Hi everyone,

I am trying to understand the forcefield type assignment precedence rules used in the Dreiding and Universal forcefields. 

I think I understand the type rules themselves, but I can't work out how the automatic type assignment system works when atoms are matched by multiple types.

For instance in the molecule glycine (NH3-CH2-COO), the CH hydrogens match both the H_ and H___A rules.  Similarly the acidic carbon matches the C_2 and the C_R rules.  The automatic assignment assigns H_ and C_R to those atoms respectively.  My question is why does the type assigner give those assignments and not the other way around?  Is it based on the order in which the rules are specified in the file or something else?

Similarly there seems to be cases where the typing rules are relaxed to allow for cases where no other atom types fit.  For instance a single oxygen atom is assigned type O_3 by Dreiding.  This type is usually for atoms that have SP3 hybridisation, but if you change the hybridisation from this the O_3 type is still assigned.  What feature of the file specifies this behaviour?  Is there a way to make the typer fail for atoms that do not exactly fit a particular rule?

I would be greatful for any input to help me work it out.

Thanks,

Bradw