.. index:: pair_style dpd/fdt pair_style dpd/fdt command ========================== pair_style dpd/fdt/energy command ================================= Syntax """""" .. parsed-literal:: pair_style style args * style = *dpd/fdt* or *dpd/fdt/energy* * args = list of arguments for a particular style .. parsed-literal:: *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 """""""" .. parsed-literal:: pair_style dpd/fdt 300.0 2.5 34387 pair_coeff * * 3.0 1.0 2.5 .. parsed-literal:: 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 .. image:: Eqs/pair_dpd_conservative.jpg :align: center where the weighting factor, omega_ij, varies between 0 and 1, and is chosen to have the following functional form: .. image:: Eqs/pair_dpd_omega.jpg :align: center 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 :doc:`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 :doc:`fix shardlow ` to perform DPD simulations under isothermal and isobaric conditions (see :ref:`(Lisal) `). The following coefficients must be defined for each pair of atoms types via the :doc:`pair_coeff ` command as in the examples above, or in the data file or restart files read by the :doc:`read_data ` or :doc:`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 :doc:`fix shardlow ` to perform DPD simulations under isoenergetic and isoenthalpic conditions (see :ref:`(Lisal) `). The following coefficients must be defined for each pair of atoms types via the :doc:`pair_coeff ` command as in the examples above, or in the data file or restart files read by the :doc:`read_data ` or :doc:`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 :doc:`fix eos ` must be specified. ---------- Restrictions """""""""""" Pair styles *dpd/fdt* and *dpd/fdt/energy* are only available if LAMMPS is built with the USER-DPD package. Pair styles *dpd/fdt* and *dpd/fdt/energy* require use of the :doc:`communicate vel yes ` option so that velocites are stored by ghost atoms. Pair style *dpd/fdt/energy* requires :doc:`atom_style dpd ` to be used in order to properly account for the particle internal energies and temperatures. Related commands """""""""""""""" :doc:`pair_coeff `, :doc:`fix shardlow ` **Default:** none ---------- .. _Lisal: **(Lisal)** M. Lisal, J.K. Brennan, J. Bonet Avalos, "Dissipative particle dynamics as isothermal, isobaric, isoenergetic, and isoenthalpic conditions using Shardlow-like splitting algorithms.", J. Chem. Phys., 135, 204105 (2011). .. _lws: http://lammps.sandia.gov .. _ld: Manual.html .. _lc: Section_commands.html#comm