# Simulations¶

## What is a simulation?¶

Simulations provide the highest-level interface in TeNPy. They represent one simulation from start (initializing the various classes from given parameters) to end (saving the results to a file). The idea is that they contain the full package of code that you run by a job on a computing cluster. (You don’t have to stick to that rule, of course.) In fact, any simulation can be run from the command line, given only a parameter file as input, like this:

python -m tenpy parameters.yml
# or alternatively, if tenpy is installed correctly:
tenpy-run parameters.yml


Of course, you need to specify somewhere what type of simulation you want to run. Often, one of the predefined ones like the GroundStateSearch for running DMRG or RealTimeEvolution for running e.g. TEBD or TDVP will suffice. The Simulation class can be specified with the simulation_class option in the yaml file, or directly as a command line argument, e.g. tenpy-run -C GroundStateSearch parameters.yml. Note that command line arguments possibly override entries in the yaml files. For more details, see tenpy.console_main() for the command-line interface.

Of course, you can also directly run the simulation from inside python, the command line call is essentially just a wrapper around the tenpy.run_simulation() python interface:

import tenpy
import yaml

# instead of using yaml, you can also define a usual python dictionary
tenpy.run_simulation(**simulation_params)


An minimal example to run finite DMRG for a Spin-1/2 Heisenberg SpinChain could be given by

simulation_class: GroundStateSearch

output_filename: results.h5

model_class:  SpinChain
model_params:
L: 32
bc_MPS: finite
Jz: 1.

initial_state_params:
method: lat_product_state
product_state: [[up], [down]]

algorithm_class: TwoSiteDMRGEngine
algorithm_params:
trunc_params:
svd_min: 1.e-10
chi_max: 100
mixer: True


## Parallelization: controlling the number of threads¶

Almost all of the TeNPy code is “only” using thread-based paralellization provided by the underlying LAPACK/BLAS package linked to by Numpy/Scipy, and/or TeNPy’s Cython code when you compile it. (A notable exception is the ThreadedStorage for caching.) In practice, you can control the number of threads in the same way as if you use just plain numpy - by default, this uses all the CPU cores on a given machine.

If you run things on a cluster, it is often required to only use a fixed number of cores. Assuming a standard Linux cluster, the easiest way to control the used number of threads is usually the OMP_NUM_THREADS environment variable, which you can set in your cluster submission script:

export OMP_NUM_THREADS=4
python -m tenpy parameters.yml


If you linked against MKL, you can use export MKL_NUM_THREADS=4 instead. In some cases, it might also be necessary to additionaly export MKL_DYNAMIC=FALSE. Universities usually have some kind of local cluster documentation with examples - try to follow those, and doulbe check that you only use the cores you request.

## Customizing parameters¶

The most straight-forward way to customize a simulation is to tweak and adjust the parameters to your needs. As you can see in the above example, the parameters are organized in a hierarchical structure, following roughly the same level structure as discussed in the Overview.

The allowed options on the top level are documented in the corresponding simulation class, e.g. the GroundStateSearch.

The allowed entries in the model_params section depend on the model_class: Clearly, the SpinChain in the example above requires a different set of specified coupling parameters than, e.g., the FermiHubbardModel. The base model classes like the CouplingMPOModel have a common set of parameters usually read out, but custom model implementations can override this and/or add additional parameters. The list of allowed parameters can hence be found in the documentation of the most specialized class that you use, e.g., the TFIChain above.

Similarly, allowed values in the algorithm_params section depend on the used algorithm_class.

To get the full set of used options, it can be convenient to simply run the algorithm (for debugging parameters to allow a very quick run) and look at the results['simulation_parameters'] returned by the simulation (or saved to file):

import tenpy
from pprint import pprint
import yaml

with open('parameters.yml', 'r') as f:
results = tenpy.run_simulation(simulation_parameters)
pprint(results['simulation_parameters'])


Note

You can find a list of all the different configs in the Config Index, and a list of all parameters in Config-Options Index.

Note

If you add extra options to your configuration that TeNPy doesn’t read out by the end of the simulation, it will (usually) issue a warning. Getting such a warnings is often an indicator for a typo in your configuration, or an option being in the wrong section.

If specified, output files are saved in a given Simulation.directory. As shown in the parameter example above, you can simply give an Simulation.output_filename parameter. Alternatively, one can specify the Simulation.output_filename_params to make the filename depend on other simulation paramters (specified as keys of the parts), e.g:

directory: results
output_filename_params:
prefix: dmrg
parts:
algorithm_params.trunc_params.chi_max: 'chi_{0:04d}'
model_params.L: 'L_{0:d}'
suffix: .h5


With the above example parameters, this would yield the output filename results/dmrg_chi_0100_L_16.h5; further examples in the documentation of output_filename_from_dict().

Note that TeNPy will not overwrite output unless you explicitly set Simulation.overwrite_output to True. Rather, it will modify the filename with extra numbers, e.g., file.h5, file_1.h5, file_2.h5, ..., or it will raise a specific Skip exception if Simulation.skip_if_output_exists is set. Further, temporary .backup.h5 files are used while saving to avoid loosing previous results in case of a crash during the save.

The option Simulation.save_psi allows to enable (default) or disable saving the full tensor network at the end of the simulation - note this drastically influences the size of the output file! For long-running simulations you can decide to save intermediate checkpoints with the option Simulation.save_every_x_seconds; see the resume_details section below.

Log files by default use the same filename as the output but with the extension .log, see Logging and terminal output for more details. In practice, it is useful only print warnings and errors to stdout to allow a simple check for errors, while the .log files can then be used to follow the details and progress of the simulation:

log_params:
to_file: INFO
to_stdout: WARN
# format: "{levelname:.4s} {asctime} {message}"


Note

Always check errors and warnings! In most simulations, there shouldn’t be any warnings left.

## Analyzing the results post simulation: output structure¶

A simulation usually generates an output file that can be loaded with the load() function. It is usually either in the pickle or HDF5 format, see Saving to disk: input/output for more details.

The ability to keep code snippets and plots together in [jupyter] notebooks makes them a very convenient environment for analyzing results. There are a bunch of jupyter notebooks in the Examples that you can look at for inspiration.

The results returned by run_simulation() are a (nested) dictionary. The general structure is listed in results. Possible entries depend on the simulation class run, and some options like save_psi or specified measurements.

Let us consider our initial DMRG example. The GroundStateSearch performs two measurements: one on the inital state (unless disabled with measure_initial and one on the final state. Further, MPS-based simulations by default measure the entanglement entropies for cutting at the various MPS bonds, such that we can read out the final half-chain entanglement entropy like this:

>>> import tenpy
>>> L = results['simulation_parameters']['model_params']['L']
>>> L
32
>>> print(results['measurements']['entropy'].shape)
(2, 31)
>>> print(results['measurements']['entropy'][-1, (L-1)//2])


Here, the shape of the entropy array is (2, 31) since 2 is the number of measurements, and 31=L-1 the number of bonds. Note that you can easily read out the simulation parameters, even default ones.

Most simulation classes have only a few default_measurements, but you can easily add more under Simulation.connect_measurements. Each measurement is simply a function that is called whenever the simulation wants to measure, e.g. with the initial state, at the end of the simulation, and for time evolutions also during the evolution. The default measurement functions are defined in the module tenpy.simulations.measurement; measurement_index() documents what arguments a measurement function should have. In the simplest case, you just specify the module and function name, but you can also add more arguments, as the following example shows:

connect_measurements:
- - tenpy.simulations.measurement
- onsite_expectation_value
- opname: Sz
- - tenpy.simulations.measurement
- psi_method
- method: correlation_function
key: '<Sp_i Sm_j>'
ops1: Sp
ops2: Sm


Note the indentation and minus signs here: this yaml syntax is equivalent to the following python structure:

{'connect_measurements': [['tenpy.simulations.measurement',
'onsite_expectation_value',
{'opname': 'Sz'}],
['tenpy.simulations.measurement',
'psi_method',
{'key': '<Sp_i Sm_j>',
'method': 'correlation_function',
'ops1': 'Sp',
'ops2': 'Sm'}]]}


The measurement functions add the values under the specified key to the results returned and saved by the simulation, e.g. for the above measurements you can now read out results['measurements']['<Sz>'] (default key) and results['measurements']['<Sp_i Sm_j>'].

## A full example with custom python code¶

While there are plenty of predefined models and algorithms, there is a good chance that you need to tweak and adjust them further by writing your own python code. Examples could be custom models and/or lattices, measurment functions, or even adjustments to any other class (tensor networks, algorithms, simulations…).

As a concrete example, let’s try to reproduce some results of , namely the $$\mathcal{O}_I$$ defined in eq. (15) of that paper. A new model class is not strictly necessary, one can also select appropriate parameters for the SpinChain, but we include it here for completeness. Details on how to define a custom model class can be found in Models.

"""This file /examples/model_custom.py illustrates customization code for simulations.

Put this file somewhere where it can be importet by python, i.e. either in the working directory
or somewhere in the PYTHON_PATH. Then you can use it with the parameters file
/examples/model_custom.yml from the terminal like this::

tenpy-run -i model_custom simulation_custom.yml
"""

import tenpy
from tenpy.networks.site import SpinSite
from tenpy.networks.mps import TransferMatrix
from tenpy.models.model import CouplingMPOModel, NearestNeighborModel
from tenpy.models.lattice import Chain
from tenpy.linalg import np_conserved as npc
import numpy as np

class AnisotropicSpin1Chain(CouplingMPOModel, NearestNeighborModel):
r"""An example for a custom model, implementing the Hamiltonian of :arxiv:1204.0704.

.. math ::
H = J \sum_i \vec{S}_i \cdot \vec{S}_{i+1} + B \sum_i S^x_i + D \sum_i (S^z_i)^2
"""
default_lattice = Chain
force_default_lattice = True

def init_sites(self, model_params):
B = model_params.get('B', 0.)
conserve = model_params.get('conserve', 'best')
if conserve == 'best':
conserve = 'Sz' if not model_params.any_nonzero(['B']) else None
self.logger.info("%s: set conserve to %s", self.name, conserve)
return SpinSite(S=1., conserve=None)

def init_terms(self, model_params):
J = model_params.get('J', 1.)
B = model_params.get('B', 0.)
D = model_params.get('D', 0.)

for u1, u2, dx in self.lat.pairs['nearest_neighbors']:
self.add_coupling(J / 2., u1, 'Sp', u2, 'Sm', dx, plus_hc=True)
self.add_coupling(J, u1, 'Sz', u2, 'Sz', dx)

for u in range(len(self.lat.unit_cell)):

def pollmann_turner_inversion(results, psi, simulation, tol=0.01):
"""Measurement function for equation 15 of :arxiv:1204.0704.

See :func:~tenpy.simulations.measurement.measurement_index for the call structure.
"""
psi2 = psi.copy()
psi2.spatial_inversion()
mixed_TM = TransferMatrix(psi, psi2, transpose=False, charge_sector=0, form='B')
evals, evecs = mixed_TM.eigenvectors(which='LM', num_ev=1)
results['pollmann_turner_inversion_eta'] = eta = evals[0]
U_I = evecs[0].split_legs().transpose().conj()
U_I *= np.sqrt(U_I.shape[0])  # previously normalized to npc.norm(U_I) = 1, need unitary
results['pollmann_turner_inversion_U_I'] = U_I
if abs(eta) < 1. - tol:
O_I = 0.
else:
O_I = npc.inner(U_I, U_I.conj(), axes=[[0, 1], [1, 0]]) / U_I.shape[0]
results['pollmann_turner_inversion'] = O_I


The corresponding simulation_custom.yml parameter file, collecting the snippets above, could then look like this:

#!/usr/bin/env -S python -m tenpy -i model_custom
# requires that model_custom.py can be imported.

simulation_class : GroundStateSearch

directory: results
output_filename_params:
prefix: dmrg
parts:
algorithm_params.trunc_params.chi_max: 'chi_{0:04d}'
model_params.B: 'B_{0:.1f}'
model_params.D: 'D_{0:.1f}'
suffix: .h5
# skip_if_output_exists: True
# overwrite_output: True
save_every_x_seconds: 1800
# save_psi: False  # don't save full wave function - less disk space, but can't resume/redo measurements!

log_params:
to_stdout: WARNING  # always check this output - empty is good
to_file: INFO
# format: "{levelname:.4s} {asctime} {message}"

model_class : AnisotropicSpin1Chain
model_params :
L: 2
bc_MPS: infinite
J: 1.
B: 0.
D: 1.5

initial_state_params:
method : lat_product_state
product_state : [[up], [down]]

algorithm_class: TwoSiteDMRGEngine
algorithm_params:
trunc_params:
svd_min: 1.e-8
chi_max: 100

connect_measurements:
- - tenpy.simulations.measurement
- onsite_expectation_value
- opname: Sz
- - tenpy.simulations.measurement
- psi_method
- method: correlation_function
key: '<Sp_i Sm_j>'
ops1: Sp
ops2: Sm
- - model_custom  # module where function is defined
- pollmann_turner_inversion


Note that we explicitly specified the module model_custom for the additional measurement; you need to adjust that if you rename the model_custom.py file. You can then run this simulation, say for three different D values specified directly on the command line:

tenpy-run -i model_custom simulation_custom.yml -o model_params.D 0.
tenpy-run -i model_custom simulation_custom.yml -o model_params.D 1.5
tenpy-run -i model_custom simulation_custom.yml -o model_params.D -1.0


Note

If you use the setup from the [TeNPyProjectTemplate] repository, the cluster_jobs.py helps to manage submiting jobs with similar parameters to a computing cluster; it includes this very example as a starting point for customization.

## Checkpoints for resuming a simulation¶

As mentioned above, you can save intermediate results with the option save_every_x_seconds. Moreover, you need to have Simulation.save_psi and Simulation.save_resume_data enabled:

save_every_x_seconds: 1800
save_psi: True
save_resume_data: True


If this is the case, the simulation will save the current status at certain “checkpoints” defined by the algorithm, e.g., in DMRG at the end of a sweep. The checkpoints are saved to the same filename as the desired final output file, and get overwritten by each following save at a checkpoint. You can check results['finished'] in the output file to see whether it finished.

You can then resume the simulation using the function resume_from_checkpoint().

Note that you can also adjust parameters for the resume. For example, if you find that a DMRG result (even a finished one) is not yet fully converged in bond dimension, you can “resume” the simulation with a larger bond dimension and a new output filename. For DMRG, this is roughly equivalent to starting a new simulation with the initial state loaded from_file(); but it can reuse more than just the state, e.g., environments and already performed measurements, or the evolved_time of a time evolution.

## Sequential simulations¶

Instead of waiting for one simulation to finish and “resuming” another one with slightly different parameters, you can also directly specify a set of “sequential” simulations where the output/results of one simulation are reused for the next one. This can be particularly useful to “adiabatically” follow the ground state when tuning model parameters, in particular for flux pump experiments, or to get a stable scaling with bond dimension.

To achieve this, you need to call run_seq_simulations() instead of just run_simulation(), and specify the sequential parameters for the simulation (at the top level of the yaml files), in particular the recursive_keys for the parameters to be changed. The values for those parameters can be specified as sequential.value_lists, or as lists in the original localtion of the yaml file.

sequential:
recursive_keys:
- algorithm_params.trunc_params.chi_max

algorithm_params:
trunc_params:
chi_max: [128, 256, 512]
`