Sirius has the capability to build molecular structures from provided fragment libraries or to modify existing structures. When building a structure from fragments, first activate the desired fragment by clicking the corresponding button. Once the fragment is active, click anywhere in the display area to add the fragment. If you click an existing hydrogen atom in a displayed structure, the fragment will attach to it repacing the hydrogen. This is the fastest way to construct a molecule. If you add fragments to the display and connect them afterwards, it requires an additional step of creating a bond.

Note that when a fragment button is active, clicking in the display adds a fragment. Also, if a non-hydrogen atom is clicked in this mode, a warning message is displayed, since fragments cannot be added to non-hydrogen atoms. If you would like to return to the regular display mode, either deactivate the fragment button or close the builder palette.