Forces

mechanica.Force: alias of mechanica.mechanica.MxForce

class mechanica.MxForce

MxForce is a metatype, in that Mechanica has lots of different instances of force functions, that have different attributes, but only have one base type.

Forces are one of the fundamental processes in Mechanica that cause objects to move.

bind_species(a_type: MxParticleType, coupling_symbol: std::string const &) → HRESULT

Bind a force to a species.

When a force is bound to a species, the magnitude of the force is scaled by the concentration of the species.

Parameters

a_type (MxParticleType) – particle type containing the species
coupling_symbol (string) – symbol of the species

Return type

int

Returns

HRESULT

static berenderson_tstat(tau: float const &) → Berendsen *

Creates a Berendsen thermostat.

The thermostat uses the target temperature \(T_0\) from the object to which it is bound. The Berendsen thermostat effectively re-scales the velocities of an object in order to make the temperature of that family of objects match a specified temperature.

The Berendsen thermostat force has the function form:

\[\frac{\mathbf{p}}{\tau_T} \left(\frac{T_0}{T} - 1 \right),\]

where \(\mathbf{p}\) is the momentum, \(T\) is the measured temperature of a family of particles, \(T_0\) is the control temperature, and \(\tau_T\) is the coupling constant. The coupling constant is a measure of the time scale on which the thermostat operates, and has units of time. Smaller values of \(\tau_T\) result in a faster acting thermostat, and larger values result in a slower acting thermostat.

Parameters: tau (float) – time constant that determines how rapidly the thermostat effects the system.
Return type: Berendsen
Returns: Berendsen*

static random(std: float const &, mean: float const &, duration: float const & = 0.01) → Gaussian *

Creates a random force.

A random force has a randomly selected orientation and magnitude.

Orientation is selected according to a uniform distribution on the unit sphere.

Magnitude is selected according to a prescribed mean and standard deviation.

Parameters

std (float) – standard deviation of magnitude
mean (float) – mean of magnitude
duration (float) – duration of force. Defaults to 0.01.

Return type

Gaussian

Returns

Gaussian*

static friction(coef: float const &, std: float const & = 0.0, mean: float const & = 0.0, duration: float const & = 0.1) → Friction *

Creates a friction force.

A friction force has the form:

\[- \frac{|| \mathbf{v} ||}{\tau} \mathbf{v} + \mathbf{f}_{r} ,\]

where \(\mathbf{v}\) is the velocity of a particle, \(\tau\) is a time constant and \(\mathbf{f}_r\) is a random force.

Parameters

coef (float) – time constant
std (float) – standard deviation of random force magnitude
mean (float) – mean of random force magnitude
duration (float) – duration of force. Defaults to 0.1.

Return type

Friction

Returns

Friction*

toString() → std::string

Get a JSON string representation

Return type: string
Returns: std::string

static fromString(str: std::string const &) → MxForce *

Create from a JSON string representation

Parameters: str (string) –
Return type: MxForce
Returns: MxForce*

__reduce__(): Helper for pickle.

mechanica.ConstantForce: alias of mechanica.mechanica.MxConstantForcePy

class mechanica.MxConstantForce(*args)

Bases: mechanica.mechanica.MxForce

A custom force function.

The force is updated according to an update frequency.

This object acts like a constant force, but also acts like a time event, in that it periodically calls a custom function to update the applied force.

property value

Current value of the force.

This can be set to a function that takes no arguments and returns a 3-component list of floats.

property period: Period of the force

static fromForce(f: MxForce) → MxConstantForce *

Convert basic force to MxConstantForce.

If the basic force is not a MxConstantForce, then NULL is returned.

Parameters: f (MxForce) –
Return type: MxConstantForce
Returns: MxConstantForce*

class mechanica.MxConstantForcePy(*args)

Bases: mechanica.mechanica.MxConstantForce

Creates an instance from an underlying custom python function

Parameters

f (PyObject) – python function. Takes no arguments and returns a three-component vector.
period (float) – period at which the force is updated.

static fromForce(f: MxForce) → MxConstantForcePy *