<P><B>Description:</B>

</P>

<P>Apply a Langevin thermostat as described in <A HREF = "#Schneider">(Schneider)</A>

to a group of atoms which models an interaction with a background

implicit solvent.  Used with <A HREF = "fix_nve_eff.html">fix nve/eff</A>, this command

performs Brownian dynamics (BD), since the total force on each atom

will have the form:

to a group of nuclei and electrons in the <A HREF = "pair_eff.html">electron force

field</A> model.  Used with <A HREF = "fix_nve_eff.html">fix nve/eff</A>,

this command performs Brownian dynamics (BD), since the total force on

each atom will have the form:

</P>

<PRE>F = Fc + Ff + Fr

Ff = - (m / damp) v

</P>

<P>The Ff and Fr terms are added by this fix on a per-particle basis.

</P>

<P>Ff is a frictional drag or viscous damping term proportional to the

particle's velocity (including the electronic radial degrees of

freedom).  The proportionality constant for each atom is computed as

m/damp, where m is the mass of the particle and damp is the damping

factor specified by the user.

</P>

<P>Fr is a force due to solvent atoms at a temperature T randomly bumping

into the particle.  As derived from the fluctuation/dissipation

theorem, its magnitude as shown above is proportional to sqrt(Kb T m /

dt damp), where Kb is the Boltzmann constant, T is the desired

temperature, m is the mass of the particle, dt is the timestep size,

and damp is the damping factor.  Random numbers are used to randomize

the direction and magnitude of this force as described in

<A HREF = "#Dunweg">(Dunweg)</A>, where a uniform random number is used (instead of

a Gaussian random number) for speed.

</P>

<P>Note that the thermostat effect of this fix is applied to the

translational and electronic radial degrees of freedom for the

particles in a system.  The translational degrees of freedom can also

have a bias velocity removed from them before thermostatting takes

place; see the description below.

</P>

<P>IMPORTANT NOTE: Unlike the <A HREF = "fix_nh_eff.html">fix nvt/eff</A> command which

performs Nose/Hoover thermostatting AND time integration, this fix

does NOT perform time integration.  It only modifies forces to effect

thermostatting.  Thus you must use a separate time integration fix,

like <A HREF = "fix_nve_eff.html">fix nve/eff</A> to actually update the velocities

(including electronic radial velocities) and positions of atoms

(nuclei and electrons) using the modified forces.  Likewise, this fix

should not normally be used on atoms that also have their temperature

controlled by another fix - e.g. by <A HREF = "fix_nh.html">fix nvt/eff</A> command.

</P>

<P>See <A HREF = "Section_howto.html#4_16">this howto section</A> of the manual for a

discussion of different ways to compute temperature and perform

thermostatting.

</P>

<P>The desired temperature at each timestep is a ramped value during the

run from <I>Tstart</I> to <I>Tstop</I>.

</P>

<P>Like other fixes that perform thermostatting, this fix can be used

with <A HREF = "compute.html">compute commands</A> that remove a "bias" from the

atom velocities.  E.g. removing the center-of-mass velocity from a

group of atoms or removing the x-component of velocity from the

calculation.  This is not done by default, but only if the

<A HREF = "fix_modify.html">fix_modify</A> command is used to assign a temperature

compute to this fix that includes such a bias term.  See the doc pages

for individual <A HREF = "compute.html">compute commands</A> to determine which ones

include a bias.  In this case, the thermostat works in the following

manner: bias is removed from each atom, thermostatting is performed on

the remaining thermal degrees of freedom, and the bias is added back

in.

</P>

<P>The <I>damp</I> parameter is specified in time units and determines how

rapidly the temperature is relaxed.  For example, a value of 0.1 means

to relax the temperature in a timespan of 100 timesteps (for the

default 0.001 timesteps and fmsec unit used in eFF - see the

<A HREF = "units.html">units</A> command).  The damp factor can be thought of as

inversely related to the viscosity of the solvent.  I.e. a small

relaxation time implies a hi-viscosity solvent and vice versa.  See

the discussion about gamma and viscosity in the documentation for the

<A HREF = "fix_viscous.html">fix viscous</A> command for more details.

</P>

<P>The random # <I>seed</I> must be a positive integer.  A Marsaglia random

number generator is used.  Each processor uses the input seed to

generate its own unique seed and its own stream of random numbers.

Thus the dynamics of the system will not be identical on two runs on

different numbers of processors.

</P>

<P>The keyword <I>scale</I> allows the damp factor to be scaled up or down by

the specified factor for atoms of that type.  This can be useful when

different atom types have different sizes or masses.  It can be used

multiple times to adjust damp for several atom types.  Note that

specifying a ratio of 2 increases the relaxation time which is

equivalent to the solvent's viscosity acting on particles with 1/2 the

diameter.  This is the opposite effect of scale factors used by the

<A HREF = "fix_viscous.html">fix viscous</A> command, since the damp factor in fix

<I>langevin</I> is inversely related to the gamma factor in fix <I>viscous</I>.

Also note that the damping factor in fix <I>langevin</I> includes the

particle mass in Ff, unlike fix <I>viscous</I>.  Thus the mass and size of

different atom types should be accounted for in the choice of ratio

values.

</P>

<P>The keyword <I>tally</I> enables the calculation of the cummulative energy

added/subtracted to the atoms as they are thermostatted.  Effectively

it is the energy exchanged between the infinite thermal reservoir and

the particles.  As described below, this energy can then be printed

out or added to the potential energy of the system to monitor energy

conservation.

<P>The operation of this fix is exactly like that described by the <A HREF = "fix_langevin.html">fix

langevin</A> command, except that the thermostatting

is also applied to the radial electron velocity for electron

particles.

</P>

<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>

</P>

[Description:]

Apply a Langevin thermostat as described in "(Schneider)"_#Schneider

to a group of atoms which models an interaction with a background

implicit solvent.  Used with "fix nve/eff"_fix_nve_eff.html, this command

performs Brownian dynamics (BD), since the total force on each atom

will have the form:

to a group of nuclei and electrons in the "electron force

field"_pair_eff.html model.  Used with "fix nve/eff"_fix_nve_eff.html,

this command performs Brownian dynamics (BD), since the total force on

each atom will have the form:

F = Fc + Ff + Fr

Ff = - (m / damp) v

The Ff and Fr terms are added by this fix on a per-particle basis.

Ff is a frictional drag or viscous damping term proportional to the

particle's velocity (including the electronic radial degrees of

freedom).  The proportionality constant for each atom is computed as

m/damp, where m is the mass of the particle and damp is the damping

factor specified by the user.

Fr is a force due to solvent atoms at a temperature T randomly bumping

into the particle.  As derived from the fluctuation/dissipation

theorem, its magnitude as shown above is proportional to sqrt(Kb T m /

dt damp), where Kb is the Boltzmann constant, T is the desired

temperature, m is the mass of the particle, dt is the timestep size,

and damp is the damping factor.  Random numbers are used to randomize

the direction and magnitude of this force as described in

"(Dunweg)"_#Dunweg, where a uniform random number is used (instead of

a Gaussian random number) for speed.

Note that the thermostat effect of this fix is applied to the

translational and electronic radial degrees of freedom for the

particles in a system.  The translational degrees of freedom can also

have a bias velocity removed from them before thermostatting takes

place; see the description below.

IMPORTANT NOTE: Unlike the "fix nvt/eff"_fix_nh_eff.html command which

performs Nose/Hoover thermostatting AND time integration, this fix

does NOT perform time integration.  It only modifies forces to effect

thermostatting.  Thus you must use a separate time integration fix,

like "fix nve/eff"_fix_nve_eff.html to actually update the velocities

(including electronic radial velocities) and positions of atoms

(nuclei and electrons) using the modified forces.  Likewise, this fix

should not normally be used on atoms that also have their temperature

controlled by another fix - e.g. by "fix nvt/eff"_fix_nh.html command.

See "this howto section"_Section_howto.html#4_16 of the manual for a

discussion of different ways to compute temperature and perform

thermostatting.

The desired temperature at each timestep is a ramped value during the

run from {Tstart} to {Tstop}.

Like other fixes that perform thermostatting, this fix can be used

with "compute commands"_compute.html that remove a "bias" from the

atom velocities.  E.g. removing the center-of-mass velocity from a

group of atoms or removing the x-component of velocity from the

calculation.  This is not done by default, but only if the

"fix_modify"_fix_modify.html command is used to assign a temperature

compute to this fix that includes such a bias term.  See the doc pages

for individual "compute commands"_compute.html to determine which ones

include a bias.  In this case, the thermostat works in the following

manner: bias is removed from each atom, thermostatting is performed on

the remaining thermal degrees of freedom, and the bias is added back

in.

The {damp} parameter is specified in time units and determines how

rapidly the temperature is relaxed.  For example, a value of 0.1 means

to relax the temperature in a timespan of 100 timesteps (for the

default 0.001 timesteps and fmsec unit used in eFF - see the

"units"_units.html command).  The damp factor can be thought of as

inversely related to the viscosity of the solvent.  I.e. a small

relaxation time implies a hi-viscosity solvent and vice versa.  See

the discussion about gamma and viscosity in the documentation for the

"fix viscous"_fix_viscous.html command for more details.

The random # {seed} must be a positive integer.  A Marsaglia random

number generator is used.  Each processor uses the input seed to

generate its own unique seed and its own stream of random numbers.

Thus the dynamics of the system will not be identical on two runs on

different numbers of processors.

The keyword {scale} allows the damp factor to be scaled up or down by

the specified factor for atoms of that type.  This can be useful when

different atom types have different sizes or masses.  It can be used

multiple times to adjust damp for several atom types.  Note that

specifying a ratio of 2 increases the relaxation time which is

equivalent to the solvent's viscosity acting on particles with 1/2 the

diameter.  This is the opposite effect of scale factors used by the

"fix viscous"_fix_viscous.html command, since the damp factor in fix

{langevin} is inversely related to the gamma factor in fix {viscous}.

Also note that the damping factor in fix {langevin} includes the

particle mass in Ff, unlike fix {viscous}.  Thus the mass and size of

different atom types should be accounted for in the choice of ratio

values.

The keyword {tally} enables the calculation of the cummulative energy

added/subtracted to the atoms as they are thermostatted.  Effectively

it is the energy exchanged between the infinite thermal reservoir and

the particles.  As described below, this energy can then be printed

out or added to the potential energy of the system to monitor energy

conservation.

The operation of this fix is exactly like that described by the "fix

langevin"_fix_langevin.html command, except that the thermostatting

is also applied to the radial electron velocity for electron

particles.

[Restart, fix_modify, output, run start/stop, minimize info:]

<P><B>Description:</B>

</P>

<P>Perform constant NVE integration to update position and velocity for

atoms in the group each timestep, including the translational and radial electron motions.  V is volume; E is energy.  This

creates a system trajectory consistent with the microcanonical

ensemble.

nuclei and electrons in the group for the <A HREF = "pair_eff.html">electron force

field</A> model.  V is volume; E is energy.  This creates a

system trajectory consistent with the microcanonical ensemble.

</P>

<P>The operation of this fix is exactly like that described by the <A HREF = "fix_nve.html">fix

nve</A> command, except that the radius and radial velocity

of electrons are also updated.

</P>

<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>

</P>

</P>

<P><B>Related commands:</B>

</P>

<P><A HREF = "fix_nh.html">fix nvt/eff</A>, <A HREF = "fix_nh.html">fix npt/eff</A>

<P><A HREF = "fix_nve.html">fix nve</A>, <A HREF = "fix_nh_eff.html">fix nvt/eff</A>, <A HREF = "fix_nh_eff.html">fix

npt/eff</A>

</P>

<P><B>Default:</B> none

</P>