Molecular dynamics

Typical computer simulations involve moving the atoms around, either to optimize a structure (energy minimization) or to do molecular dynamics. This chapter discusses molecular dynamics, energy minimization algorithms will be discussed in the ase.optimize section.

A molecular dynamics object will operate on the atoms by moving them according to their forces - it integrates Newton’s second law numerically. A typical molecular dynamics simulation will use the Velocity Verlet dynamics. You create the ase.md.verlet.VelocityVerlet object, giving it the atoms and a time step, and then you perform dynamics by calling its run() method:

dyn = VelocityVerlet(atoms, dt=5.0 * units.fs,
                     trajectory='md.traj', logfile='md.log')
dyn.run(1000)  # take 1000 steps

A number of different algorithms can be used to perform molecular dynamics, with slightly different results.

Choosing the time step

All the dynamics objects need a time step. Choosing it too small will waste computer time, choosing it too large will make the dynamics unstable, typically the energy increases dramatically (the system “blows up”). If the time step is only a little to large, the lack of energy conservation is most obvious in Velocity Verlet dynamics, where energy should otherwise be conserved.

Experience has shown that 5 femtoseconds is a good choice for most metallic systems. Systems with light atoms (e.g. hydrogen) and/or with strong bonds (carbon) will need a smaller time step.

All the dynamics objects documented here are sufficiently related to have the same optimal time step.

File output

The time evolution of the system can be saved in a trajectory file, by creating a trajectory object, and attaching it to the dynamics object. This is documented in the module ase.io.trajectory. You can attach the trajectory explicitly to the dynamics object, and you may want to use the optional interval argument, so every time step is not written to the file.

Alternatively, you can just use the trajectory keyword when instantiating the dynamics object as in the example above. In this case, a loginterval keyword may also be supplied to specify the frequency of writing to the trajectory. The loginterval keyword will apply to both the trajectory and the logfile.

Logging

A logging mechanism is provided, printing time; total, potential and kinetic energy; and temperature (calculated from the kinetic energy). It is enabled by giving the logfile argument when the dynamics object is created, logfile may be an open file, a filename or the string ‘-‘ meaning standard output. Per default, a line is printed for each timestep, specifying the loginterval argument will chance this to a more reasonable frequency.

The logging can be customized by explicitly attaching a MDLogger object to the dynamics:

from ase.md import MDLogger
dyn = VelocityVerlet(atoms, dt=2*ase.units.fs)
dyn.attach(MDLogger(dyn, atoms, 'md.log', header=False, stress=False,
           peratom=True, mode="a"), interval=1000)

This example will skip the header line and write energies per atom instead of total energies. The parameters are

header: Print a header line defining the columns.

stress: Print the six components of the stress tensor.

peratom: Print energy per atom instead of total energy.

mode: If ‘a’, append to existing file, if ‘w’ overwrite existing file.

Despite appearances, attaching a logger like this does not create a cyclic reference to the dynamics.

Note

If building your own logging class, be sure not to attach the dynamics object directly to the logging object. Instead, create a weak reference using the proxy method of the weakref package. See the ase.md.MDLogger source code for an example. (If this is not done, a cyclic reference may be created which can cause certain calculators, such as Jacapo, to not terminate correctly.)

class ase.md.MDLogger(dyn, atoms, logfile, header=True, stress=False, peratom=False, mode=’a’)[source]

Class for logging molecular dynamics simulations.

Parameters: dyn: The dynamics. Only a weak reference is kept.

atoms: The atoms.

logfile: File name or open file, “-” meaning standard output.

stress=False: Include stress in log.

peratom=False: Write energies per atom.

mode=”a”: How the file is opened if logfile is a filename.

Constant NVE simulations (the microcanonical ensemble)

Newton’s second law preserves the total energy of the system, and a straightforward integration of Newton’s second law therefore leads to simulations preserving the total energy of the system (E), the number of atoms (N) and the volume of the system (V). The most appropriate algorithm for doing this is velocity Verlet dynamics, since it gives very good long-term stability of the total energy even with quite large time steps. Fancier algorithms such as Runge-Kutta may give very good short-term energy preservation, but at the price of a slow drift in energy over longer timescales, causing trouble for long simulations.

In a typical NVE simulation, the temperature will remain approximately constant, but if significant structural changes occurs they may result in temperature changes. If external work is done on the system, the temperature is likely to rise significantly.

Velocity Verlet dynamics

class ase.md.verlet.VelocityVerlet(atoms, dt, trajectory=None, logfile=None, loginterval=1)[source]

VelocityVerlet is the only dynamics implementing the NVE ensemble. It requires two arguments, the atoms and the time step. Choosing a too large time step will immediately be obvious, as the energy will increase with time, often very rapidly.

Example: See the tutorial Molecular dynamics.

Constant NVT simulations (the canonical ensemble)

Since Newton’s second law conserves energy and not temperature, simulations at constant temperature will somehow involve coupling the system to a heat bath. This cannot help being somewhat artificial. Two different approaches are possible within ASE. In Langevin dynamics, each atom is coupled to a heat bath through a fluctuating force and a friction term. In Nosé-Hoover dynamics, a term representing the heat bath through a single degree of freedom is introduced into the Hamiltonian.

Langevin dynamics

class ase.md.langevin.Langevin(atoms, timestep, temperature, friction)[source]

The Langevin class implements Langevin dynamics, where a (small) friction term and a fluctuating force are added to Newton’s second law which is then integrated numerically. The temperature of the heat bath and magnitude of the friction is specified by the user, the amplitude of the fluctuating force is then calculated to give that temperature. This procedure has some physical justification: in a real metal the atoms are (weakly) coupled to the electron gas, and the electron gas therefore acts like a heat bath for the atoms. If heat is produced locally, the atoms locally get a temperature that is higher than the temperature of the electrons, heat is transferred to the electrons and then rapidly transported away by them. A Langevin equation is probably a reasonable model for this process.

A disadvantage of using Langevin dynamics is that if significant heat is produced in the simulation, then the temperature will stabilize at a value higher than the specified temperature of the heat bath, since a temperature difference between the system and the heat bath is necessary to get a finite heat flow. Another disadvantage is that the fluctuating force is stochastic in nature, so repeating the simulation will not give exactly the same trajectory.

When the Langevin object is created, you must specify a time step, a temperature (in energy units) and a friction. Typical values for the friction are 0.01-0.02 atomic units.

# Room temperature simulation
dyn = Langevin(atoms, 5 * units.fs, units.kB * 300, 0.002)

Both the friction and the temperature can be replaced with arrays giving per-atom values. This is mostly useful for the friction, where one can choose a rather high friction near the boundaries, and set it to zero in the part of the system where the phenomenon being studied is located.

Nosé-Hoover dynamics

In Nosé-Hoover dynamics, an extra term is added to the Hamiltonian representing the coupling to the heat bath. From a pragmatic point of view one can regard Nosé-Hoover dynamics as adding a friction term to Newton’s second law, but dynamically changing the friction coefficient to move the system towards the desired temperature. Typically the “friction coefficient” will fluctuate around zero.

Nosé-Hoover dynamics is not implemented as a separate class, but is a special case of NPT dynamics.

Berendsen NVT dynamics

class ase.md.nvtberendsen.NVTBerendsen(atoms, timestep, temperature, taut, fixcm)[source]

In Berendsen NVT simulations the velocities are scaled to achieve the desired temperature. The speed of the scaling is determined by the parameter taut.

This method does not result proper NVT sampling but it usually is sufficiently good in practise (with large taut). For discussion see the gromacs manual at www.gromacs.org.

atoms:
The list of atoms.
timestep:
The time step.
temperature:
The desired temperature, in Kelvin.
taut:
Time constant for Berendsen temperature coupling.
fixcm:
If True, the position and momentum of the center of mass is kept unperturbed. Default: True.
# Room temperature simulation (300K, 0.1 fs time step)
dyn = NVTBerendsen(atoms, 0.1 * units.fs, 300, taut=0.5*1000*units.fs)

Constant NPT simulations (the isothermal-isobaric ensemble)

class ase.md.npt.NPT(atoms, timestep, temperature, externalstress, ttime, pfactor, mask=None)[source]

Dynamics with constant pressure (or optionally, constant stress) and constant temperature (NPT or N,stress,T ensemble). It uses the combination of Nosé-Hoover and Parrinello-Rahman dynamics proposed by Melchionna et al. [1] and later modified by Melchionna [2]. The differential equations are integrated using a centered difference method [3]. Details of the implementation are available in the document XXX NPTdynamics.tex, distributed with the module.

The dynamics object is called with the following parameters:

atoms:
The atoms object.
timestep:
The timestep in units matching eV, Å, u. Use the units.fs constant.
temperature:
The desired temperature in eV.
externalstress:
The external stress in eV/Å^3. Either a symmetric 3x3 tensor, a 6-vector representing the same, or a scalar representing the pressure. Note that the stress is positive in tension whereas the pressure is positive in compression: giving a scalar p is equivalent to giving the tensor (-p. -p, -p, 0, 0, 0).
ttime:
Characteristic timescale of the thermostat. Set to None to disable the thermostat.
pfactor:
A constant in the barostat differential equation. If a characteristic barostat timescale of ptime is desired, set pfactor to ptime^2 * B (where B is the Bulk Modulus). Set to None to disable the barostat. Typical metallic bulk moduli are of the order of 100 GPa or 0.6 eV/Å^3.
mask=None:
Optional argument. A tuple of three integers (0 or 1), indicating if the system can change size along the three Cartesian axes. Set to (1,1,1) or None to allow a fully flexible computational box. Set to (1,1,0) to disallow elongations along the z-axis etc.

Useful parameter values:

  • The same timestep can be used as in Verlet dynamics, i.e. 5 fs is fine for bulk copper.
  • The ttime and pfactor are quite critical[4], too small values may cause instabilites and/or wrong fluctuations in T / p. Too large values cause an oscillation which is slow to die. Good values for the characteristic times seem to be 25 fs for ttime, and 75 fs for ptime (used to calculate pfactor), at least for bulk copper with 15000-200000 atoms. But this is not well tested, it is IMPORTANT to monitor the temperature and stress/pressure fluctuations.

It has the following methods:

NPT.run(n):

Perform n timesteps.

NPT.initialize():

Estimates the dynamic variables for time=-1 to start the algorithm. This is automatically called before the first timestep.

NPT.set_stress():

Set the external stress. Use with care. It is preferable to set the right value when creating the object.

NPT.set_mask():

Change the mask. Use with care, as you may “freeze” a fluctuation in the strain rate.

NPT.set_strain_rate(eps):

Set the strain rate. eps must be an upper-triangular matrix. If you set a strain rate along a direction that is “masked out” (see set_mask), the strain rate along that direction will be maintained constantly.

NPT.get_strain_rate():

Set the instantaneous strain rate (due to the fluctuations in the shape of the computational box).

NPT.get_gibbs_free_energy():

Gibbs free energy is supposed to be preserved by this dynamics. This is mainly intended as a diagnostic tool.

References:

[1] S. Melchionna, G. Ciccotti and B. L. Holian, Molecular Physics 78, p. 533 (1993).

[2] S. Melchionna, Physical Review E 61, p. 6165 (2000).

[3] B. L. Holian, A. J. De Groot, W. G. Hoover, and C. G. Hoover, Physical Review A 41, p. 4552 (1990).

[4] F. D. Di Tolla and M. Ronchetti, Physical Review E 48, p. 1726 (1993).

Berendsen NPT dynamics

class ase.md.nptberendsen.NPTBerendsen(atoms, timestep, temperature, taut, pressure, taup, compressibility, fixcm)[source]

In Berendsen NPT simulations the velocities are scaled to achieve the desired temperature. The speed of the scaling is determined by the parameter taut.

The atom positions and the simulation cell are scaled in order to achieve the desired pressure.

This method does not result proper NPT sampling but it usually is sufficiently good in practise (with large taut and taup). For discussion see the gromacs manual at www.gromacs.org. or amber at ambermd.org

atoms:
The list of atoms.
timestep:
The time step.
temperature:
The desired temperature, in Kelvin.
taut:
Time constant for Berendsen temperature coupling.
pressure:
The desired pressure, in bar (1 bar = 1e5 Pa).
taup:
Time constant for Berendsen pressure coupling.
compressibility:
The compressibility of the material, water 4.57E-5 bar-1, in bar-1
fixcm:
If True, the position and momentum of the center of mass is kept unperturbed. Default: True.
# Room temperature simulation (300K, 0.1 fs time step, atmospheric pressure)
dyn = NPTBerendsen(atoms, timestep=0.1*units.fs, temperature=300,
                 taut=0.1*1000*units.fs, pressure = 1.01325,
                 taup=1.0*1000*units.fs, compressibility=4.57e-5)