Perhaps you should try some spelling variations:
"quantum mechanical system" » "quantum mechanics system", "quantum mechanical systems"
"quantum chemical system" » "quantum chemistry system", "quantum chemical systems", "quantum biochemical systems", "quantum dynamical system"

You may be able to get more results by adjusting your search query.

• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"quantum chemical system\") OR (\"quantum mechanical system\"))".
• Removing quotes may allow a broader search: (( quantum chemical system ) OR ( quantum mechanical system )).
• Adding a wildcard symbol may retrieve word variants: (("quantum chemical system") OR ("quantum mechanical system"))*.