[added] alvarez covalent radii

[marco-2023]

Added the covalent radii database from: B. Cordero,.V. Gómez, A. E. Platero-Prats, M. Revés,J. Echeverría, E. Cremades, F Barragána, S. Alvarez, Dalton Trans., 2008, 2832-2838 https://doi.org/10.1039/B801115J

Observations:

• The C covalent radius value was taken as reported for Csp3
• For Mn (Z=25), Fe (Z=26), and Co (Z=27) the covalent radii values were taken as reported for the low spin electronic configuration.

• These covalent radii values can be accessed using get_cov_radii(atnums, type="alvarez")

  modified: src/grid/utils.py

[Ali-Tehrani]

Thank you for this pull request. I added a test and fixed some issues.

[FarnazH]

@Ali-Tehrani, I noticed your commit message that "Based on my previous results they didn't match my results up to 4 decimal places, probably based on the conversion factor from angstrom to bohr." Can you please share how you got your numbers? If you have a code snippet, please share.

[Ali-Tehrani]

Sure, the code snippet is here.

I took the radius values from Table 2 in the paper and converted them to Bohr using the same conversion factor that was used in the previous tests.

[FarnazH]

Thanks, @Ali-Tehrani. Your conversion factor is the same as iodata, see:

>>> from iodata.utils import angstrom
>>> angstrom
1.8897261246257702
>>> angstrom - 1.8897261339213
-9.295529901365285e-09

@marco-2023, what conversion factor were you using? I just want to make sure that was the reason your numbers were different from Ali's.

[marco-2023]

@FarnazH I revised the numbers. The numbers were different because I used 1.88973 as the conversion constant (not the one in iodata). I also made a mistake parsing the results in order to add the underscores. Sorry for that. It won't happen again.

[FarnazH]

No worries @marco-2023. I just wanted to make sure we understand what was happening. We all make mistakes, and that's why we have code reviews :-) Thanks for your contribution.