As illustrated in Figure , an MC move proposal either removes a non-base atom type from the working set of atom types or creates a more specific child type from an existing atom type, A. For creation of a new child, A', if the definition of A comprises only the primary atom, a decorator may be added to it; or an alpha substituent may be added. When a new substituent is added, the type of bond connecting it, single, double, triple, or aromatic, is also chosen. If A already has an alpha substituent, then A' may be created by adding a decorator to A; by adding a second alpha substituent; or by adding a beta substituent to any non-hydrogen alpha substituent. Finally, if A already has an alpha and a beta substituent, then a new decorator may be added to A, or a new alpha or beta substituent may be added. It is possible that in the future, move types might be added to allow the addition of decorations to alpha and beta substituents, but those moves are not included in our current SMARTY tests.