Xatu (eXcitons from ATomistic calcUlations) is a program and library designed to solve the Bethe-Salpeter equation (BSE) of any material. Starting with electronic band structures from either tight-binding or DFT based on local orbitals, the BSE is constructed. Its diagonalization then yields the exciton spectrum, which can then be postprocessed to characterize the excitons and their optical properties.
The theory behind the code plus details about the implementation and some usage examples can be found in our preprint Efficient computation of optical excitations in two-dimensional materials with the Xatu code. If you find our paper or the code useful, please consider citing us.
Xatu is built upon the Armadillo C++ library for linear algebra, which is also based on the standard libraries por linear algebra, namely BLAS, LAPACK and ARPACK.
Install the required libraries:
sudo apt-get install libopenblas-dev liblapack-dev libarpack2-dev libarmadillo-dev gfortran
The library (libxatu.a) can be automatically built running the following command:
make build
Then, the Xatu binary is built running:
make xatu
Alternatively, one can define scripts that make use of the functions defined in the library. To compile them, it suffices to put the script in the /mainfolder and run
make [script]
For MacOS the dependencies can be installed via brew. To build the library it is recommended to use brew's gcc compiler instead of clang.
brew install gcc openblas lapack arpack armadillo
Then, specify in the Makefile the new compiler as well as the location of the libraries:
CC = g++-13
INCLUDE = -I$(PWD)/include -I/opt/homebrew/include -I/opt/homebrew/opt/openblas/include
LIBS = -DARMA_DONT_USE_WRAPPER -L$(PWD) -L/opt/homebrew/lib -L/opt/homebrew/opt/openblas/lib -lxatu -larmadillo -lopenblas -llapack -fopenmp -lgfortran -larpack
In case that the libraries are not available through repositories, it is always possible to manually download and compile them. For specific instructions on how to install each library we refer to the documentation provided by each of these libraries. For instance, for Armadillo:
Clone the Armadillo library repository:
git clone https://gitlab.com/conradsnicta/armadillo-code.git
To install Armadillo run:
cd armadillo-code
cmake .
make install
Once they are compiled, to link the libraries we have to modify the Makefile to specify the directories where they are installed (e.g. Armadillo and OpenBLAS):
INCLUDE = -I/dir/armadillo/include -I/another_dir/OpenBLAS/include/
LIBS = -L/another_dir/OpenBLAS/lib
The documentation for the library is generated using Doxygen. To build it, we have to install Doxygen and then run from the /docsfolder:
doxygen docs.cfg
The documentation can be accessed then in /docs/html, by opening index.html.