This report gives a summary of the events of this meeting, held in
the Czech Republic in early June, 1994. It attracted many of leading
experts in the field, which intersects chemistry, physics, chemical
engineering, and applied mathematics. The meeting was the fourth in a
series which began in 1983, as a private initiative of Ivo Nezbeda and
his colleagues from the Academy of Sciences in Prague, Czech Republic,
and continued in 1987 and 1990. This meeting was chaired by I. Nezbeda
and co-chaired by K. Gubbins (Cornell) and D. Henderson (Mexico
City).

Typically, the meeting venue is outside main residential
areas. This time it was held in a remote resort in the heart of the
Czech-Moravian Highlands, and had its standard format: an opening
lecture on Monday evening given by a renowned specialist, followed by
eight morning, three evening sessions, and a poster session.

An integral part of this particular meeting was the workshop *
"Novel trends in the simulation of complex liquids"* sponsored by
CECAM (European Center for Atomic and Molecular Computations) and
chaired by D. Frenkel (FOM Amsterdam).

The conference was opened with a review talk "Teaching an Old Dog
New Tricks: Some Recent Monte Carlo Stunts", given by J. Valleau
(Toronto), and focussing primarily on the latest development of
techniques for computing the chemical potential. Two Tuesday's morning
sessions were taken up by the CECAM workshop, within which 14
contributions were presented. The following evening session delt with
purely theoretical topics. Wednesday's morning sessions were devoted
to inhomogeneous fluids. Polar, ionic, and associating fluids covered
the following two sessions and the CECAM workshop continued on
Thursday morning. The panel discussion took place on Thursday
evening. The last two sessions on Friday were devoted to normal and
chain-molecule fluids.

In addition to the total of 48 oral presentations, 78 posters,
covering the above topics, were shown. The closing speech was
delivered by E. Glandt who briefly summarized the main ideas which
emerged during the meeting and invited all participants to the fifth
conference which will be held in June 1998.

The workshop covered both methodological and applied aspects of computer simulations and the contributions therein may be grouped as follows:

- Simulation of phase equilibria, including systems with chemical reaction
- liquid crystals and liquid crystalline polymers
- fluids in confined media
- massively parallel simulations

The Gibbs ensemble simulation technique (GEMC) and recent related
advances were reviewed by T. Panagiotopoulos (Cornell). In particular,
he addressed the problem of finite size corrections to critical
behavior, and described a method to extrapolate grand-canonical
ensemble Monte Carlo results to infinite system sizes. He also
discussed simulations of liquid-vapor coexistence in systems of chain
molecules, using the incremental chemical potential scheme of Kumar et
al. B. Smit (SHELL Amsterdam) later discussed how the Configurational
Bias MC (CBMC) method could be used to perform * direct* GEMC
simulations of liquid-vapor coexistence in systems of long alkanes (up
to C_{48}H_{98} !). He also discussed several other problems that had
become tractable with CBMC, i.e. the problem of alkanes in
zeolites. Another contribution that focussed on phase behavior was due
to D. Kofke (Buffalo), who discussed the Gibbs-Duhem integration
technique to trace out coexistence curves. He discussed the
generalization of the technique to trace coexistence curves as a
function of the Hamiltonian of the system. In particular, he showed
how the melting pressure of simple models with an r^{-n}
potential could be followed as a function of *n*. Somewhat
surprisingly, he found that the dependence on *n* is not
monotonic. The problem of simultaneous phase and reaction equilibria
has received considerable attention during the past few years, and was
discussed by W. Smith (Guelph) who focussed primarily on a new
reaction ensemble technique developed by him and B. Triska. This
easily treats several cases which have proven difficult in the past,
including multiple reactions and reactions that do not conserve the
total number of molecules. He also showed how the Gibbs ensemble
technique could be viewed as a special case of the reaction ensemble
approach.

P. Cummings (Oak Ridge) gave an overview of the work performed in his research group in recent years aimed at predicting phase equilibria in pure water and aqueous mixtures (water/methanol and water/methanol/NaCl mixtures) using GEMC methods. The results pointed to deficiencies in the intermolecular potentials. He also described the use of molecular dynamics (MD) simulation to study pure supercritical water and supercritical aqueous solutions. It was shown that simple point charge models fitted to ambient conditions have limited utility at supercritical state points. Both the GEMC and MD results suggest that water--water potentials needed to include polarizability to be applicable over the wide range of state conditions covered by vapor--liquid equilibrium and supercritical states.

Within the liquid crystal and liquid crystalline polymer topic, B. Mulder (FOM Amsterdam) reviewed recent advances in the theoretical description of liquid crystalline polymers. He showed how it is possible to generalize techniques previously used to describe main chain liquid crystal polymers, in order to treat more complicated systems like side-chain liquid crystal polymers in such a way that the influence of parameters like backbone- and spacer- length and flexibility can be systematically explored. E. Miguel (Sevilla) reported on recent simulations of the Gay-Berne model. This model has been shown to exhibit a variety of liquid crystalline phases. He showed that his simulations reveal isotropic, nematic and several smectic (A, B, hexatic-B ?) phases. R. Holyst (Warszawa) discussed the peculiar properties of wedge dislocation in smectics. Due to the strong coupling with the surface free energy, these dislocations are repelled by the surface. He reviewed the possibility of dislocation un-binding in the context of the Nelson-Toner theory.

Another topic that arose in several talks was the numerical
simulation of fluids in confined media. This problem was discussed by
W. Steele (Penn State U), who reported on simulations of a fluid in
straight and modulated `corrugated' pores. He discussed both the
structure of the confined fluid and the adsorption isotherm, as a
function of the pore shape. Liquid-vapor coexistence in pores and the
effect of the pore diameter on the location of the coexistence point
was the topic of E. Piotrovskaya's (St. Petersburg) contribution. She
showed how, from such a simulation, one can deduce the curvature
corrections to the liquid-vapor interfacial tension. Another aspect of
molecules in confined media is their unusual dynamics. J. Talbot
(Purdue) considered the motion of non-spherical molecules in the
presence of fixed obstacles (Lorentz gas). His simulation seem to
indicate that the dynamical behavior of such a molecular Lorentz gas
is quite different from that of its atomic counterpart. Another
contribution that focussed on the dynamics was given by J. Fischer
(Bochum), who considered the dynamics of liquid evaporation from a
free surface.

Two contributions were mainly focused on the problems associated
with massively parallel simulation. J. Perram (Odense) discussed
strategies for the parallel simulation of polymers, while I. Snook
(Melbourne) discussed his experience with parallel simulations of
large atomic systems. The message of both talks was that, in order to
benefit from parallelism, a complete rethinking of the problem and a
willingness to adopt existing parallelization techniques, is
essential.

The panel discussion *"Perspectives for CECAM initiatives on
simulation on complex fluids"* was chaired by N. Quirke (BIOSYM
Orsay) and the panel speakers were K. Gubbins (Cornell), A. Haymet
(Sydney), P. Cummings (Oak Ridge), J. Perram (Odense), and D. Frenkel
(FOM Amsterdam). Apart from discussion attempting to identify topics
as possible subjects for future CECAM activities, a number of speakers
expressed their interest in improved intermolecular potentials, both
for small molecules (water, alcohols) and for macro-molecules. It was
also stressed that there is a great need for workshop style training
meetings that provide the participants with hands-on experience with
state-of-the-art computational techniques.

In this session, chaired by J. Fischer (Bochum), five contributions
were delivered. Y. Rosenfeld (Beer-Sheva) discussed his concept of
the "ideal liquid". To illustrate, he considered in detail the case
of a one-component plasma. J. Percus (New York) delivered a
stimulating lecture on entropy functionals. P. Monson (Amherst)
described his recent work with X. Cottin on the development of a cell
theory for the thermodynamic properties of solid
solutions. Applications to binary hard sphere mixtures forming both
substitutionally disordered and substitutionally ordered ('compounds')
solid solutions were considered. This approach correctly describes the
trends in the solid-fluid phase diagrams as the molecular size ratio
between the species is changed. D. Evans (Canberra) presented one of
the few talks on nonequilibrium statistical mechanics, and discussed
relations between phase space stability and thermophysical
properties. Finally, S. Labik (Prague) spoke about recent work with
Smith and Malijevsky on a new integral equation hierarchy for the
background correlation functions of hard-body fluids. It yields the
most accurate first-principles equation of state for hard-sphere
systems, and very accurate pair and triplet background correlation
functions.

The session on Inhomogeneous Fluids was chaired by W. Steele (Penn
State U) and was opened by a talk by M. Rosinberg (Paris), who
described the recent work he and his colleagues have done on extending
density functional theory to inhomogeneous polyatomic fluids. Several
talks were concerned with the effects of confinement on fluid
behavior. S. Sokolowski (Lublin) spoke on the effect of pore closure
on capillary condensation, again using density functional
theory. K. Gubbins described recent simulation work on adsorption of
both simple fluids and water in model carbon (graphitic and activated)
pores. While these last two talks were concerned with precisely
characterised model pores, E. Glandt (Philadelphia) described recent
work of his on molecular fluids in random media.

G. Stell (Stony Brook) described simulations of quenched-annealed
systems. Tha last three talks in this session were concerned with the
behavior of charged interfaces. L. Blum (Puerto Rico) described
theoretical and experimental work on structured charged
interfaces. A. Haymet (Sydney) discussed the behavior of water and
electrolytes near charged surfaces, and recent results for wholly
molecular theories of electrical double layers were the topic of the
last talk by G. Torrie (Kingston).

The contributions in these sessions, chaired by J. Perram and
K. Gubbins, were approximately equally balanced between molecular
simulation (Patey, van Leeuwen, Kusalik and Mezei) and integral
equations (Nishimura, Yakub, Holovko and Blum). Haymet presented both
molecular simulation and integral equation results for a new model of
liquid water. Debenedetti's presentation concerned a lattice model
theory for associating fluids.

G. Patey (Vancouver) described the first demonstration that dipolar
forces alone can create an orientationally ordered liquid phase. MD
simulations were used to show that strongly interacting dipolar
spheres (both soft and hard) can form a ferroelectric phase (the first
time that the existence of a ferroelectric nematic phase has been
established for a model fluid) and that liquid crystals with columnar
order could be obtained. The influence of boundary conditions and
"dynamically induced" transitions was discussed. M. van Leeuwen
(SHELL Amsterdam) presented the results of GEMC simulation results for
the prediction of vapor-liquid equilibrium in pure methanol using
three three-site united-atom models published in the literature. The
GEMC calculations proved quite sensitive to details of the
intermolecular potentials, none of which was completely
satisfactory. P. Kusalik (Dalhousie) reviewed the difficulties
encountered in determining the dielectric constant of polar liquids
from computer simulation. He showed that an examination of the
distribution of fluctuations of the total dipole moment of the sample
can be very useful in understanding long-standing problems, such as
system-size and boundary condition effects. Knowledge of the
functional form of this distribution can lead to significant
reductions in the computational resources required to estimate the
dielectric constant. In a wide-ranging talk, M. Mezei (New York)
addressed several aspects of modeling liquid water. He discussed dimer
potential surfaces and aspects of intramolecular potentials in the
context of the existence of low-energy trifurcated structures and the
importance and conformation dependence of the basis-set-superposition
error and the zero-point vibration energy. He also described recently
derived analytical expressions for the average contribution of
molecules beyond a cutoff sphere and results on the free energy of
solvation of sodium and lithium ions, indicating that the Born
correction is still about a factor of two in error in the
10-14 A range. The discussion was concluded with the demonstration
of the feasibility of generating effective pairwise additive
potentials based on the configurational energy of representative
assemblies of molecules.

While simultaneously entertaining the audience with reflections and
slides from a trip to the Antarctic to collect samples of a protein
from a particular fish found near water/ice interfaces, A. Haymet
(Sydney) described a model for water, called CF1, which is a
modification of Stillinger's central force model designed to make the
pressure be closer to that of real water. Classical MD simulations of
the CF1 model were used to calculate the dissociation of water with
the quite satisfactory result of *p*H = 8.5 +- 0.7 at ambient
conditions. MD calculations of the structure of the bulk water were
then used as input to an approximate integral equation theory for the
structure and properties of water next to a planar
interface. Predictions from the theory include the oxygen and hydrogen
density profiles perpendicular to the interface, the mean
electrostatic potential, the potential of zero charge and the
differential capacitance.

H. Nishimura (Tokyo) described his approach to solving the hypernetted chain approximation (HNCA) for dipolar hard spheres. As he explained the approximations he invoked, it became clear that he was describing the linearized HNCA. Moreover, to solve the linearized HNCA he had rediscovered the invariant expansion approach of Wertheim. E. Yakub (Odessa) presented an integral equation theory for chemically associating molecules under the assumption that the number of other molecules to which each molecule can bond is limited. This property of saturation in the chemical bonding is explicitly introduced into the theory in a similar fashion to that proposed by Wertheim. and extended by Kalyuzhnyi and Stell. M. Holovko (Lwow) presented the integral equation theory derived by him and Y. Kalyuzhnyi to account for association effects in ionic fluids. The theory is an extension of the Kalyuzhnyi and Stell formalism noted above and it involves separating the potential into associating and non-associating parts with each part handled by different approximations. Both numerical and analytical solutions of HNCA-like and MSA-like approximations were presented for models of 2-2 and highly asymmetric electrolyte solutions. L. Blum (Puerto Rico) described a simple potential model for water, originally proposed by Bratko, Blum and Luzar, consisting of hard spheres with a point dipole and a potential well with the symmetry of a tetrahedral quadrupole. Structural results from the model were presented and the *g*_{OH}(r) and *g*_{HH}(r) were found to compare quite favorably with neutron scattering experiments. The agreement for *g*_{OO}(r) is not as good and reflects the hard core nature of the model.

P. Debenedetti (Princeton) described the use of a lattice model
with orientation-dependent interactions to study the phase behavior
and limits of stability of network-forming fluids, such as silica and
water. These interactions give rise to a competition between bonded
states of low energy, entropy, and density, and non-bonded states of
high energy, entropy, and density. By suitable choice of the
parameters, the mean-field solution of the model can be used to
explain the anomalous behavior of supercooled water. The model's
generality suggests that similar behavior can occur in other
network-forming fluids.

These topics covered the last two sessions, which were chaired by
W. Smith (Guelph) and E. Glandt (Philadelphia), respectively.

M. Wertheim (Haughton) gave a talk on the geometry of hard convex
bodies in connection with the problem of determining the third virial
coefficient and contributions to higher coefficients. He introduced
geometric functionals of the pairs of hard bodies and discussed ways
of their determination. D. Henderson (Mexico City) spoke on the bridge
and correlation functions for hard spheres using inhomogeneous
integral equations. T. Boublik (Prague) also dealt with the third
virial coefficients of hard body fluids. In addition to the
well-defined parameter of nonsphericity, alpha, he introduced another
parameter in order to match the virial coefficient of both
prolate and oblate bodies. R. Lustig (Aachen) concentrated on
an `exact' way of treating the "MD microcanonical ensemble",
i.e. microcanonical ensemble in which the total momentum is conserved
in contrast to the usual approach which uses the canonical
expressions. The problem of simulating the systems of molecules with a
surface adhesion (i.e. intinitely narrow range of attraction) was
addressed by B. Borstnik (Ljubljana) who presented a variant of
molecular dynamics tailored to this special case. J. Vrabec (Bochum)
described the extension of the {NpT+test particle} simulation to
mixtures. The method was applied to the system {argon+methane} and
good agreement with real experimental data was reported. F. del Rio
(Mexico City) gave an overview of the development reached by his
research group for the square-well fluid and square-well fluid
mixtures. C. Hall (Raleigh) reviewed the development of a class of
successful equations of state for hard-chain-molecule fluids based on
the generalized Flory dimer theory, in which the pressure is
calculated in terms of the chain insertion probability. The last two
talks, presented by Y. Chiew (Piscataway) and M. Banaszak (Annandale)
dealt with realistic chain-molecule fluids both from the point of
theory and simulations. Theories for equations of state were discussed
and a variational theory for Lennard-Jones chains was also
presented. Good agreement between the theory and simulation data for
pressure of Lennard-Jones chains is observed.

Following the conference opening lecture, a social mixer was held in the dining room of the hotel, giving an excellent opportunity for the conference participants and their guests to renew old and make new acquaintances. On Tuesday, a group of enthusiasts took the opportunity to admire the mountains and castles of Bohemia from the bird-eye view. On Wednesday afteroon, the conference attendees were taken by bus to the "Moravian Karst", where they toured the magnificent caves, and took a cruise on the underground river. The planned barbecue party on Thursday evening was held indoors due to the inclement weather, but this did not dampen the festivities. From Tuesday through Thursday, an excellent Accompanying Persons Program involved visits to historic sites in the surrounding countryside and to the city of Brno.