Ramp model

The ramp model, proposed by Jagla and sometimes known as the Jagla model, is described by:

${\mathrm{\Phi }}_{12}(r)=\{\begin{array}{cc}\mathrm{\infty }& r<\sigma \\ W_r-(W_r-W_a)\frac{r-\sigma }{d_a-\sigma }& \sigma \le r\le d_a\\ W_a-W_a\frac{r-d_a}{d_c-d_a}& d_a<r\le d_c\\ 0& r>d_c\end{array}$

where ${\mathrm{\Phi }}_{12}(r)$ is the intermolecular pair potential, $r:=|{\mathbf{r}}_1-{\mathbf{r}}_2|$, $W_r>0$ and $W_a<0$.

Graphically, one has:

where the red line represents an attractive implementation of the model, and the green line a repulsive implementation.

Critical points

For the particular case $W_r^{*}=3.5;W_a^{*}=-1.0,d_a^{*}=1.72,d_c^{*}=3.0$, the liquid-vapour critical point is located at :

$T_c^{*}=1.487\pm 0.003$

${\rho }_c{\sigma }^3=0.103\pm 0.001$

$p_c^{*}\simeq 0.042$

and the liquid-liquid critical point:

$T_c^{*}\simeq 0.378\pm 0.003$

${\rho }_c{\sigma }^3\simeq 0.380\pm 0.002$

$p_c^{*}/T_c^{*}\simeq 0.49\pm 0.01$

Repulsive Ramp Model

In the repulsive ramp case, where $W_a=0$, neither liquid-vapor nor liquid-liquid stable equilibria occur . However, for this model a low density crystalline phase has been found. This solid phase presents re-entrant melting, i.e. this solid melts into the fluid phase as the pressure is increased.

Lattice gas version

Recently, similar behaviour has been found in a three-dimensional Repulsive Ramp Lattice Gas model The system is defined on a simple cubic lattice. The interaction is that of a lattice hard sphere model with exclusion of nearest neighbours of occupied positions plus a repulsive interaction with next-to-nearest neighbours. The total potential energy of the system is then given by:

$U=ϵ{\sum }_{[ij]}{S}_i{S}_j$

where $ϵ>0$ ; $[ij]$ refers to all the pairs of sites that are second neighbors, and $S_k$ indicates the occupation of site $k$ (0 indicates an empty site, 1 indicates an occupied site).

See also

• Polyamorphism: Ramp model

References

Related literature

• Limei Xu, Sergey V. Buldyrev, C. Austen Angell, and H. Eugene Stanley "Thermodynamics and dynamics of the two-scale spherically symmetric Jagla ramp model of anomalous liquids", Physical Review E 74 031108 (2006)
• Limei Xu, Sergey V. Buldyrev, Nicolas Giovambattista, C. Austen Angell, and H. Eugene Stanley "A monatomic system with a liquid-liquid critical point and two distinct glassy states", Journal of Chemical Physics 130 054505 (2009)