Kpoints Sampling Saved as Float When Writing Kpoints file

[yutoml]

Python version

Python 3.10.14

Pymatgen version

Version: 2024.7.18

Operating system version

Red Hat Enterprise Linux Ver 8.6

Current behavior

When using the Kpoints class from pymatgen.io.vasp.inputs to read a Gamma-centered KPOINTS file and then write it back, the fourth line of the KPOINTS file is converted to a float string. This causes VASP5.4.4 calculations to fail when using this file. Specifically, in the from_str method of the Kpoints class, the fourth line of the KPOINTS file is cast to kpts as Sequence[tuple[float, float, float]]. The kpts attribute is then handled as a sequence of floats throughout the process, and it is written back as floats.

Expected Behavior

When writting a Gamma-centered KPOINTS file, the number of K-point divisions should be cast to a int, or it should be explicitly cast to an integer when writing the file if the style is Gamma-centered or Monkhorst-Pack.
Initially, the kpts attribute of the Kpoints class was handled as list[int] in from_str method, but it was changed to Sequence[tuple[float, float, float]] in #3758. While this change itself is not problematic, it would be more convenient if kpts were an integer sequence for Gamma-centered or Monkhorst-Pack.

Minimal example

#The following source code is a simplified code that reproduces this problem.
#The fourth line of the KPOINTS file resulting from the output of this code is `3.0 3.0 3.0`.

from pymatgen.io.vasp.inputs import Kpoints

Kpoints_str = """\
An example
0
Gamma
3    3    3
0.0  0.0  0.0"""

kpoints = Kpoints.from_str(Kpoints_str)
kpoints.write_file("KPOINTS")

Relevant files to reproduce this bug

No response

[DanielYang59]

Hi thanks for reporting and diagnosing and sorry for the trouble caused. I could confirm the issue and would fix ASAP.