handle negative and zero charge

[timosachsenberg]

Description

fixes #6280

Checklist

• Make sure that you are listed in the AUTHORS file
• Add relevant changes and new features to the CHANGELOG file
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• Updated or added python bindings for changed or new classes (Tick if no updates were necessary.)

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Advanced commands (admins / reviewer only)

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[cbielow]

looks good.

maybe renaming charge to abs_charge would help?!