special_pair

Special pair force classes apply a force and virial on every particle in the simulation state commensurate with the potential energy:

\[U_\mathrm{special~pairs} = \sum_{(j,k) \in \mathrm{special~pairs}} U_{jk}(r)\]

Special pairs are used to implement interactions between designated pairs of particles. They act much like bonds, except that the interaction potential is typically a pair potential, such as LJ.

Each special pair is defined by an ordered pair of particle tags in the hoomd.State member pair_group. HOOMD-blue does not compute special pair groups, users must explicitly define special pairs in the initial condition.

In the special pair group (j,k), \(r\) is the length of the vector between the particle positions \(r= |\mathrm{minimum\_image}(\vec{r}_k - \vec{r}_j)|\).

Special pair force classes assign 1/2 of the potential energy to each of the particles in the bond group:

\[U_i = \frac{1}{2} \sum_{(j,k) \in \mathrm{special~pairs}} U_{jk}(r) [i=j \lor i=k]\]

and similarly for virials.

Classes

• Coulomb
• LJ
• SpecialPair