FRaNKS

Welcome to FRaNKS — First-principles Relaxation and Non-collinear Kohn-Sham!
FRaNKS is a plane-wave ab initio Density Functional Theory (DFT) code.

This is a simple, all-electron, plane-wave DFT code for calculating electronic structure.
It is primarily a toy / educational project, originally started during my PhD, and as such there are no guarantees of quantitative accuracy. The code is not intended to be a full-featured production DFT package.

At present, FRaNKS does not contain explicit crystal or atomic information. There are no atoms as such, although atomic effects can be included via a user-provided external potential. The focus is instead on illustrating the core ideas behind Kohn–Sham DFT in a plane-wave basis, including relaxation and non-collinear effects.

I must also appologise for the tortured acronym. This is named for my grandfather (Frank for those of you not paying attention...) so some linguistic acrobatics had to be performed.

Prerequisites

This program relies on several external libraries that must be installed before building:

1. FFTW3 — https://www.fftw.org
2. OpenBLAS — https://www.openblas.net

These libraries are required for fast Fourier transforms and efficient linear algebra operations.
OpenBLAS is currently the only supported BLAS/LAPACK implementation, though support for additional libraries may be added in the future.

FRaNKS can optionally be compiled with MPI (Message Passing Interface), allowing the code to run in parallel. This is particularly useful for larger calculations involving many electrons. A working MPI library compatible with your Fortran compiler is required for this.

You will also need working Fortran and C compilers. In principle, any standards-compliant compiler should work, but testing has primarily been carried out using the GCC toolchain.

FRaNKS relies on LibXC for exchange–correlation functionals. LibXC is bundled with the FRaNKS source and does not need to be compiled separately.

Installation

Once all prerequisites are installed, building FRaNKS is straightforward.

To build with the default settings, simply run:

GNU Make is recommended. The configure script will automatically detect the locations of FFTW, OpenBLAS, and other required dependencies. By default, the Makefile builds the MPI-enabled version of the code if MPI is available.

There are three main build flags available for customisation:

1. gmake COMMS_ARCH=serial/mpi
   Specifies whether to build a serial or MPI-parallel executable.

2. gmake BUILD=fast/debug
   Controls compiler optimisation levels.
   fast (the default) should be used for production calculations, while debug enables additional runtime checks.

3. gmake CODATA_YEAR=2014/2018
   Selects which international standard of physical constants (CODATA) is used in calculations.