TwoSiteTDVPEngine¶

full name: tenpy.algorithms.tdvp.TwoSiteTDVPEngine
parent module: tenpy.algorithms.tdvp
type: class

Inheritance Diagram

Methods

`TwoSiteTDVPEngine.__init__`(psi, model, ...)
`TwoSiteTDVPEngine.environment_sweeps`(N_sweeps)	Perform N_sweeps sweeps without optimization to update the environment.
`TwoSiteTDVPEngine.evolve`(N_steps, dt)	Evolve by `N_steps * dt`.
`TwoSiteTDVPEngine.evolve_step`(dt)
`TwoSiteTDVPEngine.free_no_longer_needed_envs`()	Remove no longer needed environments after an update.
`TwoSiteTDVPEngine.get_resume_data`([...])	Return necessary data to resume a `run()` interrupted at a checkpoint.
`TwoSiteTDVPEngine.get_sweep_schedule`()	Slightly different sweep schedule than DMRG
`TwoSiteTDVPEngine.init_env`([model, ...])	(Re-)initialize the environment.
`TwoSiteTDVPEngine.make_eff_H`()	Create new instance of self.EffectiveH at self.i0 and set it to self.eff_H.
`TwoSiteTDVPEngine.one_site_update`(i, dt)
`TwoSiteTDVPEngine.post_update_local`(err, ...)	Algorithm-specific actions to be taken after local update.
`TwoSiteTDVPEngine.prepare_evolve`(dt)	Do nothing.
`TwoSiteTDVPEngine.prepare_update_local`()	Prepare self for calling `update_local()`.
`TwoSiteTDVPEngine.reset_stats`([resume_data])	Reset the statistics.
`TwoSiteTDVPEngine.resume_run`()	Resume a run that was interrupted.
`TwoSiteTDVPEngine.run`()	Perform a (real-)time evolution of `psi` by N_steps * dt.
`TwoSiteTDVPEngine.run_evolution`(N_steps, dt)	Perform a (real-)time evolution of `psi` by N_steps * dt.
`TwoSiteTDVPEngine.sweep`([optimize])	One 'sweep' of a sweeper algorithm.
`TwoSiteTDVPEngine.switch_engine`(other_engine, *)	Initialize algorithm from another algorithm instance of a different class.
`TwoSiteTDVPEngine.update_env`(**update_data)	Do nothing; super().update_env() is called explicitly in `update_local()`.
`TwoSiteTDVPEngine.update_local`(theta, **kwargs)	Perform algorithm-specific local update.

Class Attributes and Properties

`TwoSiteTDVPEngine.engine_params`
`TwoSiteTDVPEngine.n_optimize`	The number of sites to be optimized at once.
`TwoSiteTDVPEngine.time_dependent_H`	whether the algorithm supports time-dependent H
`TwoSiteTDVPEngine.verbose`

class tenpy.algorithms.tdvp.TwoSiteTDVPEngine(psi, model, options, **kwargs)[source]¶

Bases: TDVPEngine

Engine for the two-site TDVP algorithm.

Parameters

psi – Same as for Algorithm.
model – Same as for Algorithm.
options – Same as for Algorithm.
**kwargs – Same as for Algorithm.

Options

config TDVP¶

option	summary
active_sites in OldTDVPEngine	The number of active sites to be used for the time evolution. [...]
dt (from TimeEvolutionAlgorithm) in TimeEvolutionAlgorithm	Minimal time step by which to evolve.
lanczos_options in OldTDVPEngine	Lanczos options as described in :cfg:config:`Lanczos`.
N_steps (from TimeEvolutionAlgorithm) in TimeEvolutionAlgorithm	Number of time steps `dt` to evolve by in :meth:`run`. [...]
preserve_norm (from TimeEvolutionAlgorithm) in TimeEvolutionAlgorithm	Whether the state will be normalized to its initial norm after each time step.
start_time (from TimeEvolutionAlgorithm) in TimeEvolutionAlgorithm	Initial value for :attr:`evolved_time`.
trunc_params in OldTDVPEngine	Truncation parameters as described in :func:`~tenpy.algorithms.truncation.t [...]

option trunc_params: dict¶: Truncation parameters as described in truncate()

option lanczos_options: dict¶: Lanczos options as described in Lanczos.

options¶

Optional parameters.

Type: dict

evolved_time¶

Indicating how long psi has been evolved, psi = exp(-i * evolved_time * H) psi(t=0).

Type: float | complex

psi¶

The MPS, time evolved in-place.

Type: MPS

env¶

The environment, storing the LP and RP to avoid recalculations.

Type: MPOEnvironment

lanczos_options¶

Options passed on to LanczosEvolution.

Type: Config

EffectiveH[source]¶: alias of TwoSiteH

get_sweep_schedule()[source]¶: Slightly different sweep schedule than DMRG

update_local(theta, **kwargs)[source]¶

Perform algorithm-specific local update.

For two-site algorithms with n_optimize = 2, this always optimizes the sites i0 and i0 + 1. For single-site algorithms, the effective H only acts on site i0, but afterwards it also updates the bond to the right if move_right is True, or the bond to the left if move_right is False. Since the svd for truncation gives tensors to be multiplied into the tensors on both sides of the bond, tensors of two sites are updated even for single-site algorithms: when right-moving, site i0 + 1 is also updated; site i0 - 1 when left-moving.

Parameters: theta (Array) – Local single- or two-site wave function, as returned by prepare_update_local().
Returns: update_data – Data to be processed by update_env() and post_update_local(), e.g. containing the truncation error as err. If combine is set, it should also contain the U and VH from the SVD.
Return type: dict

update_env(**update_data)[source]¶: Do nothing; super().update_env() is called explicitly in update_local().

post_update_local(err, **update_data)[source]¶

Algorithm-specific actions to be taken after local update.

An example would be to collect statistics.

environment_sweeps(N_sweeps)[source]¶

Perform N_sweeps sweeps without optimization to update the environment.

Parameters: N_sweeps (int) – Number of sweeps to run without optimization

evolve(N_steps, dt)[source]¶

Evolve by N_steps * dt.

Parameters: N_steps (int) – The number of steps to evolve.

free_no_longer_needed_envs()[source]¶

Remove no longer needed environments after an update.

This allows to minimize the number of environments to be kept. For large MPO bond dimensions, these environments are by far the biggest part in memory, so this is a valuable optimiztion to reduce memory requirements.

get_resume_data(sequential_simulations=False)[source]¶

Return necessary data to resume a run() interrupted at a checkpoint.

At a checkpoint, you can save psi, model and options along with the data returned by this function. When the simulation aborts, you can resume it using this saved data with:

eng = AlgorithmClass(psi, model, options, resume_data=resume_data)
eng.resume_run()

An algorithm which doesn’t support this should override resume_run to raise an Error.

Parameters: sequential_simulations (bool) – If True, return only the data for re-initializing a sequential simulation run, where we “adiabatically” follow the evolution of a ground state (for variational algorithms), or do series of quenches (for time evolution algorithms); see run_seq_simulations().
Returns: resume_data – Dictionary with necessary data (apart from copies of psi, model, options) that allows to continue the simulation from where we are now. It might contain an explicit copy of psi.
Return type: dict

init_env(model=None, resume_data=None, orthogonal_to=None)[source]¶

(Re-)initialize the environment.

This function is useful to (re-)start a Sweep with a slightly different model or different (engine) parameters. Note that we assume that we still have the same psi. Calls reset_stats().

Parameters

model (MPOModel) – The model representing the Hamiltonian for which we want to find the ground state. If None, keep the model used before.
resume_data (None | dict) – Given when resuming a simulation, as returned by get_resume_data(). Can contain another dict under the key init_env_data; the contents of init_env_data get passed as keyword arguments to the environment initializaiton.
orthogonal_to (None | list of MPS | list of dict) – List of other matrix product states to orthogonalize against. Instead of just the state, you can specify a dict with the state as ket and further keyword arguments for initializing the MPSEnvironment; the psi to be optimized is used as bra. Works only for finite or segment MPS; for infinite MPS it must be None. This can be used to find (a few) excited states as follows. First, run DMRG to find the ground state, and then run DMRG again while orthogonalizing against the ground state, which yields the first excited state (in the same symmetry sector), and so on. Note that resume_data['orthogonal_to'] takes precedence over the argument.

Options

Deprecated since version 0.6.0: Options LP, LP_age, RP and RP_age are now collected in a dictionary init_env_data with different keys init_LP, init_RP, age_LP, age_RP

Deprecated since version 0.8.0: Instead of passing the init_env_data as a option, it should be passed as dict entry of resume_data.

option Sweep.init_env_data: dict¶: Dictionary as returned by self.env.get_initialization_data() from get_initialization_data(). Deprecated, use the resume_data function/class argument instead.

option Sweep.orthogonal_to: list of MPS¶: Deprecated in favor of the orthogonal_to function argument (forwarded from the class argument) with the same effect.

option Sweep.start_env: int¶: Number of sweeps to be performed without optimization to update the environment.

Raises: ValueError – If the engine is re-initialized with a new model, which legs are incompatible with those of hte old model.

make_eff_H()[source]¶: Create new instance of self.EffectiveH at self.i0 and set it to self.eff_H.

property n_optimize¶

The number of sites to be optimized at once.

Indirectly set by the class attribute EffectiveH and it’s length. For example, TwoSiteDMRGEngine uses the TwoSiteH and hence has n_optimize=2, while the SingleSiteDMRGEngine has n_optimize=1.

prepare_evolve(dt)[source]¶: Do nothing.

prepare_update_local()[source]¶

Prepare self for calling update_local().

Returns: theta – Current best guess for the ground state, which is to be optimized. Labels are 'vL', 'p0', 'p1', 'vR', or combined versions of it (if self.combine). For single-site DMRG, the 'p1' label is missing.
Return type: Array

reset_stats(resume_data=None)[source]¶

Reset the statistics. Useful if you want to start a new Sweep run.

This method is expected to be overwritten by subclass, and should then define self.update_stats and self.sweep_stats dicts consistent with the statistics generated by the algorithm particular to that subclass.

Parameters: resume_data (dict) – Given when resuming a simulation, as returned by get_resume_data(). Here, we read out the sweeps.

Options

option Sweep.chi_list: None | dict(int -> int)¶: By default (None) this feature is disabled. A dict allows to gradually increase the chi_max. An entry at_sweep: chi states that starting from sweep at_sweep, the value chi is to be used for trunc_params['chi_max']. For example chi_list={0: 50, 20: 100} uses chi_max=50 for the first 20 sweeps and chi_max=100 afterwards.

resume_run()[source]¶

Resume a run that was interrupted.

In case we saved an intermediate result at a checkpoint, this function allows to resume the run() of the algorithm (after re-initialization with the resume_data). Since most algorithms just have a while loop with break conditions, the default behaviour implemented here is to just call run().

run()[source]¶

Perform a (real-)time evolution of psi by N_steps * dt.

You probably want to call this in a loop along with measurements. The recommended way to do this is via the RealTimeEvolution.

run_evolution(N_steps, dt)[source]¶

Perform a (real-)time evolution of psi by N_steps * dt.

This is the inner part of run() without the logging. For parameters see TimeEvolutionAlgorithm.

sweep(optimize=True)[source]¶

One ‘sweep’ of a sweeper algorithm.

Iteratate over the bond which is optimized, to the right and then back to the left to the starting point. If optimize=False, don’t actually diagonalize the effective hamiltonian, but only update the environment.

Parameters: optimize (bool, optional) – Whether we actually optimize to find the ground state of the effective Hamiltonian. (If False, just update the environments).
Returns: max_trunc_err – Maximal truncation error introduced.
Return type: float

classmethod switch_engine(other_engine, *, options=None, **kwargs)[source]¶

Initialize algorithm from another algorithm instance of a different class.

You can initialize one engine from another, not too different subclasses. Internally, this function calls get_resume_data() to extract data from the other_engine and then initializes the new class.

Note that it transfers the data without making copies in most case; even the options! Thus, when you call run() on one of the two algorithm instances, it will modify the state, environment, etc. in the other. We recommend to make the switch as engine = OtherSubClass.switch_engine(engine) directly replacing the reference.

Parameters

cls (class) – Subclass of Algorithm to be initialized.
other_engine (Algorithm) – The engine from which data should be transfered. Another, but not too different algorithm subclass-class; e.g. you can switch from the TwoSiteDMRGEngine to the OneSiteDMRGEngine.
options (None | dict-like) – If not None, these options are used for the new initialization. If None, take the options from the other_engine.
**kwargs – Further keyword arguments for class initialization. If not defined, resume_data is collected with get_resume_data().

time_dependent_H = False¶: whether the algorithm supports time-dependent H