A class of linear viscoelastic models based on Bessel functions

In this paper we investigate a general class of linear viscoelastic models whose creep and relaxation memory functions are expressed in Laplace domain by suitable ratios of modified Bessel functions of contiguous order. In time domain these functions are shown to be expressed by Dirichlet series (that is infinite Prony series). It follows that the corresponding creep compliance and relaxation modulus turn out to be characterized by infinite discrete spectra of retardation and relaxation time respectively. As a matter of fact, we get a class of viscoelastic models depending on a real parameter \(\nu > -1\). Such models exhibit rheological properties akin to those of a fractional Maxwell model (of order 1/2) for short times and of a standard Maxwell model for long times.     

High Magnetic Field Consequences on the NMR Hyperfine Shifts in Solution

Pseudocontact shifts arise from the isotropic reorientational average of the dipolar coupling between unpaired electron and nuclei, in the presence of magnetic susceptibility anisotropy. The effect of residual orientation due to high magnetic fields on pseudocontact shifts is evaluated here. The effect is found to be smaller and of opposite sign with respect to another novel effect of high magnetic fields on hyperfine shifts due to saturation of the electron spin magnetic moment as described by the Brillouin equation.     

Trajectory surface hopping within linear response time-dependent density-functional theory

A fewest switches trajectory surface hopping algorithm based on linear response time-dependent density-functional theory is developed and implemented into the plane wave ab initio molecular dynamics package CPMD. A scheme to calculate nonadiabatic couplings using a multi determinantal approximation of the excited state wave function is introduced. The method is applied to the study of the photorelaxation of protonated formaldimine, a minimal model of the rhodopsin chromophore retinal. A good agreement of the structural and dynamic behavior is found with respect to state averaged multiconfiguration self consistent field based trajectory surface hopping.     

Vibrational characterization of dinaphthylpolyynes: a model system for the study of end-capped sp carbon chains

We perform a systematic investigation of the resonance and vibrational properties of naphthyl-terminated sp carbon chains (dinaphthylpolyynes) by combined multi-wavelength resonant Raman (MWRR) spectroscopy, ultraviolet-visible spectroscopy, and Fourier-transform infrared (FT-IR) spectroscopy, plus ab initio density functional theory (DFT) calculations. We show that the MWWR and FT-IR spectroscopies are particularly suited to identify chains of different lengths and different terminations, respectively. By DFT calculations, we further extend those findings to sp carbon chains end-capped by other organic structures. The present analysis shows that combined MWRR and FT-IR provide a powerful tool to draw a complete picture of chemically stabilized sp carbon chains.     

Time-dependent density functional theory molecular dynamics simulation of doubly charged uracil in gas phase

We use time-dependent density functional theory and Born-Oppenheimer molecular dynamics methods to investigate the fragmentation of doubly ionized uracil in gas phase. Different initial electronic excited states of the dication are obtained by removing electrons from different inner-shell orbitals of the neutral species. We show that shape-equivalent orbitals lead to very different fragmentation patterns revealing the importance of the intramolecular chemical environment. The results are in good agreement with ionion coincidence measurements of uracil collision with 100 keV protons.     

CT-MQC – a coupled-trajectory mixed quantum/classical method including nonadiabatic quantum coherence effects

Upon photoexcitation by a short light pulse, molecules can reach regions of the configuration space characterized by strong nonadiabaticity, where the motion of the nuclei is strongly coupled to the motion of the electrons. The subtle interplay between the nuclear and electronic degrees of freedom in such situations is rather challenging to capture by state-of-the-art nonadiabatic dynamics approaches, limiting therefore their predictive power. The Exact Factorization of the molecular wavefunction, though, offers new perspectives in the solution of this longstanding issue. Here, we investigate the performance of a mixed quantum/classical (MQC) limit of this theory, named Coupled Trajectory-MQC, which was shown to reproduce the excited-state dynamics of small systems accurately. The method is applied to the study of the photoinduced ring opening of oxirane and the results are compared with two other nonadiabatic approaches based on different Ansätze for the molecular wavefunction, namely Ehrenfest dynamics and Ab Initio Multiple Spawning (AIMS). All simulations were performed using linear-response time-dependent density functional theory. We show that the CT-MQC method can capture the (de)coherence effects resulting from the dynamics through conical intersections, in good agreement with the results obtained with AIMS and in contrast with ensemble Ehrenfest dynamics.     

NMR properties of sedimented solutes

This perspective paper is intended to give some insights into the recently proposed technique of NMR of solutes sedimented by ultracentrifugation in a rotor used for solid state NMR experiments. Sedimented "states" correspond to molecules with very little reorientational capability in extremely concentrated solutions. They provide solid state NMR spectra comparable in quality with those of the best microcrystalline samples. Here we report some experiments to look for chemicals which affect the properties of the sediment, and we show that it is possible to fill the rotor in a true ultracentrifuge and then record the spectra. The latter possibility opens new horizons for NMR of sedimented systems.     

Carbon sp chains in graphene nanoholes

Nowadays sp carbon chains terminated by graphene or graphitic-like carbon are synthesized routinely in several nanotech labs. We propose an ab initio study of such carbon-only materials, by computing their structure and stability, as well as their electronic, vibrational and magnetic properties. We adopt a fair compromise of microscopic realism with a certain level of idealization in the model configurations, and predict a number of properties susceptible to comparison with experiment.