1-dimensional hard rods (sometimes known as a Tonks gas [1]) consist of non-overlapping line segments of length
who all occupy the same line which has length
. One could also think of this model as being a string of hard spheres confined to 1 dimension (not to be confused with 3-dimensional hard rods). The model is given by the intermolecular pair potential:

where
is the position of the center of the k-th rod, along with an external potential. Thus, the Boltzmann factor is

The whole length of the rod must be inside the range:

Canonical Ensemble: Configuration Integral[edit]
The statistical mechanics of this system can be solved exactly.
Consider a system of length
defined in the range
. The aim is to compute the partition function of a system of
hard rods of length
.
Consider that the particles are ordered according to their label:
;
taking into account the pair potential we can write the canonical partition function
of a system of
particles as:

Variable change:
; we get:

Therefore:


Thermodynamics[edit]
Helmholtz energy function

In the thermodynamic limit (i.e.
with
, remaining finite):
![{\displaystyle A\left(N,L,T\right)=Nk_{B}T\left[\log \left({\frac {N\Lambda }{L-N\sigma }}\right)-1\right].}](https://wikimedia.org/api/rest_v1/media/math/render/svg/a3e69427cc1e3db932ca4534de09c9fca9d0b31f)
Equation of state[edit]
Using the thermodynamic relations, the pressure (linear tension in this case)
can
be written as:

The compressibility factor is

where
; is the fraction of volume (i.e. length) occupied by the rods. 'id' labels the ideal and 'ex' the excess part.
It was shown by van Hove [2] that there is no fluid-solid phase transition for this system (hence the designation Tonks gas).
Chemical potential[edit]
The chemical potential is given by

with ideal and excess part separated:

Isobaric ensemble: an alternative derivation[edit]
Adapted from Reference [3]. If the rods are ordered according to their label:
the canonical partition function can also be written as:

where
does not appear one would have
analogous expressions
by permuting the label of the (distinguishable) rods.
is the Boltzmann factor
of the hard rods, which is
if
and
otherwise.
A variable change to the distances between rods:
results in

the distances can take any value as long as they are not below
(as enforced
by
) and as long as they add up to
(as enforced by the Dirac delta). Writing the later as the inverse Laplace transform of an exponential:
![{\displaystyle Z=\int _{0}^{\infty }dy_{0}\int _{0}^{\infty }dy_{1}\cdots \int _{0}^{\infty }dy_{N-1}f(y_{0})f(y_{1})\cdots f(y_{N-1}){\frac {1}{2\pi i}}\int _{-\infty }^{\infty }ds\exp \left[-s\left(\sum _{i=0}^{N-1}y_{i}-L\right)\right].}](https://wikimedia.org/api/rest_v1/media/math/render/svg/c7e5dbcd5658343abecf597044a800367b803cee)
Exchanging integrals and expanding the exponential the
integrals decouple:

We may proceed to invert the Laplace transform (e.g. by means of the residues theorem), but this is not needed: we see our configuration integral is the inverse Laplace transform of another one,

so that

This is precisely the transformation from the configuration integral in the canonical (
) ensemble to the isobaric (
) one, if one identifies
. Therefore, the Gibbs energy function is simply
, which easily evaluated to be
. The chemical potential is
, and by means of thermodynamic identities such as
one arrives at the same equation of state as the one given above.
Confined hard rods[edit]
[4]
References[edit]
Related reading
- L. van Hove "Quelques Propriétés Générales De L'intégrale De Configuration D'un Système De Particules Avec Interaction", Physica, 15 pp. 951-961 (1949)
- Zevi W. Salsburg, Robert W. Zwanzig, and John G. Kirkwood "Molecular Distribution Functions in a One-Dimensional Fluid", Journal of Chemical Physics 21 pp. 1098-1107 (1953)
- Robert L. Sells, C. W. Harris, and Eugene Guth "The Pair Distribution Function for a One-Dimensional Gas", Journal of Chemical Physics 21 pp. 1422-1423 (1953)
- Donald Koppel "Partition Function for a Generalized Tonks' Gas", Physics of Fluids 6 609 (1963)
- J. L. Lebowitz, J. K. Percus and J. Sykes "Time Evolution of the Total Distribution Function of a One-Dimensional System of Hard Rods", Physical Review 171 pp. 224-235 (1968)
- Gerardo Soto-Campos, David S. Corti, and Howard Reiss "A small system grand ensemble method for the study of hard-particle systems", Journal of Chemical Physics 108 pp. 2563-2570 (1998)
- Paolo V. Giaquinta "Entropy and Ordering of Hard Rods in One Dimension", Entropy 10 pp. 248-260 (2008)