SingleSiteMixer

class tenpy.algorithms.dmrg.SingleSiteMixer(mixer_params)[source]

Bases: tenpy.algorithms.dmrg.Mixer

Mixer for single-site DMRG.

Performs a subspace expansion following [Hubig2015].

Methods

perturb_svd(self, engine, theta, i0, …)

Mix extra terms to theta and perform an SVD.

subspace_expand(self, engine, theta, i0, …)

Expand the MPS subspace, to allow the bond dimension to increase.

update_amplitude(self, sweeps)

Update the amplitude, possibly disable the mixer.
perturb_svd(self, engine, theta, i0, move_right, next_B)[source]

Mix extra terms to theta and perform an SVD.

We calculate the left and right reduced density matrix using the mixer (which might include applications of H). These density matrices are diagonalized and truncated such that we effectively perform a svd for the case mixer.amplitude=0.

Parameters
engineEngine

The DMRG engine calling the mixer.

thetaArray

The optimized wave function, prepared for svd.

i0int

The site index where theta lives.

move_rightbool

Whether we move to the right (True) or left (False).

next_BArray

The subspace expansion requires to change the tensor on the next site as well. If move_right, it should correspond to engine.psi.get_B(i0+1, form='B'). If not move_right, it should correspond to engine.psi.get_B(i0-1, form='A').

Returns
UArray

Left-canonical part of tensordot(theta, next_B). Labels '(vL.p0)', 'vR'.

S1D ndarray

(Perturbed) singular values on the new bond (between theta and next_B).

VHArray

Right-canonical part of tensordot(theta, next_B). Labels 'vL', '(p1.vR)'.

errTruncationError

The truncation error introduced.

subspace_expand(self, engine, theta, i0, move_right, next_B)[source]

Expand the MPS subspace, to allow the bond dimension to increase.

This is the subspace expansion following [Hubig2015].

Parameters
engineSingleSiteDMRGEngine | TwoSiteDMRGEngine

‘Engine’ for the DMRG algorithm

thetaArray

Optimized guess for the ground state of the effective local Hamiltonian.

i0int

Site index at which the local update has taken place.

move_rightbool

Whether the next i0 of the sweep will be right or left of the current one.

next_BArray

The subspace expansion requires to change the tensor on the next site as well. If move_right, it should correspond to engine.psi.get_B(i0+1, form='B'). If not move_right, it should correspond to engine.psi.get_B(i0-1, form='A').

Returns
theta :

Local MPS tensor at site i0 after subspace expansion.

next_B :

MPS tensor at site i0+1 or i0-1 (depending on sweep direction) after subspace expansion.

update_amplitude(self, sweeps)

Update the amplitude, possibly disable the mixer.

Parameters
sweepsint

The number of performed sweeps, to check if we need to disable the mixer.

Returns
mixerMixer | None

Returns self if we should continue mixing, or None, if the mixer should be disabled.