Code: refactor `charge_mixing.cpp`

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[ ] Identify the specific code file or section with the code quality issue.
[ ] Investigate the issue and determine the root cause.
[ ] Research best practices and potential solutions for the identified issue.
[ ] Refactor the code to improve code quality, following the suggested solution.
[ ] Ensure the refactored code adheres to the project's coding standards.
[ ] Test the refactored code to ensure it functions as expected.
[ ] Update any relevant documentation, if necessary.
[ ] Submit a pull request with the refactored code and a description of the changes made.

deepmodeling / abacus-develop