Research Activity

I was born in Rome, I have lived there for most of my life and I have studied Physics at the university La Sapienza, finally achieving a M.Sc. degree. But still feeling uneasy I decided to become a PhD student in Chemistry at the University of Amsterdam. Keywords for my activities in science are statistical mechanics and computer simulations. More specifically, on the "roman" side I have worked on Nonextensive Statistical Mechanics, and on the "dutch" side I work on Rare Events. Have a look at them (and something more) in the rest of the page ...

Rare Events

Whenever you have a system where the free energy you decide to study displays two minima separated by a high barrier, transitions between the minima become rare. A simulation started in one of them will eventually go into the other only after a very long time.

The group of David Chandler in Berkeley, California has developed a method to overcome this problem. The idea is to concentrate the simulation only on the moment of the transition and study the set of all possible paths connecting the states. In simulative jargon it is a Monte Carlo sampling of the transition path space. The method was devised for chemical reactions. The free energy as a function of the reaction coordinate shows a barrier between reactants and products. The rate constant can be obtained with the method and the transition state as well, without any prior knowledge of it. The method is called Transition Path Sampling and further general informations can be found here.

The system I study is a very small cluster of 7 Lennard-Jones particles and the reactions I am concerned with are the conformational rearrangements between the potential energy minima. Since the system is so small the potential energy is completely known ¹ , and rate constants have also been computed for a range of energies ² . I have faced it as a test case to acquire the technique and I have computed the rate constant for the transition between the two lowest energy minima. But at variance with the literature I could lower the total energy to values very close to that of the transition state, being able to detect transitions with rate constant of even 10^-12 s^-1. You can find more details in an early presentation I made to my group in Mar 2001 or in the poster I presented at the conference Fourth International Discussion Meeting on Relaxation in Complex Systems, held in Crete, Greece in Jun 2001.

Group Presentation

Poster in Crete

Nonextensive Statistical Mechanics

The name identifies an extension of ordinary Boltzmann-Gibbs statistical mechanics started by Constantino Tsallis in 1988 ³ . Since then a large amount of articles has been produced on the subject and you can find an updated list here. The basis of the theory is a generalized entropy, depending on a parameter q, which has turned out to have several interesting properties. Most of the formal structure of the standard theory, recovered for q=1, has a corresponding generalization. Besides mathematical consistency, there have been numerous theoretical and experimental evidences for the validity of the theory in those cases where standard statistical mechanics fails to give a proper explanation ⁴.

One of these cases are systems with long-range interactions. When I was still a student in search for a subject for my thesis, I bumped into a classical XY model on a d-dimensional lattice with pairwise interaction of the form r ^-a where 0 < a < d. Proper rescaling is necessary to have a well-defined thermodynamics. Together with my promoter Andrea Giansanti and my second supervisor Alessandro Campa, I solved the model analytically and investigated its dynamical properties through computer simulations. My thesis was written and a number of articles produced. I gave two presentations of my work on this subject. The first was a talk given to my present group in Amsterdam when I was applying for the position and the second is a poster I presented at a conference in Lunteren, The Netherlands.

My Thesis

Presentation in
Amsterdam

Poster in
Lunteren

Something more

In our group at the University of Amsterdam we use to have periodic meetings with two presentations, one on the progress of our work, and one is a report of an article from scientifical literature not directly connected with our area of research. Every friday we do it in turn. When I did the literature talk, I spoke about the NEAR experiment. What to know more ? Have a look at the wonderful website of the experiment or at my presentation.

Presentation

Bibliography

¹ C. Tsai and K. Jordan, J. Phys. Chem. 97, 11227 (1993)
² M. A. Miller and D. J. Wales, J. Chem. Phys. 107, 8568 (1997)
³ C. Tsallis, J. Stat. Phys. 52, 479 (1988)
⁴ C. Tsallis, Braz. J. Phys. 29, 1 (1999)

Workgroup

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