## Learn to Use ITensor

# Sweeps

The Sweeps class makes it easy to specify the "accuracy parameters" used when doing DMRG calculations. After setting the number of sweeps in the constructor, you can set parameters such as the truncation error cutoff to use for each sweep by providing the cutoffs in a comma-separated list.

## Synopsis

```
auto sweeps = Sweeps(5); //do 5 sweeps
//set max bond/link dimension for each sweep
sweeps.maxdim() = 10,20,40,80,160;
//set truncation error cutoff for each sweep
sweeps.cutoff() = 1E-5,1E-8,1E-10;
//if fewer parameters are given than number of
//sweeps (as above) then the last value is used
//for all remaining sweeps.
```

## Names and meaning of "accuracy parameters"

`maxdim`

— the bond dimension of any bond of the MPS cannot exceed the maxdim value.`mindim`

— the bond dimension of any bond of the MPS will be at least as large as mindim, however singular values that are exactly zero will still be truncated.`cutoff`

— the truncation error cutoff to use when computing SVD or density matrix diagonalizations. For more information on how the cutoff is defined, see the page on tensor decompositions.`niter`

— controls maximum number of Davidson ("exact diagonalization") iterations to use in the core step of DMRG. (Regrettably, for historical reasons within ITensor, niter actually corresponds to the number of basis states used, so the minimum value of niter that will result in any optimization happening must be niter >= 2. )`noise`

— magnitude of the noise term to be added to the density matrix to aid convergence. If noise is set to zero, no noise term will be computed.

## Constructors

Sweeps(int nsweeps, int mindim = 1, int maxdim = 500, Real cutoff = 1E-8, Real noise = 0.)

Construct a Sweeps object specifying the number of sweeps (nsweeps).

You can optionally provide fixed values of mindim, maxdim, cutoff to use on every sweep (these can be changed later).

Sweeps(Args const& args)

Construct a Sweeps object by specifying a set of named arguments.

Required arguments:

- "Nsweep" — number of sweeps
- "MaxDim" — maximum bond dimension of MPS
- "Cutoff" — truncation error threshold used for density matrix truncation

Optional arguments:

- "MinDim" — maximum bond dimension of MPS
- "Noise" — coefficient of density matrix noise term
- "Niter" — maximum number of Davidson iterations

Show Exampleauto sweeps = Sweeps({"Nsweep=",5, "MaxDim=",100, "Cutoff=",1E-8});

`Sweeps(int nsweeps, InputGroup & sweep_table)`

Construct a Sweeps object from an InputGroup of the following form:

input_group_name { ... table_name { maxdim mindim cutoff niter noise 20 20 1E-8 4 1E-8 40 20 1E-8 3 1E-9 80 20 1E-10 2 1E-10 160 20 1E-12 2 0 240 20 1E-12 2 0 } ... }

Then the following code can be used to read in this table and construct a sweeps object:

auto input = InputGroup(filename,"input_group_name"); auto sw_table = InputGroup(input,"table_name"); auto sweeps = Sweeps(5,sw_table);

## Comma-separated list input methods

For the following methods, the returned type is a "SweepSetter" object which collects numbers in a comma-separated list, then provides these numbers to the associated Sweeps object.

If fewer values are provided than the number of sweeps, the last value will be used for all remaining sweeps.

`.maxdim() -> SweepSetter<int>`

Set the maxdim value used for each sweep from a comma-separated list.

Show Examplesweeps.maxdim() = 10,40,80,160,160,800;

`.cutoff() -> SweepSetter<Real>`

Set the cutoff value used for each sweep from a comma-separated list.

Show Examplesweeps.cutoff() = 1E-8,1E-10;

`.mindim() -> SweepSetter<int>`

Set the mindim value used for each sweep from a comma-separated list.

`.niter() -> SweepSetter<int>`

Set the niter value used for each sweep from a comma-separated list.

`.noise() -> SweepSetter<Real>`

Set the noise value used for each sweep from a comma-separated list.

## Other methods

`.nsweep() -> int`

`.size() -> int`

Retrieve the number of sweeps represented by this Sweeps object.

`.mindim(int sw) -> int`

Retrieve the mindim value set for sweep number

`sw`

.`.maxdim(int sw) -> int`

Retrieve the maxdim value set for sweep number

`sw`

.`.cutoff(int sw) -> Real`

Retrieve the cutoff value set for sweep number

`sw`

.`.noise(int sw) -> Real`

Retrieve the noise value set for sweep number

`sw`

.`.niter(int sw) -> int`

Retrieve the niter value set for sweep number

`sw`

.`.setmindim(int sw, int newmindim)`

Set a new mindim value for sweep number

`sw`

.`.setmaxdim(int sw, int newmaxdim)`

Set a new maxdim value for sweep number

`sw`

.`.setcutoff(int sw, Real newcutoff)`

Set a new cutoff value for sweep number

`sw`

.`.setnoise(int sw, Real newnoise)`

Set a new noise value for sweep number

`sw`

.`.setniter(int sw, int newniter)`

Set a new niter value for sweep number

`sw`

.`.read(std::istream & s)`

`.write(std::ostream & s)`

Read or write this Sweeps object from/to a binary stream.

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