long distance behavior

[Ersin Yurtsever]

Good evening

 

I am calculating BSSE corrected potential energy surfaces from standard CCSD(T) for molecules with He or H2. In case of H2 at large distances the interaction energy becomes positive ( a few cm-1) even for large basis sets. This does not happen for He even though they both are 2-electron systems.

 

The input is simple:

basis=avqz                                                                      
set,charge=-1   
hf
ccsd(t),nocheck                                                              
dummy,h1,h2                                                                     
set charge=-1                                                                  
hf                                                                             
ccsd(t),nocheck                                                                 
dummy,n,c,o                                                                    
set,charge=0                                                                   
hf                                                                              
ccsd(t),nocheck                                                                                                                                    

I appreciate any suggestions