I grant trust to molecular with 100 usd, but I have only 0.5 in my ripple acc now

You sure you want to do that? That means that if you ever hold USD in the system, molecular or someone else can take them from you and in exchange give you molecular IOUs, which he is under no obligation to honor.
Under no obligation? Maybe not face to face, but if molecular is owed $100 via ripple, I can swap IOUs with that person, yes?