to_cube function has a bug

[giovannipizzi]

Describe the bug
The to_cube function of VolumetricData has a bug: it stores in lines 4-6 the real-space vectors instead of the size of a voxel.

To Reproduce

1. Get any cube file
2. run the following code, that should return the same file (apart from cosmetics):

   from pymatgen.io.common import VolumetricData
   cubefile = VolumetricData.from_cube(IN_FNAME)
   cubefile.to_cube(OUT_FNAME)

3. check the content of the two files. You will see that lines 4-6 are wrong.

E.g. in my case, original file (lines 4-6):

   72    0.170589    0.000000    0.000000
   80    0.000000    0.163473    0.000000
  108   -0.114105    0.000000    0.125017

Output file:

	 72 12.282408 0.000000 0.000000 
	 80 0.000000 13.077840 0.000000 
	 108 -12.323340 0.000000 13.501836 

Expected behavior
The correct values are written in the cube file.

It should be as simple as changing the following lines:

pymatgen/pymatgen/io/common.py

Lines 349 to 365 in e513400

	f"\t {self.dim[0]} "
	f"{self.structure.lattice.matrix[0][0] * ang_to_bohr :.6f} "
	f"{self.structure.lattice.matrix[0][1] * ang_to_bohr :.6f} "
	f"{self.structure.lattice.matrix[0][2] * ang_to_bohr :.6f} \n"
	)
	file.write(
	f"\t {self.dim[1]} "
	f"{self.structure.lattice.matrix[1][0] * ang_to_bohr :.6f} "
	f"{self.structure.lattice.matrix[1][1] * ang_to_bohr :.6f} "
	f"{self.structure.lattice.matrix[1][2] * ang_to_bohr :.6f} \n"
	)
	file.write(
	f"\t {self.dim[2]} "
	f"{self.structure.lattice.matrix[2][0] * ang_to_bohr :.6f} "
	f"{self.structure.lattice.matrix[2][1] * ang_to_bohr :.6f} "
	f"{self.structure.lattice.matrix[2][2] * ang_to_bohr :.6f} \n"
	)

to something like (untested, but should work)

            file.write(
                f"\t {self.dim[0]} "
                f"{self.structure.lattice.matrix[0][0] / self.dim[0] * ang_to_bohr :.6f} "
                f"{self.structure.lattice.matrix[0][1] / self.dim[0] * ang_to_bohr :.6f} "
                f"{self.structure.lattice.matrix[0][2] / self.dim[0] * ang_to_bohr :.6f} \n"
            )

and analogously for the following two lines.

[janosh]

Thanks for reporting! 👍