Potential energy surface with dynamics simulation by Dmol3

Hi everyone, 

              Is there anybody who knows how to get the potential energy surface for an ab initio dynamics by Dmol3?  My system is about a gas molecule adsorbed on a surface.    
             The second question is that for an ab initio dynamics by Dmol3,  how long should be calculated?  10 ps or longer?  Is there some guidance?

Cheers
water