Need some helps for adsorption energy

Hi all
 I try to apply the computational calculation for adsorption field. First, I
 try to calculate adsorption interaction (at zero coverage) between noble
 gases (He, Ne, Ar and Kr) and the graphite lattice (7 rings) as shown in
 Figure below.
                     ____
                    /    \
               ____/      \____
              /    \      /    \
             /      \____/      \
             \      /    \      /
              \____/   o  \____/
              /    \      /    \
             /      \____/      \
             \      /    \      /
              \____/      \____/
                   \      /
                    \____/
 Where (o) in the center denotes the atom of noble gas above the ring
 cluster representing the graphite.
 To do this, I optimized and calculated the energy of the graphite
 lattice(E1), each noble gas (E2) and the merged system between graphite
 lattice and each noble gas (E3). For each step, the methods and basis sets
 are the same for calculating E1, E2 and E3.  Then I simply calculated the
 adsorption interaction energy by E3-(E1+E2). The optimization methods basis
 sets (I used gaussian 98, A.7) and the results are shown below.
                                          Adsorption Interaction Energy
 (E3-(E1+E2)), cal/mole
 Methods and basis sets                 He                Ne
 Ar               Kr
 RHF/6-311g(d)                      -2.07078           -19.39004
 -17.06826         -13.93071
 RHF/6-311g(d)//B3LYP/6-311g(d)    -308.92293          -17.50751
 234.50030         793.98777
 B3LYP/STO-3g                       -4.64357           -2.51004
 112.95171         48.82024
 B3LYP/6-311g(d,p)                  -22.40209          -570.34338
 -22.08833         -44.11392
 For the expected result, we should obtain the linear relationship between
 the adsorption energy and the atomic no. of noble gas because the
 dispersion interaction is dominant (theorrtically, the dispersion
 interaction depends on polarizability). Also the sign of adsorption
 interaction should be negative because it supposes to be released energy. I
 got no luck. As you can see from my results, there are no methods and basis
 sets that the results will follow the expected result. Can anybody give me
 some suggestion about the method or basis set that I used  or anything that
 I should aware of when I do this kind of calculation?
 Thanks in advance
 dom
 Sompop Sanongraj
 208 Church Street
 Hancock MI 49930.
 TEL: (906) 482 9377