.. index:: pair_style dpd pair_style dpd command ====================== pair_style dpd/gpu command ========================== pair_style dpd/omp command ========================== pair_style dpd/tstat command ============================ pair_style dpd/tstat/gpu command ================================ pair_style dpd/tstat/omp command ================================ Syntax """""" .. parsed-literal:: pair_style dpd T cutoff seed pair_style dpd/tstat Tstart Tstop cutoff seed * T = temperature (temperature units) * Tstart,Tstop = desired temperature at start/end of run (temperature units) * cutoff = global cutoff for DPD interactions (distance units) * seed = random # seed (positive integer) Examples """""""" .. parsed-literal:: pair_style dpd 1.0 2.5 34387 pair_coeff * * 3.0 1.0 pair_coeff 1 1 3.0 1.0 1.0 .. parsed-literal:: pair_style dpd/tstat 1.0 1.0 2.5 34387 pair_coeff * * 1.0 pair_coeff 1 1 1.0 1.0 Description """"""""""" Style *dpd* computes a force field for dissipative particle dynamics (DPD) following the exposition in :ref:`(Groot) `. Style *dpd/tstat* invokes a DPD thermostat on pairwise interactions, which is equivalent to the non-conservative portion of the DPD force field. This pair-wise thermostat can be used in conjunction with any :doc:`pair style `, and in leiu of per-particle thermostats like :doc:`fix langevin ` or ensemble thermostats like Nose Hoover as implemented by :doc:`fix nvt `. To use *dpd/tstat* as a thermostat for another pair style, use the :doc:`pair_style hybrid/overlay ` command to compute both the desired pair interaction and the thermostat for each pair of particles. For style *dpd*\ , the force on atom I due to atom J is given as a sum of 3 terms .. image:: Eqs/pair_dpd.jpg :align: center where Fc is a conservative force, Fd is a dissipative force, and Fr is a random force. Rij is a unit vector in the direction Ri - Rj, Vij is the vector difference in velocities of the two atoms = Vi - Vj, alpha is a Gaussian random number with zero mean and unit variance, dt is the timestep size, and w(r) is a weighting factor that varies between 0 and 1. Rc is the cutoff. Sigma is set equal to sqrt(2 Kb T gamma), where Kb is the Boltzmann constant and T is the temperature parameter in the pair_style command. For style *dpd/tstat*\ , the force on atom I due to atom J is the same as the above equation, except that the conservative Fc term is dropped. Also, during the run, T is set each timestep to a ramped value from Tstart to Tstop. For style *dpd*\ , the pairwise energy associated with style *dpd* 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 all 3 terms. For style *dpd/tstat* there is no pairwise energy, but the last two terms of the formula make a contribution to the virial. For style *dpd*\ , 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) * gamma (force/velocity units) * cutoff (distance units) The last coefficient is optional. If not specified, the global DPD cutoff is used. Note that sigma is set equal to sqrt(2 T gamma), where T is the temperature set by the :doc:`pair_style ` command so it does not need to be specified. For style *dpd/tstat*\ , the coefficiencts defined for each pair of atoms types via the :doc:`pair_coeff ` command is the same, except that A is not included. The GPU-accelerated versions of these styles are implemented based on the work of :ref:`(Afshar) ` and :ref:`(Phillips) `. .. note:: If you are modeling DPD polymer chains, you may want to use the :doc:`pair_style srp ` command in conjuction with these pair styles. It is a soft segmental repulsive potential (SRP) that can prevent DPD polymer chains from crossing each other. .. note:: The virial calculation for pressure when using this pair style includes all the components of force listed above, including the random force. ---------- Styles with a *cuda*\ , *gpu*\ , *intel*\ , *kk*\ , *omp*\ , or *opt* suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed in :doc:`Section_accelerate ` of the manual. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues. These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL, KOKKOS, USER-OMP and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the :ref:`Making LAMMPS ` section for more info. You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the :ref:`-suffix command-line switch ` when you invoke LAMMPS, or you can use the :doc:`suffix ` command in your input script. See :doc:`Section_accelerate ` of the manual for more instructions on how to use the accelerated styles effectively. ---------- **Mixing, shift, table, tail correction, restart, rRESPA info**\ : These pair styles do not support mixing. Thus, coefficients for all I,J pairs must be specified explicitly. These pair styles do not support the :doc:`pair_modify ` shift option for the energy of the pair interaction. Note that as discussed above, the energy due to the conservative Fc term is already shifted to be 0.0 at the cutoff distance Rc. The :doc:`pair_modify ` table option is not relevant for these pair styles. These pair style do not support the :doc:`pair_modify ` tail option for adding long-range tail corrections to energy and pressure. These pair styles writes their information to :doc:`binary restart files `, so pair_style and pair_coeff commands do not need to be specified in an input script that reads a restart file. Note that the user-specified random number seed is stored in the restart file, so when a simulation is restarted, each processor will re-initialize its random number generator the same way it did initially. This means the random forces will be random, but will not be the same as they would have been if the original simulation had continued past the restart time. These pair styles can only be used via the *pair* keyword of the :doc:`run_style respa ` command. They do not support the *inner*\ , *middle*\ , *outer* keywords. The *dpd/tstat* style can ramp its target temperature over multiple runs, using the *start* and *stop* keywords of the :doc:`run ` command. See the :doc:`run ` command for details of how to do this. ---------- Restrictions """""""""""" The default frequency for rebuilding neighbor lists is every 10 steps (see the :doc:`neigh_modify ` command). This may be too infrequent for style *dpd* simulations since particles move rapidly and can overlap by large amounts. If this setting yields a non-zero number of "dangerous" reneighborings (printed at the end of a simulation), you should experiment with forcing reneighboring more often and see if system energies/trajectories change. These pair styles requires you to use the :doc:`comm_modify vel yes ` command so that velocites are stored by ghost atoms. These pair styles will not restart exactly when using the :doc:`read_restart ` command, though they should provide statistically similar results. This is because the forces they compute depend on atom velocities. See the :doc:`read_restart ` command for more details. Related commands """""""""""""""" :doc:`pair_coeff `, :doc:`fix nvt `, :doc:`fix langevin `, :doc:`pair_style srp ` **Default:** none ---------- .. _Groot: **(Groot)** Groot and Warren, J Chem Phys, 107, 4423-35 (1997). .. _Afshar: **(Afshar)** Afshar, F. Schmid, A. Pishevar, S. Worley, Comput Phys Comm, 184, 1119-1128 (2013). .. _Phillips: **(Phillips)** C. L. Phillips, J. A. Anderson, S. C. Glotzer, Comput Phys Comm, 230, 7191-7201 (2011). .. _lws: http://lammps.sandia.gov .. _ld: Manual.html .. _lc: Section_commands.html#comm