If you have a permanent legal separation or a divorce decree, you will have no problem with claiming head of household in the same structure as your former spouse.
However, if you are still considered married, making a claim of "living separate from your spouse for the last 6 months of the year" might cause some issues. When the IRS examines tax returns, the agents often take a "facts and circumstances" approach with rules like dependents, filing statuses, and itemized deductions. This means that the IRS will take a rule like living separate from your spouse for the last 6 months of the year, and disallow your use of the head of household filing status.
