional (DFT) calculations are used for the characterization of intramolecular modes. Coupling between low-frequency molecular vibrations and lattice phonons is accounted for. Molecule-to-molecule transfer integrals, as well as the Peierls and Holstein (non-local and local) coupling constants, are evaluated through the semiempirical method INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization).

The nature of the interaction of a series of α-oligothiophenes (1T up to 6T) with a Si(001) surface was studied by means of density functional theory (DFT) structural optimizations. For 1T, we foun...

A molecular dynamic computer simulation of the cooling of liquid silica to form an amorphous solid (glassy) phase was performed using the potential of Tsuneyuki, Tsukuda, Aoki, and Matsui, which was found to be consistent with the structure of several crystalline phases of silica. In the simulations, at low temperatures the atoms formed an almost perfect random tetrahedral network, and on subsequent annealing the material formed a perfect network, with each silicon having four oxygen neighbors and each oxygen having two silicon neighbors. The simulated material at low temperatures has a structure that is in close agreement with experimental neutron scattering measurements on amorphous silica. Several properties of the material were calculated and compared with experiment, and the nature of the ‘‘inherent structure’’ of the material as a function of temperature was investigated. The potential of Tsuneyuki et al. provides an excellent model for amorphous silica as well as crystalline silica.

Lattice dynamics calculations have been performed for crystalline SiO2, using the two‐body potential model developed by Tsuneyuki et al. [Phys. Rev. Lett. 61, 869 (1988)]. The computed phonon frequencies agree reasonably well with experiment. Minor deficiencies suggest that the Si–O force constant obtained from the model is too weak.

We have systematically sampled the potential energy surface of crystalline tetracene to identify its local minima. These minima represent all possible stable configurations and constitute the "inherent structures" of the system. The crystal is described in terms of rigid molecules with Coulombic and atom-atom interactions. Hundreds of distinct minima are identified, mostly belonging to the space groups P-1 (triclinic) and P21/c (monoclinic), with a variety of structural arrangements. The deepest minimum corresponds to the high temperature-low pressure polymorph. This is the only polymorph with a completely described X-ray structure, which is satisfactorily described by the calculations. The next deep minimum is likely to correspond to the low temperature-high pressure polymorph, which has been experimentally identified but not yet fully described.

The competition between cation-pi interaction and aqueous solvation for the Na+ ion has been investigated by molecular dynamics simulations, using the phenylalanine amino acid as the test pi system. Starting from one of the best standard force fields, we have developed new parameters that significantly improve the agreement with experimental and high quality quantum mechanical results for the complexes of Na+ with phenylalanine, benzene, and water. The modified force field performs very well in forecasting energy and geometry of cation coordination for the complexes. Next, analysis of MD trajectories and steered MD simulations indicate that the Na+-phenylalanine complex survives for a significant time in aqueous solution and that the free energy barrier opposing dissociation of the complex is sizable. Finally, we analyze the role of different intermolecular interactions in determining the preference for cation-pi bonding with respect to aqueous solvation. We thus confirm that the Na+-phenylalanine stabilization energy may overcome the interactions with water.

We have performed a lattice dynamics calculation to compute the "inherent structures" of minimum potential energy for pentacene, starting from available X-ray data. The calculation shows that two distinct bulk crystalline phases of pentacene exist, with very subtle structural differences but clearly different phonon spectra. The method of crystal growth (from solution or vapor) is not the determining factor for obtaining either structure.

Catastrophe theory has been applied to classify the properties of the potential surfaces of excimer states in some anthracene derivatives. The observed dependence of the excimer emission frequencies with pressure has been reproduced using as model for the excited state surface an anharmonic quartic potential modulated by pressure. A discussion is given in terms of “stable” and “unstable” systems on the basis of the deformation of the anharmonic potential surface when pressure is changed. Unstable systems, like 9,10-dichloro- and 9,10-dibromo-anthracene, undergo pressure-induced phase transitions.

We present Molecular Dynamics (MD), Quasi Harmonic Lattice Dynamics (QHLD) and Energy Minimization (EM) calculations for the crystal structure of Ne, Ar, Kr and Xe as a function of pressure and temperature. New Lennard-Jones (LJ) parameters are obtained for Ne, Kr and Xe to reproduce the experimental pressure dependence of the density. We employ a simple method which combines results of QHLD and MD calculations to achieve densities in good agreement with experiment from 0 K to melting. Melting is discussed in connection with intrinsic instability of the solid as given by the QHLD approximation. (See http://www.fci.unibo.it/~valle for related papers), Comment: 7 pages, 5 figures, REVtex

We discuss the double-time Green's-function approach for the phonon-phonon interaction in anharmonic solids. Using a certain interpretation of the phonon operators, the crystal Hamiltonian may be rewritten in a compact form. The equation of motion for the Green's function is tremendously simplified with this formalism. The self-energy can be obtained in a very simple way, to any desired perturbation order. Our results reproduce the corresponding terms of the available literature results, which are limited to the fourth order in the perturbation parameter.

The lattice stability of YBa2Cu3Ox is studied using a rigid ion model. The semiconducting compound, YBa2Cu3O6, is stable, while superconducting YBa2Cu3O7 is unstable. The vibrational eigenstates of the phonon modes which destabilize YBa2Cu3O7 have properties which may greatly enhance the electron-phonon coupling.

The lattice dynamics of α-CO is studied assuming a fully-ordered antiferro P213 structure, using a potential model which includes electrostatic, dispersion and exchange interactions. The coupling of translational and librational motions leads to mixed modes at finite wave vectors and to anomalous acoustic dispersion. Anharmonic frequency shifts and the damping due to phonon—phonon interactions are evaluated for the zero wave vector modes. The single-particle potential wells for the high-frequency librational modes are investigated and suggest that translation—rotation coupling plays a primary role in the molecular reorientations.

Nonlinear four-wave light scattering (FWLS) on a three-level system is analyzed, adopting the density operator approach. A strong field Ω 1 is mixed by the material system with two fields Ω 2 and Ω 3 to yield an output field peaked at the frequency Ω 4 = Ω 1 + Ω 3 - Ω 2 . The theory is developed within the dipole and Markov approximations. The strong resonant field is treated “exactly”, while the other fields are regarded as perturbations. The generated intensity is obtained as the expectation value of the photon number operator, estimated at the sixth order in the weak-field expansion. The generation of a coherent field at the frequency Ω 4 is due to processes where either a single molecule or a couple of molecules are involved. The total intensity is interpreted as the superposition of four contributions: a four-photon scattering process (FPS), a three-photon process (TPS), and two more processes related to resonance Raman scattering (RRS) and resonance fluorescence (RF). The method allows the deduction of nonlinearities in the generated intensity which are not included in fully perturbative treatment, and which, in the strong-field limit, originate the Rabi intensity dip of the lines.

We discuss the transient evolution of the coherent and incoherent emission from a three-level system driven by a resonant laser field of arbitrary intensity. The effects due to the levels finite lifetimes and dephasings and to the laser power are considered, within the Markov and rotating wave approximations. An analytical expression for the time-dependent generated signal is derived, enabling the use of time-resolved spectroscopy to extract the parameters characterizing the level dynamics.

The anharmonic relaxation time of the totally symmetric ring stretching mode of crystalline benzene has been calculated at room temperature and at different pressures between 0.2 and 4 GPa; the perturbative method adopted for the calculation accounts for the three-phonon scattering decay processes. The pressure dependence of the calculated linewidth is in qualitative agreement with experiment, although the calculated values are consistently lower than the experimental ones.

Unlike translational stability, torsional stability in molecular crystals can be extremely sensitive to the details of the adopted potential model. The

In resonance Raman scattering experiments both coherent and incoherent processes occur, their relative intensities being determined by the magnitudes of the statistical fluctuations and laser beam intensity. In the weak-field limit coherent processes dominate, and the competition between resonance fluorescence and inverse Raman scattering is determinant. In the strong-field limit incoherent processes are enhanced, and the emitted radiation is essentially fluorescent in character. In particular we show that, under stimulated conditions, inverse resonance Raman scattering can, in principle, be observed directly.

The steady-state response of a three-level system in the presence of a strong laser field is described with the density operator formalism, in order to derive an analytical expression for the intensity of resonance Raman scattering. The Liouville equation for the density operator is written in quantum form for both the molecular system and the radiation field, making use of the dipole and rotating-wave approximations for the matter-radiation interaction, and of the Markov approximation for the molecular decays. The equation is solved exactly for the laser field, and in perturbative way for the generated field. The results account for power broadening of the generated line. Rabi intensity-dip of the generated signal, and for the excitation profiles in the Franck-Condom approximation.