pair_style dpd/fdt command
pair_style dpd/fdt/energy command
Syntax
pair_style style args
- style = dpd/fdt or dpd/fdt/energy
- args = list of arguments for a particular style
dpd/fdt args = T cutoff seed T = temperature (temperature units) cutoff = global cutoff for DPD interactions (distance units) seed = random # seed (positive integer) dpd/fdt/energy args = cutoff seed cutoff = global cutoff for DPD interactions (distance units) seed = random # seed (positive integer)
Examples
pair_style dpd/fdt 300.0 2.5 34387
pair_coeff * * 3.0 1.0 2.5
pair_style dpd/fdt/energy 2.5 34387
pair_coeff * * 3.0 1.0 0.1 2.5
Description
Styles dpd/fdt and dpd/fdt/energy set the fluctuation-dissipation theorem parameters and compute the conservative force for dissipative particle dynamics (DPD). The conservative force on atom I due to atom J is given by

where the weighting factor, omega_ij, varies between 0 and 1, and is chosen to have the following functional form:

where Rij is a unit vector in the direction Ri - Rj, and Rc is the cutoff. Note that alternative definitions of the weighting function exist, but would have to be implemented as a separate pair style command.
These pair style differ from the other dpd styles in that the dissipative and random forces are not computed within the pair style. This style can be combined with the fix shardlow to perform the stochastic integration of the dissipative and random forces through the Shardlow splitting algorithm approach.
The pairwise energy associated with styles dpd/fdt and dpd/fdt/energy is only due to the conservative force term Fc, and is shifted to be zero at the cutoff distance Rc. The pairwise virial is calculated using only the conservative term.
For style dpd/fdt, the fluctuation-dissipation theorem defines gamma to be set equal to sigma*sigma/(2 T), where T is the set point temperature specified as a pair style parameter in the above examples. This style can be combined with fix shardlow to perform DPD simulations under isothermal and isobaric conditions (see (Lisal)). The following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands:
- A (force units)
- sigma (force*time^(1/2) units)
- cutoff (distance units)
The last coefficient is optional. If not specified, the global DPD cutoff is used.
For style dpd/fdt/energy, the fluctuation-dissipation theorem defines gamma to be set equal to sigma*sigma/(2 dpdTheta), where dpdTheta is the average internal temperature for the pair. Furthermore, the fluctuation-dissipation defines alpha*alpha to be set equal to 2*kB*kappa, where kappa is the mesoparticle thermal conductivity parameter. This style can be combined with fix shardlow to perform DPD simulations under isoenergetic and isoenthalpic conditions (see (Lisal)). The following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands:
- A (force units)
- sigma (force*time^(1/2) units)
- kappa (1/time units)
- cutoff (distance units)
The last coefficient is optional. If not specified, the global DPD cutoff is used.
For style dpd/fdt/energy, the particle internal temperature is related to the particle internal energy through a mesoparticle equation of state. Thus, an an additional fix eos must be specified.
Restrictions
These commands are part of the USER-DPD package. They are only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.
Pair styles dpd/fdt and dpd/fdt/energy require use of the communicate vel yes option so that velocites are stored by ghost atoms.
Pair style dpd/fdt/energy requires atom_style dpd to be used in order to properly account for the particle internal energies and temperatures.