TwoSiteMixer

• full name: tenpy.algorithms.dmrg.TwoSiteMixer

• parent module: tenpy.algorithms.dmrg

• type: class

Inheritance Diagram

Methods

TwoSiteMixer.__init__(options)	Initialize self.
TwoSiteMixer.perturb_svd(engine, theta, i0, …)	Mix extra terms to theta and perform an SVD.
TwoSiteMixer.subspace_expand(engine, theta, …)	Expand the MPS subspace, to allow the bond dimension to increase.
TwoSiteMixer.update_amplitude(sweeps)	Update the amplitude, possibly disable the mixer.

class tenpy.algorithms.dmrg.TwoSiteMixer(options)

Bases: tenpy.algorithms.dmrg.SingleSiteMixer

Mixer for two-site DMRG.

This is the two-site version of the mixer described in [hubig2015]. Equivalent to the DensityMatrixMixer, but never construct the full density matrix.

perturb_svd(engine, theta, i0, move_right)

Mix extra terms to theta and perform an SVD.

Parameters

• engine (Engine) – The DMRG engine calling the mixer.

• theta (Array) – The optimized wave function, prepared for svd.

• i0 (int) – Site index; theta lives on i0, i0+1.

• update_LP (bool) – Whether to calculate the next env.LP[i0+1].

• update_RP (bool) – Whether to calculate the next env.RP[i0].

Returns

• U (Array) – Left-canonical part of theta. Labels '(vL.p0)', 'vR'.

• S (1D ndarray | 2D Array) – Without mixer just the singluar values of the array; with mixer it might be a general matrix; see comment above.

• VH (Array) – Right-canonical part of theta. Labels 'vL', '(vR.p1)'.

• err (TruncationError) – The truncation error introduced.

subspace_expand(engine, theta, i0, move_right, next_B)

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

This is the subspace expansion following [hubig2015].

Parameters

• engine (SingleSiteDMRGEngine | TwoSiteDMRGEngine) – ‘Engine’ for the DMRG algorithm

• theta (Array) – Optimized guess for the ground state of the effective local Hamiltonian.

• i0 (int) – Site index at which the local update has taken place.

• move_right (bool) – Whether the next i0 of the sweep will be right or left of the current one.

• next_B (Array) – 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(sweeps)

Update the amplitude, possibly disable the mixer.

Parameters

sweeps (int) – The number of performed sweeps, to check if we need to disable the mixer.

Returns

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

Return type

Mixer | None