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# fix mvv/tdpd command

## Syntax

fix ID group-ID mvv/dpd lambda

fix ID group-ID mvv/edpd lambda

fix ID group-ID mvv/tdpd lambda

• ID, group-ID are documented in fix command

• mvv/dpd, mvv/edpd, mvv/tdpd = style name of this fix command

• lambda = (optional) relaxation parameter (unitless)

## Examples

fix 1 all mvv/dpd
fix 1 all mvv/dpd 0.5
fix 1 all mvv/edpd
fix 1 all mvv/edpd 0.5
fix 1 all mvv/tdpd
fix 1 all mvv/tdpd 0.5


## Description

Perform time integration using the modified velocity-Verlet (MVV) algorithm to update position and velocity (fix mvv/dpd), or position, velocity and temperature (fix mvv/edpd), or position, velocity and concentration (fix mvv/tdpd) for particles in the group each timestep.

The modified velocity-Verlet (MVV) algorithm aims to improve the stability of the time integrator by using an extrapolated version of the velocity for the force evaluation:

$\begin{split}v(t+\frac{\Delta t}{2}) = & v(t) + \frac{\Delta t}{2}\cdot a(t) \\ r(t+\Delta t) = & r(t) + \Delta t\cdot v(t+\frac{\Delta t}{2}) \\ a(t+\Delta t) = & \frac{1}{m}\cdot F\left[ r(t+\Delta t), v(t) +\lambda \cdot \Delta t\cdot a(t)\right] \\ v(t+\Delta t) = & v(t+\frac{\Delta t}{2}) + \frac{\Delta t}{2}\cdot a(t+\Delta t)\end{split}$

where the parameter $$\lambda$$ depends on the specific choice of DPD parameters, and needs to be tuned on a case-by-case basis. Specification of a lambda value is optional. If specified, the setting must be from 0.0 to 1.0. If not specified, a default value of 0.5 is used, which effectively reproduces the standard velocity-Verlet (VV) scheme. For more details, see Groot.

Fix mvv/dpd updates the position and velocity of each atom. It can be used with the pair_style mdpd command or other pair styles such as pair dpd.

Fix mvv/edpd updates the per-atom temperature, in addition to position and velocity, and must be used with the pair_style edpd command.

Fix mvv/tdpd updates the per-atom chemical concentration, in addition to position and velocity, and must be used with the pair_style tdpd command.

## Restart, fix_modify, output, run start/stop, minimize info

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

## Restrictions

These fixes are part of the DPD-MESO package. They are only enabled if LAMMPS was built with that package. See the Build package page for more info.

## Default

The default value for the optional lambda parameter is 0.5.

(Groot) Groot and Warren, J Chem Phys, 107: 4423-4435 (1997). DOI: 10.1063/1.474784