9-3 Lennard-Jones potential

Functional form

The 9-3 Lennard-Jones potential is related to the standard Lennard-Jones potential.

It takes the form:

$\Phi (r)={\frac {3{\sqrt {3}}}{2}}\epsilon \left[\left({\frac {\sigma }{r}}\right)^{9}-\left({\frac {\sigma }{r}}\right)^{3}\right].$

where $\Phi (r)$ is the intermolecular pair potential. The minimum value of $\Phi (r)$ is obtained at $r=r_{min}$, with

• $\Phi \left(r_{min}\right)=-\epsilon$,

• ${\frac {r_{min}}{\sigma }}=3^{1/6}$

Applications

It is commonly used to model the interaction between the particles of a fluid with a flat structureless solid wall.

Interaction between a solid and a fluid molecule

Let us consider the space divided in two regions:

• $x<0$: this region is occupied by a diffuse solid with density $\rho _{s}$ composed of 12-6 Lennard-Jones atoms

with parameters $\sigma _{s}$ and $\epsilon _{a}$

Our aim is to compute the total interaction between this solid and a molecule located at a position $x_{f}>0$. Such an interaction can be computed using cylindrical coordinates.

The interaction will be:

$\Phi _{W}\left(x\right)=4\epsilon _{sf}\rho _{s}\int _{0}^{2\pi }d\phi \int _{-\infty }^{-x}dz\int _{0}^{\infty }{\textrm {dr}}\left[\sigma ^{12}{\frac {r}{(r^{2}+z^{2})^{6}}}-\sigma ^{6}{\frac {r}{(r^{2}+z^{2})^{3}}}\right].$

$\Phi _{W}\left(x\right)=8\pi \epsilon _{sf}\rho _{s}\int _{-\infty }^{-x}{{\textrm {d}}z}\left[{\frac {\sigma ^{12}}{10(r^{2}+z^{2})^{5}}}-{\frac {\sigma ^{6}}{4(r^{2}+z^{2})^{2}}}\right]_{r=\infty }^{r=0}.$

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Phi_{W} \left( x \right) = 8 \pi \epsilon_{sf} \rho_{s} \int_{-\infty}^{-x} {\textrm d z} \left[ \frac{ \sigma^{12}} { 10 z^{10} } - \frac{\sigma^6 }{ 4 z^4 } \right]; }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Phi_{W} \left( x \right) = 8 \pi \epsilon_{sf} \rho_s \left[ - \frac{ \sigma^{12}} { 90 z^{9} } + \frac{\sigma^6 }{ 12 z^3 } \right]_{z=-\infty}^{z=-x}; }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Phi_{W} \left( x \right) = \frac{4 \pi \epsilon_{sf} \rho_s \sigma^3}{3} \left[ \frac{ \sigma^{9}} { 15 x^{9} } - \frac{\sigma^3 }{ 2 x^3 } \right] }