Hi there

I have a guest@host system optimized with CASTEP without any constraints (with Fine setting), however, the binding energy or the interaction energy between the host and guest seems positive. I believe it supposes to be negative for a system in reality so my question is there any possibility that the binding energy of a guest@host complex is positive...? 

I have used the following equation to find binding energy: E_bind = E_guest@host - [E_guest + E_host]

-Monu