Dear Prof Gerard: In fact I have read your Model Validation on-line, and I have to say it is youe lecture that made me to think about this problem, though I have used it as a common stand for the quality of the model without understand it for a long time. And the http://en.wikipedia.org/wiki/Molecular_geometry and http://en.wikipedia.org/wiki/Dihedral_angle, I think both from your lecture, I have checked them today before I post my mail, but unable to access them. For some reason, wiki is forbiddon here. I really coouldnot find the best answer and have discussed with others before, so I just want to make it clear, though maybe nothing to do with practicals. Thanks for your reply again.
Best Regards!