pair_style square/well

Syntax

pair_style square/well cutoff

  • cutoff = cutoff for square/well interactions (distance units)

Examples

pair_style    square/well 2.0 
pair_coeff    1 2 square/well 5.0 0.33 0.005 1.32

Description

The square/well style computes a continuous version of the square-well potential given by:

\[ U_{wp} (r)=-\frac{1}{2} \epsilon_w\left[1-\tanh\left(\frac{r-r_w}{\alpha}\right)\right] \quad \text{for} \quad r<r_{c} \]

where \(r_{c}\) is the cutoff.

The following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the example above, or in the data file or restart files read by the read_data or read_restart commands:

  • \(\epsilon_w\) (energy units)

  • \(r_{w}\) (distance units)

  • \(\alpha\) (distance units)

  • \(r_{c}\) (distance units)

This continuous square-well potential function was proposed in Espinosa et al.1 to calculate liquid-crystal interfacial free energies through the Mold Integration, and later extended to the Lattice Mold technique (Espinosa et al.2 and Sanchez-Burgos et al.3). Also, it can be used to simulate attractive patches distributed along the surface of a sphere, known as “patchy particles” (Espinosa et al.4).

Warning

The value of \(\alpha\) cannot be too small to avoid strong forces acting on the particles trapped inside the wells. If small values are needed (steep potential), the simulation timestep must be reduced (Sanchez-Burgos and Espinosa5).

Note

In case \(\epsilon\) is set to a negative value, the well potential would become fully repulsive.

Note

In the *cpp and *h files you may find the word “pocillo” with the meaning of “small well” in Spanish.

Restrictions

This pair style can only be used if LAMMPS was built with the Mold package. See the Build package doc page for more info.

1

JR Espinosa, C Vega, and E Sanz. The mold integration method for the calculation of the crystal-fluid interfacial free energy from simulations. The Journal of chemical physics, 141(13):134709, 2014.

2

Jorge R Espinosa, Pablo Sampedro, Chantal Valeriani, Carlos Vega, and Eduardo Sanz. Lattice mold technique for the calculation of crystal nucleation rates. Faraday discussions, 195:569–582, 2016.

3

Ignacio Sanchez-Burgos, Andres R Tejedor, Carlos Vega, Maria M Conde, Eduardo Sanz, Jorge Ramirez, and Jorge R Espinosa. Homogeneous ice nucleation rates for mw and tip4p/ice models through lattice mold calculations. The Journal of Chemical Physics, 157(9):094503, 2022.

4

Jorge R Espinosa, Adiran Garaizar, Carlos Vega, Daan Frenkel, and Rosana Collepardo-Guevara. Breakdown of the law of rectilinear diameter and related surprises in the liquid-vapor coexistence in systems of patchy particles. The Journal of chemical physics, 150(22):224510, 2019.

5

Ignacio Sanchez-Burgos and Jorge R Espinosa. Direct calculation of the interfacial free energy between nacl crystal and its aqueous solution at the solubility limit. Physical Review Letters, 130(11):118001, 2023.