While reading this paper on molecular graph generators using deep learning algorithms, I encountered the terms syntactically and semantically valid molecular graphs:
$\dots$ to ensure the generation of syntactically and semantically valid graphs.
The output molecular graph should at least satisfy the following constraints:
Valency: Number of incident edges to an atom shall not exceed the valency of the atom on that node
Connectivity: The output graph should be a single connected graph.
I was wondering what semantic and syntactic validity means in this context? Which type are each of these constraints?
Some concepts from computer science:
Syntax is the structure or form of expressions, statements, and program units but Semantics is the meaning of those expressions, statements, and program units.
But I can't make an exact connection to molecular graph's case. Also,
In computer science, the syntax of a computer language is the set of rules that defines the combinations of symbols that are considered to be a correctly structured document or fragment in that language
Then, valency and connectivity, unlike the linked paper in the answer below, should be syntactic constraints.