I need a word to describe something like this:
Disclaimer: I'm not a Smallville fan.
A half-smile or a sappy smile.
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Collins is happy with half-smile
a smile that is uncertain or short-lived
A related term is sly smile
a smile that shows you know something that other people do not: "I know why Chris didn't come home yesterday," she said with a sly smile.
More often than not, such a smile has a more knowing look to the eyes and is a bit less innocent in appearance.
Another term is wry smile
using or expressing dry, especially mocking, humor
While often subtle in appearance, such a smile is often described as slightly twisted
Presumably you're looking for positive connotations, and linked to romanticism.
A timid / tender smile
A warm / sympathetic / fond smile
An amiable / engaging / thoughtful smile
A knowing / charming / suggestive smile
A quasi smile.
It's called an Archaic Smile
Archaic Smile, from Wikipedia
The Archaic smile was used by Greek Archaic sculptors, especially in the second quarter of the 6th century BCE, possibly to suggest that their subject was alive, and infused with a sense of well-being. To viewers habituated to realism, the smile is flat and quite unnatural looking, although it could be seen as a movement towards naturalism.
Yeah, I've squeezed my mind and scoured the internet, but I don't think you'll find a word in English for a smile like that. The closest you can get is a combination of words, like in Mari-Lou's list.
Other languages do have words for various degrees of smiles - beyond smirks and grins - but not English, apparently.