Only undertake this is “conventional” uninstall fails.
Version 3
Delete Registry Keys
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Word\Addins\Chem4Word V3
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\Word\Addins\Chem4Word V3
HKEY_CURRENT_USER\Software\Chem4Word V3
Delete Folders
C:\ProgramData\Chem4Word.V3
C:\Program Files (x86)\Chem4Word V3
C:\Program Files\Chem4Word V3
C:\Users\Owner\AppData\Local\Chem4Word.V3
Version 2
Delete Registry Keys
HKEY_LOCAL_MACHINE\Software\Microsoft\Office\Word\Addins\Chemistry Add-in for Word
HKEY_LOCAL_MACHINE\Software\WOW6432Node\Microsoft\Office\Word\Addins\Chemistry Add-in for Word
HKEY_CURRENT_USER\Software\Chem4Word
Delete Folders
C:\Program Files (x86)\Chem4Word
C:\Program Files\Chem4Word
C:\Users\Owner\AppData\Local\Chemistry Add-in for Word
Note:
Some registry keys and file locations will not exist depending on whether your version of Windows in 32bit or 64 bit.