# ABACUS

Copyright (c) [J.-S. Caux](https://jscaux.org).


ABACUS is a general set of algorithms for dealing with Bethe Ansatz-solvable systems including:

* Lieb-Liniger (delta-)interacting bosons
* Heisenberg spin chains
  * Isotropic antiferromagnet (XXX)
  * Anisotropic gapless antiferromagnet (XXZ, 0 < Delta < 1)
  * Anisotropic gapped antiferromagnet (XXZ_gpd, Delta > 1)
* Interacting fermions (Jordan-Wigner'd XXZ)

The principal purposes of ABACUS are to:

* provide structs and classes to construct Bethe eigenstates
* provide structs and classes to solve TBA equations
* provide functions to compute matrix elements and other ABA-obtained quantities (overlaps, ...)
* provide Hilbert space scanning functions, enabling the computation of dynamical correlation functions of basic observables
* provide a set of executables to perform such computations, either serially from the command-line or in parallel on a cluster.

The ABACUS codebase is written in C++.
The serial executables are OpenMP-aware. Parallel versions use MPICH.

## Installation
From the base directory, simply run

``` shell
$ make
```

This will produce all serial executables, together with a library `ABACUS_[vn].a` where vn is of the form [digit][character]. The digit marks a release number, with the optional letter marking a subversion.

##### Serial executables
All executables are in the `bin/` folder. Invoking them with no arguments will print out usage instructions.

##### Parallel executables
To compile, invoke
``` shell
$ make parallel
```
Three executables must be run sequentially: `[prefix]_par_Prepare`, `[prefix]_par_Run` and `[prefix_par_Wrapup`, in which `prefix` is `Heis_DSF`, `LienLin_DSF` or `LiebLin_DSF_General_State`.

## Documentation
Documentation for the ABACUS codebase is automatically generated using Doxygen. To generate the documentation, invoke `make docs` (you will need to have Doxygen installed on your system). The configuration file is `ABACUS_Doxygen`.  The HTML version is anchored at `docs/html/index.html`. The PDF version is located at `docs/ABACUS_[vn]_manual.pdf`.

An outdated description of ABACUS can be found in J.-S. Caux, J. Math. Phys. 50, 095214 (2009), [doi:10.1063/1.3216474](https://doi.org/10.1063/1.3216474).

## Note on versions and branches
Versions are labeled `ABACUS_[vn][subvn]`, where `vn` is a version number and `subvn` is a letter subversion label. Branch `master` sits on the latest version/subversion available.

In the repo, development takes place in branches labeled `dev_from[vn][subvn]` where vn marks the version from which development takes place. Sub-branches for specific features are labeled `dev_from[vn]_[feature]`, where feature is a short descriptor. Completed and accepted features are merged into the dev branch, before this is ultimately merged into master and released as a new [sub]version.


## Warnings
* The ODSLF part (for one-dimensional spinless fermions) is not functional: it is based on the old Young Tableaux ids, and must be upgraded to `State_Label`s.
* The Richardson part is not implemented; what exists is old and long deprecated.

## Acknowledgements
_(in historical order)_

__Antoine Klauser__ provided functions for computing neighbour-operator-product matrix elements in XXX: `ln_Szz_ME`, `ln_Szm_p_Smz_ME` and `ln_Smm_ME`.

__Miłosz Panfil__ contributed to code to help in the calculation of finite-temperature correlations of Lieb-Liniger.

__Jacopo De Nardis__ contributed code for the `ln_g2_ME` function for Lieb-Liniger.

__Teun Zwart__ has given much useful advice concerning C++ code organization.


## Version history
Development of ABACUS was started by [J.-S. Caux](https://jscaux.org) around 2004 and has known an embarrassingly large number of versions. Some notes on the more recent versions can be found in file [ABACUS_version_history.md](ABACUS_version_history.md) in this directory.