eig_occ.txt: Occupation numbers below 1e-15 should be printed as 0 instead of tiny values (machine precision noise)

[xinchenhou]

Describe the bug

In the eig_occ.txt output file, electronic occupation numbers that are effectively zero (due to machine precision) are printed as very small numbers like 1e-27, 1e-58, etc. These values have no physical meaning and are just numerical noise from the solver. They make the output file harder to read and parse automatically.

Expected behavior

Maybe we can add a small threshold (e.g., 1.0e-15) when writing occupation numbers to eig_occ.txt. For example, if occupation < 1e-15, print 0.0 instead of the tiny numerical value.

To Reproduce

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Environment

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Additional Context

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