Stopping a DMRG Run "Gracefully"

Often it is helpful to set DMRG to do more sweeps than is needed, monitor its convergence, then stop it manually when converged. But killing the process would not allow any cleanup, final measurements, or saving of the wavefunction.

Stopping DMRG

Fortunately the DMRG code included with ITensor can be stopped gracefully (meaning, it finishes whichever sweep it is on then exits normally) by simply putting a file named STOP_DMRG in the directory where the DMRG process is running. A nice unix command to use for creating this file is:

touch STOP_DMRG

The touch command creates an empty file if it doesn't already exist (otherwise it updates the file's time stamp).

Stopping iDMRG

The iDMRG code included with ITensor works a bit differently from the finite DMRG code, in that it uses the finite DMRG as an inner loop. Creating the STOP_DMRG file will only exit from this inner loop and won't stop the iDMRG process.

To stop iDMRG, do the following:

touch STOP_DMRG_ALL

Internally, this keeps the object governing the DMRG algorithm's behavior in a permanent "stop now" state, causing iDMRG to quit after it finishes the step it is currently on.

Advanced Customization

The above STOP_DMRG and STOP_DMRG_ALL file functionality is handled in ITensor by the DMRGObserver class. You can customize this class by creating your own subclass of it. The (virtual) class method of DMRGObserver responsible for ending a DMRG calculation is called checkDone, and in DMRGObserver, checkDone just looks for the STOP_DMRG and STOP_DMRG_ALL files, signaling done (by returning true) if they are found.

In your own subclass of DMRGObserver you have a few options:

• Not overriding checkDone just inherits the same behavior as DMRGObserver.

• Overriding checkDone but calling DMRGObserver::checkDone() allows you to combine your own custom "done" conditions with the default behavior.

• Finally, overriding and ignoring DMRGObserver::checkDone() lets you turn off the STOP_DMRG file checking, allowing you to replace it with other custom "done" conditions.