MonteCarloMembraneBarostat

class OpenMM::MonteCarloMembraneBarostat : public OpenMM::Force

This is a Monte Carlo barostat designed specifically for membrane simulations. It assumes the membrane lies in the XY plane. The Monte Carlo acceptance criterion includes a term to model isotropic pressure, which depends on the volume of the periodic box, and a second term to model surface tension, which depends on the cross sectional area of the box in the XY plane. Note that pressure and surface tension are defined with opposite senses: a larger pressure tends to make the box smaller, but a larger surface tension tends to make the box larger.

There are options for configuring exactly how the various box dimensions are allowed to change:

• The X and Y axes may be treated isotropically, in which case they always scale by the same amount and remain in proportion to each other; or they may be treated anisotropically, in which case they can vary independently of each other.

• The Z axis can be allowed to vary independently of the other axes; or held fixed; or constrained to vary in inverse proportion to the other two axes, so that the total box volume remains fixed.

This class assumes the simulation is also being run at constant temperature, and requires you to specify the system temperature (since it affects the acceptance probability for Monte Carlo moves). It does not actually perform temperature regulation, however. You must use another mechanism along with it to maintain the temperature, such as LangevinIntegrator or AndersenThermostat.

Public Types

enum XYMode

This is an enumeration of the different behaviors for the X and Y axes.

Values:

enumerator XYIsotropic

The X and Y axes are always scaled by the same amount, so the ratio of their lengths remains constant.

enumerator XYAnisotropic

The X and Y axes are allowed to vary independently of each other.

enum ZMode

This is an enumeration of the different behaviors for Z axis.

Values:

enumerator ZFree

The Z axis is allowed to vary freely, independent of the other two axes.

enumerator ZFixed

The Z axis is held fixed and does not change.

enumerator ConstantVolume

The Z axis is always scaled in inverse proportion to the other two axes so the box volume remains fixed. Note that in this mode pressure has no effect on the system, only surface tension.

Public Functions

MonteCarloMembraneBarostat(double defaultPressure, double defaultSurfaceTension, double defaultTemperature, XYMode xymode, ZMode zmode, int frequency = 25)

Create a MonteCarloMembraneBarostat.

Parameters

• defaultPressure – the default pressure acting on the system (in bar)

• defaultSurfaceTension – the default surface tension acting on the system (in bar*nm)

• defaultTemperature – the default temperature at which the system is being maintained (in Kelvin)

• xymode – the mode specifying the behavior of the X and Y axes

• zmode – the mode specifying the behavior of the Z axis

• frequency – the frequency at which Monte Carlo volume changes should be attempted (in time steps)

inline double getDefaultPressure() const

Get the default pressure acting on the system (in bar).

Returns

the default pressure acting on the system, measured in bar.

inline void setDefaultPressure(double pressure)

Set the default pressure acting on the system. This will affect any new Contexts you create, but not ones that already exist.

Parameters

pressure – the default pressure acting on the system, measured in bar.

inline double getDefaultSurfaceTension() const

Get the default surface tension acting on the system (in bar*nm).

Returns

the default surface tension acting on the system, measured in bar*nm.

inline void setDefaultSurfaceTension(double surfaceTension)

Set the default surface tension acting on the system. This will affect any new Contexts you create, but not ones that already exist.

Parameters

surfaceTension – the default surface tension acting on the system, measured in bar.

inline int getFrequency() const

Get the frequency (in time steps) at which Monte Carlo volume changes should be attempted. If this is set to 0, the barostat is disabled.

inline void setFrequency(int freq)

Set the frequency (in time steps) at which Monte Carlo volume changes should be attempted. If this is set to 0, the barostat is disabled.

inline double getDefaultTemperature() const

Get the default temperature at which the system is being maintained, measured in Kelvin.

inline void setDefaultTemperature(double temp)

Set the default temperature at which the system is being maintained. This will affect any new Contexts you create, but not ones that already exist.

Parameters

temp – the system temperature, measured in Kelvin.

inline XYMode getXYMode() const

Get the mode specifying the behavior of the X and Y axes.

inline void setXYMode(XYMode mode)

Set the mode specifying the behavior of the X and Y axes.

inline ZMode getZMode() const

Get the mode specifying the behavior of the Z axis.

inline void setZMode(ZMode mode)

Set the mode specifying the behavior of the Z axis.

inline int getRandomNumberSeed() const

Get the random number seed. See setRandomNumberSeed() for details.

inline void setRandomNumberSeed(int seed)

Set the random number seed. It is guaranteed that if two simulations are run with different random number seeds, the sequence of Monte Carlo steps will be different. On the other hand, no guarantees are made about the behavior of simulations that use the same seed. In particular, Platforms are permitted to use non-deterministic algorithms which produce different results on successive runs, even if those runs were initialized identically.

If seed is set to 0 (which is the default value assigned), a unique seed is chosen when a Context is created from this Force. This is done to ensure that each Context receives unique random seeds without you needing to set them explicitly.

inline virtual bool usesPeriodicBoundaryConditions() const

Returns whether or not this force makes use of periodic boundary conditions.

Returns

true if force uses PBC and false otherwise

Public Static Functions

static inline const std::string &Pressure()

This is the name of the parameter which stores the current pressure acting on the system (in bar).

static inline const std::string &SurfaceTension()

This is the name of the parameter which stores the current surface tension acting on the system (in bar*nm).

static inline const std::string &Temperature()

This is the name of the parameter which stores the current temperature at which the system is being maintained (in Kelvin)