Hard ellipsoid model

A prolate ellipsoid.

Interaction Potential

The general ellipsoid, also called a triaxial ellipsoid, is a quadratic surface which is given in Cartesian coordinates by

${\frac {x^{2}}{a^{2}}}+{\frac {y^{2}}{b^{2}}}+{\frac {z^{2}}{c^{2}}}=1$

where $a$, $b$ and $c$ define the lengths of the axis.

Overlap algorithm

The most widely used overlap algorithm is that of Perram and Wertheim:

• John W. Perram and M. S. Wertheim "Statistical mechanics of hard ellipsoids. I. Overlap algorithm and the contact function", Journal of Computational Physics 58 pp. 409-416 (1985)

Geometric properties

The mean radius of curvature is given by (Ref. 2)

$R={\frac {a}{2}}\left[{\sqrt {\frac {1+\epsilon _{b}}{1+\epsilon _{c}}}}+{\sqrt {\epsilon }}_{c}\left\{{\frac {1}{\epsilon _{c}}}F(\varphi ,k_{1})+E(\varphi ,k_{1})\right\}\right],$

and the surface area is given by

$S=2\pi a^{2}\left[1+{\sqrt {\epsilon _{c}(1+\epsilon _{b})}}\left\{{\frac {1}{\epsilon _{c}}}F(\varphi ,k_{2})+E(\varphi ,k_{2})\right\}\right],$

where $F(\varphi ,k)$ is an elliptic integral of the first kind and $E(\varphi ,k)$ is an elliptic integral of the second kind, with the amplitude being

$\varphi =\tan ^{-1}({\sqrt {\epsilon }}_{c}),$

and the moduli

$k_{1}={\sqrt {\frac {\epsilon _{c}-\epsilon _{b}}{\epsilon _{c}}}},$

and

$k_{2}={\sqrt {\frac {\epsilon _{b}(1+\epsilon _{c})}{\epsilon _{c}(1+\epsilon _{b})}}},$

where the anisotropy parameters, $\epsilon _{b}$ and $\epsilon _{c}$, are

$\epsilon _{b}=\left({\frac {b}{a}}\right)^{2}-1,$

and

$\epsilon _{c}=\left({\frac {c}{a}}\right)^{2}-1.$

The volume of the ellipsoid is given by the well known

$V={\frac {4\pi }{3}}abc.$

See also

• Hard ellipsoid equation of state

References

1. Carl McBride and Enrique Lomba "Hard biaxial ellipsoids revisited: Numerical results", Fluid Phase Equilibria 255 pp. 37-45 (2007)
2. G. S. Singh and B. Kumar "Geometry of hard ellipsoidal fluids and their virial coefficients", Journal of Chemical Physics 105 pp. 2429-2435 (1996)