Dr Jean-Jacques Weis, one of the pillars of the development of the modern theory of liquids, recently turned sixty-five, and on this occasion it is more than fitting that the large international liquids community celebrate one of its most distinguished members through this Special Issue of Molecular Physics. The journal has always been at the forefront of theoretical developments of all aspects of the liquid state, and Weis has been one of its most dedicated authors, having published 28 papers in Molecular Physics over the 40 years of his very productive career.
Weis was born and educated in Luxembourg up to his baccalauréat. He then moved to France to read Physics at the Sorbonne in Paris and obtained his first degree (Licence ès Sciences) in 1966, followed by a Master's degree (Diplôme d’Etudes Approfondies) in Theoretical Physics. Thereafter he joined Loup Verlet's group in Orsay to work on Statistical Mechanics and simulation of ‘simple’ liquids under Verlet's supervision. Verlet was one of the pioneers of Monte Carlo (MC) and Molecular Dynamics (MD) simulations and, although small, his group was leading the field in Europe. Weis immediately felt at home in this thriving environment, and interacted strongly with Verlet and his former students, Dominique Levesque (to whom our journal dedicated a Special Issue, vol. 101 (2003)), Daniel Schiff, Jacques Vieillard-Baron and Jean-Pierre Hansen. The scientific atmosphere was extremely creative and personal relations between all members of the team particularly friendly and warm. Computers were incredibly slow in the late 1960s and early 1970s compared with present-day standards, and central memories did not exceed 1 Mbyte! To run the most advanced MD code of the time (capable of handling up to 1000 particles over a few picosecond trajectories!) the Orsay group coded in Assembly Language and took over the UNIVAC computers of the University during nights and week-ends to run the codes as long and efficiently as possible. Weis, like his colleagues, enjoyed the challenge and the competitive atmosphere heightened by regular visits of the leading scientists in the ‘liquids’ field, including Berni Alder (the ‘father’ of MD), the late Anees Rahman, Mal Kalos, Mike Klein, Ian McDonald, George Stell and John Valleau.
After being recruited as a junior researcher at the Centre National de la Recherche Scientifique (CNRS), Weis obtained his PhD (Doctorat ès Sciences) in 1973; he remained on the CNRS staff throughout his carreer, and was promoted to a senior level (Directeur de Recherche) as early as 1978.
Weis’ contributions to Statistical Mechanics of liquids and solids cover a broad range of topics, invariably at the forefront, and have a lasting impact in the field. His early work dealt with thermodynamic perturbation theory of atomic liquids (which are ‘simple’ only in name). In his PhD dissertation, Weis formulated a very accurate semi-analytic version of the Weeks–Chandler–Andersen theory applied to Lennard–Jones fluids; his classic paper with Verlet is one of the most cited references in the liquids literature. Weis then turned to a particularly challenging subject, namely polar fluids, which were then poorly understood. In a series of ground-breaking papers (frequently in collaboration with D. Levesque and G. Patey, or G. Stell, and later J.M. Caillol), Weis laid the foundations of a quantitative description of the anisotropic pair structure, thermodynamic and dielectric properties of models of dense polar fluids, using integral equations (mostly extensions of the hypernetted-chain (HNC) theory) and simulation. Meanwhile, with B.J. Alder and H. Strauss (Berkeley), Weis wrote a series of papers reporting MD calculations of the band shapes of depolarized light scattered by atomic fluids, in excellent agreement with Strauss’ pioneering experiments.
At about the same time Weis, in collaboration with M.L. Klein and D. Levesque, was among the first to use MD to explore the collective dynamics of molecular liquids and solids, including N2, O2, HCl and NH4. Using path integral quantum MC simulations the same authors later determined the high pressure phase diagram of helium at and above room temperature.
In the late 1980s, Weis turned his attention to ionic solutions in explicit polar solvents, and to the electrical properties of polarizable ionic solutions. With L. Reatto and D. Levesque he proposed a powerful iterative method, combining fluid integral equations and MC simulations, to solve the long-standing ‘inverse problem’, i.e. to extract effective pair potentials from measured pair distribution functions. Around the same time Weis turned to the theory of confined fluids (e.g. in porous media), combining simulation, generalized integral equations and the recently developed classical density functional theory (DFT) of inhomogeneous fluids.
In the 1990s, Weis’ interests shifted to ‘complex fluids’, in particular to dipolar liquid crystals and ferrofluids. He and Levesque, together with their student G.J. Zarragoicoechea pioneered the investigations to characterize the influence of dipolar interactions on the orientational order of rod-like or discotic liquid crystals. As to ferrofluids, Weis and Levesque were the first to show convincingly the chain formation of dipolar spheres (at low densities) and to characterize the ferroelectric phases of the same model at higher densities. In parallel Weis and collaborators explored the phase transitions of a continuum model of the classical Heisenberg magnet (Heisenberg spin fluid), combining MC simulations and DFT.
In the mid-1990s, Caillol, Levesque and Weis made a key contribution to the characterization of the phase separation of the restricted primitive model of ionic solutions into high and low concentration phases, which had been predicted theoretically by G. Stell and by M. Fisher and their collaborators. Using advanced MC and finite size scaling techniques, they were able to obtain the first reliable estimates of the critical temperature and concentration, and to show that the critical exponents are Ising-like.
Weis’ more recent work deals with several timely problems, including the orientational structure of dipolar colloids adsorbed at an interface, the structure of dipolar fluids adsorbed into disordered porous media, or the liquid–vapour transition of fluids or particles interacting via long-range Yukawa potentials. In collaboration with an experimental group in Paris, Weis is presently investigating the self-organization of confined dipolar particles in an external field. Jean-Jacques Weis is planning to dedicate his Emeritus years to a comprehensive understanding of the complete phase diagram of dipolar systems.
This brief overview gives only an incomplete idea of the breadth and originality of Jean-Jacques Weis’ work. Although he spent all of his carreer in Orsay, Weis has collaborated with many of the major players in the field, and made several long visits to leading groups, notably Berni Alder in Berkeley, George Stell at Stony Brook, Mike Klein at NRC (Ottawa), Gren Patey at UBC (Vancouver), Enrique Lomba in Madrid, Margarida Telo da Gama in Lisbon and Gerhard Kahl in Wien. With his quiet and reserved personality, Weis shies away from the limelight of large meetings and conferences, but his friendly and deeply dedicated attitude, and his exemplary reliability and highly professional competence are greatly appreciated by his numerous collaborators who owe him so much.
Although very dedicated to his research, Weis has many other interests in life, including trecking and mountaineering in the Alps, the Andes or the Himalayas, and travelling around remote parts of Asia. He is a keen photographer, and the pictures he brings back from his expeditions are stunningly beautiful. Classical music and archeology are among his more reflective hobbies at home.
The present-day understanding of the liquid state of matter would not be as advanced without Jean-Jacques Weis’ contributions. He has more plans for the future, and we wish him a most fruitful and active ‘retirement’.