Salvatore Califano

Lattice dynamics of molecular crystals

1 Lattice Dynamics.- 1.1 Introduction.- 1.2 The Dynamical Equations in Cartesian Coordinates.- 1.3 Dispersion Curves. Acoustic and Optical Modes.- 1.4 Invariance Conditions.- 1.5 Molecular Coordinates.- 1.6 The Dynamical Equations in Molecular Coordinates.- 2 Symmetry.- 2.1 Space Group Symmetry.- 2.2 Irreducible Representations of the Translational Group.- 2.3 Irreducible Representations of the Space Groups.- 2.4 Time Reversal.- 2.5 Symmetry of the Dynamical Matrix.- 2.6 Symmetry Properties of Vibrational States.- 2.7 Selection Rules.- 3 Intermolecular Potentials.- 3.1 The Crystal Potential.- 3.2 The Intramolecular Potential.- 3.3 The Intermolecular Potential.- 3.4 Intermolecular Force Constants.- 3.5 Lattice Sums and Ewalds Method.- 2.6 Calculation of Phonon Frequencies.- 4 Anharmonic Interactions.- 4.1 Introduction.- 4.2 The Crystal Hamiltonian.- 4.3 Quantum Field Treatment of Phonons.- 4.4 The Hamiltonian Renormalization Procedure.- 4.5 The Self-Consistent Phonon Method.- 4.6 The Method of the Greens Functions.- 4.7 Thermal Strain.- 4.8 Anharmonic Calculations.- 5 Two-Phonon Spectra of Molecular Crystals.- 5.1 Introduction.- 5.2 General Considerations.- 5.3 Two-phonon Density of States.- 5.4 Two-phonon Absorption Coefficients.- 6 Infrared and Raman Intensities in Molecular Crystals.- 6.1 Introduction.- 6.2 Historical Survey.- 6.3 Electrostatic Model of Infrared and Raman Intensities of Molecular Crystals.

Electrical response in chemical potential equalization schemes

In this paper we compare the polarization response given by two different chemical potential equalization schemes to be applied to molecular dynamics simulations: the standard fluctuating point charge model (FQ) and the atom–atom charge transfer model (AACT). We have tested the transferability of FQ and AACT parameters, fitted to the polarizability of small size alkanes and polyenes, to large size homologues. We show that the FQ scheme is not adequate for the n-alkanes as it strongly overestimates the polarizability tensor components as the number of carbon atoms increases. The FQ approach has been found more predictive for highly conjugated systems like polyenes, although still unsatisfactory. The AACT parameters tuned on ethane are instead perfectly transferable to alkanes of any length and conformation. The AACT scheme satisfactorily reproduces the polarization response also for highly conjugated systems.

Anharmonic processes in molecular crystals. Calculation of the anharmonic shifts, bandwidths and energy decay processes in crystalline naphthalene

Abstract The anharmonic frequencies and the bandwidths of the lattice phonons of naphthalene at 4 K have been calculated using an intermolecular potential which includes atom-atom and quadrupole-quadrupole contributions. The calculated bandwidths agree well with the available experimental data. The contribution of the cubic and quartic terms of the crystal hamiltonian to the anharmonic shifts is discussed in terms of phonon diagrams. The calculated quartic shifts are found to be positive whereas the cubic shifts are smaller and negative. The total anharmonic shifts are then positive and it is shown that this is in agreement with the observed dependence of the phonon frequencies with temperature. The mechanism of energy transfer between the optical lattice phonons and the two-phonon manifold of the crystal is discussed in terms of phonon-phonon coupling processes. Up and down conversions are analyzed and their relative efficiency is evaluated. The temperature dependence of both the anharmonic shifts and bandwidths is studied in the range 4–68 K at constant volume. Comparison with the available experimental data at constant pressure is discussed.

Glycerol condensed phases Part II.A molecular dynamics study of the conformational structure and hydrogen bonding

An analysis of the conformational properties and hydrogen bonding in the condensed phases of glycerol is reported using the same model as adopted in Part I (Phys. Chem. Chem. Phys., 1999, 1, 871). Structural properties of the liquid and glassy states are analyzed in relation to the molecular backbone conformation of the glycerol molecule. The effects of hydrogen bonding and of temperature on the conformational distribution are analyzed. The structural and dynamical properties of hydrogen bonding in glycerol are also investigated. The results are consistent with available experimental observations and clarify many important and interrelated aspects of the microscopic structure of liquid, glassy and crystalline phases of glycerol.

Vibrational relaxation in molecular crystals

Abstract The relaxation processes responsible for the finite lifetime of excited vibrational states in molecular crystals are discussed on the basis of recent experimental and theoretical developments. Experimental information on the lifetime of optically active phonons is obtained in the time domain by time-resolved CARS spectroscopy or in the frequency domain by high-resolution infrared and Raman spectroscopy. The experimental results are then interpreted in terms of many-body perturbation techniques or of molecular dynamics simulation. The elementary mechanisms involved in vibrational energy relaxation processes are discussed in detail. These can be summarized as: (a) depopulation processes of phonon states via energy transfer to other phonon states; (b) pure dephasing processes due to interaction with thermal bath phonons; and (c) depopulation or scattering processes due to impurities and crystal defects. The contributions to the phonon lifetime and band profile of these different mechanisms are analy...

Lattice dynamics of molecular crystals using atom—atom and multipole—multipole potentials

Abstract The theory of the lattice dynamics of a molecular crystal assuming an atom—atom and a multipole—multipole potential is developed. Expressions are d

Glycerol condensed phases Part I. A molecular dynamics study

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.

Raman Spectrum of a Single Crystal of Benzene

The Raman spectrum of a single crystal of benzene at 140°K has been obtained using an argon ion laser source. The single crystals were oriented with the aid of a polarizing microscope. This technique was found suitable for low temperature work. Despite some cracking of the sample, the polarization was defined well enough to make the assignment of most librational lattice modes certain. The internal vibrations were also investigated, and for ν6, ν9, and ν10 the observed peaks have been assigned to their proper factor group species. The agreement between observed frequencies and those calculated in this laboratory is extremely good for the lattice vibrations, and acceptable for the internal modes.

Simulated structure, dynamics, and vibrational spectra of liquid benzene

A classical molecular dynamics simulation of liquid benzene is performed, using a potential model which allows for full molecular flexibility. The short range intermolecular radial distribution function is on average reminiscent of the crystalline structure, although practically no preferential orientation can be found for the molecules in the first coordination shell. The average cage lifetime and its vibrational dynamics are obtained from appropriate time correlation functions. The intramolecular vibrations are investigated by calculating the vibrational density of states and the infrared and Raman spectra, achieving an excellent agreement with the experimental data. Finally, the dephasing of the ν1(A1g) ring breathing mode and of the ν6(E2g) in-plane bending mode is analyzed on the basis of the Kubo dephasing function. For ν1 mode the Kubo correlation time of 516 fs agrees with the experimental value, and is consistent with a relaxation mechanism involving the cage reorganization. In contrast, ν6 has a...

Lattice dynamics of crystalline ammonia and deutero-ammonia

Abstract The theory of lattice dynamics of molecular crystals using atom-atom and multipole-multipole potentials developed in a previous paper is applied to crystalline ammonia. It is found that the electrostatic terms play an important part in calculations of the crystal energy, structure and lattice vibrations.