What’s new in Kwant 0.2

This article explains the user-visible changes in Kwant 0.2. Kwant 0.2 was released on 29 November 2012.

Improved performance

This has been the main focus of this release. Through optimization a level of performance has been reached that we consider satisfactory: runs of Kwant for mid-sized (100x100 say) systems now typically spend most time in highly optimized libraries and not anymore in Python-implemented code. For large, truly performance-critical systems almost all time is now spent in optimized libraries.

An important optimization has been replacing NumPy for most uses within Kwant by tinyarray. tinyarray provides a subset of NumPy’s functionality in a way that is highly optimized for small arrays such as the tags of sites in Kwant.

New MUMPS-based solver

The code for sparse matrix solvers has been reorganized and a new solver has been added next to kwant.solvers.sparse: kwant.solvers.mumps. The new solver uses the MUMPS software package and is much (typically several times) faster than the UMFPACK-based old solver. In addition, MUMPS uses considerably less memory for a given system while at the same time it is able to take advantage of more than 2 GiB of RAM.

New tutorial dealing with superconductivity

Superconductors: orbital degrees of freedom, conservation laws and symmetries

New plotter module

plotter has been rewritten using matplotlib, which allows plot post-processing, basic 3D plotting and many other features. Due to the possibility to easily modify a matplotlib plot after it has been generated, function plot has much fewer input parameters, and is less flexible than its previous implementation. Its interface is also much more similar to that of matplotlib. For the detailed interface and input description check plot documentation.

The behavior of plot with low level systems has changed. Arguments of plot which are functions are given site numbers in place of Site objects when plotting a low level system. This provides an easy way to make the appearance of lines and symbols depend on computation results.

A new function map was implemented. It allows to show a map of spatial dependence of a function of a system site (e.g. density of states) without showing the sites themselves.

TranslationalSymmetry is used differently

When constructing an instance of TranslationalSymmetry a sole parameter used to be expected: A sequence of sequences of 1d real space vectors. Now TranslationalSymmetry can take an arbitrary number of parameters, each of them a 1d real space vector. This reduced the number of parantheses necessary in the common case where there is just a single parameter

Example of old usage:

sym = kwant.TranslationalSymmetry([(-1, 0)])

New usage:

sym = kwant.TranslationalSymmetry((-1, 0))

Band structure functionality has been moved

The functionality that used to be provided by the method energies of kwant.system.InfiniteSystem has been moved to the kwant.physics package. See the documentation of kwant.physics.Bands and Beyond transport: Band structure and closed systems.

Calculation of the local density of states

The new function of sparse solvers ldos allows the calculation of the local density of states.

Calculation of wave functions in the scattering region

(Kwant 0.3 update: wave_func has been renamed to wave_function.)

The new function of sparse solvers wave_func allows the calculation of the wave function in the scattering region due to any mode of any lead.

Return value of sparse solver

The function solve of sparse solvers now always returns a single instance of BlockResult. The latter has been generalized to include more information for leads defined as infinite systems.