fix polarize/functional command¶

Syntax¶

fix ID group-ID style nevery tolerance ...

• ID, group-ID are documented in fix command

• style = polarize/bem/gmres or polarize/bem/icc or polarize/functional

• Nevery = this fixed is invoked every this many timesteps

• tolerance = the relative tolerance for the iterative solver to stop

Examples¶

fix 2 interface polarize/bem/gmres 5 0.0001
fix 1 interface polarize/bem/icc 1 0.0001
fix 3 interface polarize/functional 1 0.001


Used in input scripts:

examples/PACKAGES/dielectric/in.confined
examples/PACKAGES/dielectric/in.nopbc


Description¶

These fixes compute induced charges at the interface between two impermeable media with different dielectric constants. The interfaces need to be discretized into vertices, each representing a boundary element. The vertices are treated as if they were regular atoms or particles. atom_style dielectric should be used since it defines the additional properties of each interface particle such as interface normal vectors, element areas, and local dielectric mismatch. These fixes also require the use of pair_style and kspace_style with the dielectric suffix. At every time step, given a configuration of the physical charges in the system (such as atoms and charged particles) these fixes compute and update the charge of the interface particles. The interfaces are allowed to move during the simulation with appropriate time integrators (for example, with fix_rigid).

Consider an interface between two media: one with dielectric constant of 78 (water), the other of 4 (silica). The interface is discretized into 2000 boundary elements, each represented by an interface particle. Suppose that each interface particle has a normal unit vector pointing from the silica medium to water. The dielectric difference along the normal vector is then 78 - 4 = 74, the mean dielectric value is (78 + 4) / 2 = 41. Each boundary element also has its area and the local mean curvature (which is used by these fixes for computing a correction term in the local electric field). To model charged interfaces, the interface particle will have a non-zero charge value, coming from its area and surface charge density.

For non-interface particles such as atoms and charged particles, the interface normal vectors, element area, and dielectric mismatch are irrelevant. Their local dielectric value is used to rescale their actual charge when computing the Coulombic interactions. For instance, for a cation carrying a charge of +2 (in charge unit) in an implicit solvent with dielectric constant of 40 would have actual charge of +2, and a local dielectric constant value of 40. It is assumed that the particles cannot pass through the interface during the simulation so that its local dielectric constant value does not change.

There are some example scripts for using these fixes with LAMMPS in the examples/PACKAGES/dielectric directory. The README file therein contains specific details on the system setup. Note that the example data files show the additional fields (columns) needed for atom_style dielectric beyond the conventional fields id, mol, type, q, x, y, and z.

For fix polarize/bem/gmres and fix polarize/bem/icc the induced charges of the atoms in the specified group, which are the vertices on the interface, are computed using the equation:

$\sigma_b(\mathbf{s}) = \dfrac{1 - \bar{\epsilon}}{\bar{\epsilon}} \sigma_f(\mathbf{s}) - \epsilon_0 \dfrac{\Delta \epsilon}{\bar{\epsilon}} \mathbf{E}(\mathbf{s}) \cdot \mathbf{n}(\mathbf{s})$
• $$\sigma_b$$ is the induced charge density at the interface vertex $$\mathbf{s}$$.

• $$\bar{\epsilon}$$ is the mean dielectric constant at the interface vertex: $$\bar{\epsilon} = (\epsilon_1 + \epsilon_2)/2$$.

• $$\Delta \epsilon$$ is the dielectric constant difference at the interface vertex: $$\Delta \epsilon = \epsilon_1 - \epsilon_2$$

• $$\sigma_f$$ is the free charge density at the interface vertex

• $$\mathbf{E}(\mathbf{s})$$ is the electrical field at the vertex

• $$\mathbf{n}(\mathbf{s})$$ is the unit normal vector at the vertex pointing from medium with $$\epsilon_2$$ to that with $$\epsilon_1$$

Fix polarize/bem/gmres employs the Generalized Minimum Residual (GMRES) as described in (Barros) to solve $$\sigma_b$$.

Fix polarize/bem/icc employs the successive over-relaxation algorithm as described in (Tyagi) to solve $$\sigma_b$$.

The iterative solvers would terminate either when the maximum relative change in the induced charges in consecutive iterations is below the set tolerance, or when the number of iterations reaches iter_max (see below).

Fix polarize/functional employs the energy functional variation approach as described in (Jadhao) to solve $$\sigma_b$$.

More details on the implementation of these fixes and their recommended use are described in (NguyenTD).

Restart, fix_modify, output, run start/stop, minimize info¶

The fix_modify command provides certain options to control the induced charge solver and the initial values of the interface elements:

itr_max arg
arg = maximum number of iterations for convergence
dielectrics ediff emean epsilon area charge
ediff = dielectric difference
emean = dielectric mean
epsilon = local dielectric value
aree = element area
charge = real interface charge

polarize/bem/gmres or polarize/bem/icc compute a global 2-element vector which can be accessed by various output commands. The first element is the number of iterations when the solver terminates (of which the upper bound is set by iter_max). The second element is the RMS error.

Restrictions¶

These fixes are part of the DIELECTRIC package. It is only enabled if LAMMPS was built with that package, which requires that also the KSPACE package is installed. See the Build package page for more info.

Note that the polarize/bem/gmres and polarize/bem/icc fixes only support units lj, real, metal, si and nano at the moment.

Default¶

iter_max = 20

(Barros) Barros, Sinkovits, Luijten, J. Chem. Phys, 140, 064903 (2014)

(Tyagi) Tyagi, Suzen, Sega, Barbosa, Kantorovich, Holm, J Chem Phys, 132, 154112 (2010)

(Jadhao) Jadhao, Solis, Olvera de la Cruz, J Chem Phys, 138, 054119 (2013)

(NguyenTD) Nguyen, Li, Bagchi, Solis, Olvera de la Cruz, Comput Phys Commun 241, 80-19 (2019)