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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.

Changed in version 29Aug2024.

This fix is incompatible with deformation controls that remap velocity, for instance the remap v option of fix deform.

## 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