[gmx-users] tutorial for "dual" alchemical transformation in gromacs?

[Sebastian Stolzenberg]

Dear Gromacs experts,

I am looking for a Gromacs tutorial to perform a "dual" alchemical 
transformation in a water box, i.e. a mutation or any other alchemical 
transformation, in which:
the nonbonded interactions between the environment and a molecule A 
disappear as lambda goes from 0 to 1, and
the nonbonded interactions between the environment and a molecule B 
appears simultaneously.
Interactions between atoms A and B are to be excluded.

So far, I have been trying to use the Gromacs user guide to extend the 
ethanol tutorial
http://www.gromacs.org/@api/deki/files/196/=fe-tutorial-4.6.pdf
into such a "dual" transformation, but also because I am currently faced 
with manually manipulating my topology file, this option is highly prone 
to my own errors.

Would you be able to help?

With Thanks and Best Regards,
Sebastian