Radial distribution function (RDF) for a pair

Dear LAMMPS users,

I have a system containing a water-hexadecane mixture on top of a membrane. And I would like to compute the radial distribution function (RDF) of hexadecane from the membrane surface. I’ve researched on the forum and from the LAMMPS documentation and I tried to write the code as below. But I think it is for the RDF of Hexadecane only. Can someone please help me how to compute the RDF for the pair of Hexadecane-Membrane.
Thank you.

#Atom definition

read_data Data.lmpdat

group Mem id <= 5900 #Mem: Membrane

group Water id <> 5900 17901

group Hex id >= 17901 #Hex: Hexadecane

#RDF Computation
compute myRDF Hex rdf 100

fix 1 Hex ave/time 100 1 100 c_myRDF[*] file rdfoutput.rdf mode vector

Kind regards
Tan Vu Bui