Quantum Diffusive Dynamics of Macromolecular Transitions

We study the role of quantum fluctuations of atomic nuclei in the real-time dynamics of non-equilibrium macro-molecular transitions. To this goal we introduce an extension of the Dominant Reaction Pathways (DRP) formalism, in which the quantum corrections to the classical overdamped Langevin dynamics are rigorously taken into account to order h^2 . We first illustrate our approach in simple cases, and compare with the results of the instanton theory. Then we apply our method to study the C7_eq to C7_ax transition of alanine dipeptide. We find that the inclusion of quantum fluctuations can significantly modify the reaction mechanism for peptides. For example, the energy difference which is overcome along the most probable pathway is reduced by as much as 50%.Comment: Final version, to appear in the Journal of Chemical Physic

Fluctuations in the Ensemble of Reaction Pathways

The dominant reaction pathway (DRP) is a rigorous framework to microscopically compute the most probable trajectories, in non-equilibrium transitions. In the low-temperature regime, such dominant pathways encode the information about the reaction mechanism and can be used to estimate non-equilibrium averages of arbitrary observables. On the other hand, at sufficiently high temperatures, the stochastic fluctuations around the dominant paths become important and have to be taken into account. In this work, we develop a technique to systematically include the effects of such stochastic fluctuations, to order k_B T. This method is used to compute the probability for a transition to take place through a specific reaction channel and to evaluate the reaction rate

Ab-initio Dynamics of Rare Thermally Activated Reactions

We introduce a framework to investigate ab-initio the dynamics of rare thermally activated reactions. The electronic degrees of freedom are described at the quantum-mechanical level in the Born-Oppenheimer approximation, while the nuclear degrees of freedom are coupled to a thermal bath, through a Langevin equation. This method is based on the path integral representation for the stochastic dynamics and yields the time evolution of both nuclear and electronic degrees of freedom, along the most probable reaction pathways, without spending computational time to explore metastable states. This approach is very efficient and allows to study thermally activated reactions which cannot be simulated using ab-initio molecular dynamics techniques. As a first illustrative application, we characterize the dominant pathway in the cyclobutene to butadiene reaction.Comment: 4 pages, 4 figure

Garigliano nuclear power plant: seismic evaluation of the turbine building

The Italian Garigliano Nuclear Power Plant (NPP) started its energy production in 1963. At present it is in the decommissioning stage. In order to get a proper management of the radioactive waste that will be produced during the dismantling operations it has been considered convenient to convert the turbine building of the plant into a temporary waste repository. This decision posed a remarkable seismic safety assessment issue. As a matter of fact, the challenge was to extend, in satisfactory safety conditions, the use of an important facility that has reached the end of its designed lifetime and to have this extended use approved by nuclear safety agencies. In this context many tasks have been accomplished, of which the most important are: (a) a new appraisal of site seismic hazard; (b) the execution of many investigations and testing on the construction materials; (c) the set up of a detailed 3D finite element model including the explicit representation of foundation piles and soil; (d) consideration of soil structure kinematic and dynamic nteraction effects. This paper describes the adopted seismic safety assessment criteria which are based on a performance objectives design approach. While performance based design is the approach currently recommended by European Regulations to manage seismic risk and it is fully incorporated in the Italian code for conventional buildings, bridges and plants, NPP are not explicitly considered. Therefore it was necessary to delineate a consistent interpretation of prescribed rules in order to properly select the maximum and operating design earthquakes on one side and corresponding acceptable limit states on the other side. The paper further provides an outline of the numerical analyses carried out, of the main results obtained and of the principal retrofitting actions that will be realized

Investigating Biological Matter with Theoretical Nuclear Physics Methods

The internal dynamics of strongly interacting systems and that of biomolecules such as proteins display several important analogies, despite the huge difference in their characteristic energy and length scales. For example, in all such systems, collective excitations, cooperative transitions and phase transitions emerge as the result of the interplay of strong correlations with quantum or thermal fluctuations. In view of such an observation, some theoretical methods initially developed in the context of theoretical nuclear physics have been adapted to investigate the dynamics of biomolecules. In this talk, we review some of our recent studies performed along this direction. In particular, we discuss how the path integral formulation of the molecular dynamics allows to overcome some of the long-standing problems and limitations which emerge when simulating the protein folding dynamics at the atomistic level of detail.Comment: Prepared for the proceedings of the "XII Meeting on the Problems of Theoretical Nuclear Physics" (Cortona11

Effective Field Theory for the Quantum Electrodynamics of a Graphene Wire

We study the low-energy quantum electrodynamics of electrons and holes, in a thin graphene wire. We develop an effective field theory (EFT) based on an expansion in p/p_T, where p_T is the typical momentum of electrons and holes in the transverse direction, while p are the momenta in the longitudinal direction. We show that, to the lowest-order in (p/p_T), our EFT theory is formally equivalent to the exactly solvable Schwinger model. By exploiting such an analogy, we find that the ground state of the quantum wire contains a condensate of electron-hole pairs. The excitation spectrum is saturated by electron-hole collective bound-states, and we calculate the dispersion law of such modes. We also compute the DC conductivity per unit length at zero chemical potential and find g_s =e^2/h, where g_s=4 is the degeneracy factor.Comment: 7 pages, 2 figures. Definitive version, accepted for publication on Phys. Rev.