Subject: Need some helps for adsorption energy

From: Sompop Sanongraj <[email-masked]>
Date: Mon, 28 Feb 2000 15:02:48 -0500

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