Firmi: Get a firm grasp on your Fermi surface!
Software utility to prepare calculated Fermi surface of metal for 3D printing.
30 November 2015, Materials Research Society's Materials Hackathon 2015
Dr. David Strubbe, Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, dstrubbe@mit.edu
Dr. Ryan Cooper, Materials Science and Technology Division, Oak Ridge National Laboratory, cooperrc@ornl.gov

INSTALLATION:
Requires Fortran90 compiler to compile.
Requires OpenSCAD (GPL-2 license) installed to create STL files (available from http://www.openscad.org and many package managers).
Edit FC and FCFLAGS in Makefile as necessary to adapt to your Fortran90 compiler, if not called 'gfortran'.
Type 'make' to install.

USAGE:
1. Obtain 'bxsf' from an electronic structure calculation, e.g. Quantum ESPRESSO, Wannier90, etc.
       (in XCrySDen Fermi surface file format, http://www.xcrysden.org/doc/XSF.html#__toc__14)
2. Run 'bxsf2scad.x' on 'bxsf' producing 'bxsf.scad'. (Must have the two .data files present in working directory.)
3. Open 'bxsf.scad' in OpenSCAD software.
4. In OpenSCAD menu, Design->Compile, for quick view.
5. In OpenSCAD menu, Design->Compile and Render (CGAL) [may take a while!].
6. In OpenSCAD menu, File->Export->STL to save file.
7. Open STL file in (e.g.) MakerBot software, adjust size, rotation, position as desired for printing.
8. Print, and enjoy!

NOTES:
- Adjust center (via origin line in 'bxsf', as integer number of points in k-grid to displace) to make a closed
  surface if the Fermi surface is not around the \Gamma point.
- Using too fine a k-grid can make a surface that OpenSCAD will run out of memory trying to render, so
  if that happens, use a coarser k-grid. Of course, the resolution of the 3D print might be less.
- You can use the Fermi surface viewer of XCrysDen (www.xcrysden.org, GPL-2 license) to check out the
  Fermi surface before trying to create the SCAD file.
- The code will plot the first band in 'bxsf' that crosses the Fermi level. If you want to plot a different
  one, then include only the band you want in 'bxsf'.
- Fermi surfaces with open orbits may not render in OpenSCAD; further development needed for this.
- Currently, the Fermi surface is plotted in the primitive unit cell, not the Brillouin zone.
- The printed object is a surface enclosing a volume of lesser energy, i.e. an electron pocket.

EXAMPLES:
Calculated from examples in Wannier90 2.0.0, GPL-2 license, interpolating to finer k-grid from DFT-LDA calculations with Quantum ESPRESSO
fcc lead, 20x20x20 k-grid, Wannier90 example02. OpenSCAD step may take ~7 minutes.
fcc copper, 20x20x20 k-grid, Wannier90 example04. OpenSCAD step may take ~19 minutes.

License = GPL-2 (see LICENSE file)
Code based in part on Octopus version 5.0.0, www.tddft.org/programs/octopus, from feature for output of isosurface
of density, wavefunction, or potential from code's internal real-space mesh in 3D-printable format,
which implements Marching Cubes algorithm for creating isosurface, with linear interpolation. originally by Xavier Andrade.
Modified to use general mesh of (non-orthogonal) parallelipiped reciprocal lattice vectors, use periodic boundary conditions,
and allow translation of the cell to close the surface.