Space-time curvature is caused by huge objects in space like black holes, merging black holes, or planets. This curvature is what causes gravity. Can molecules cause at least a really tiny curve ?
In a nutshell, yes.
Think about Newton's gravity. Even you and I have some gravitational pull, even though it is tiny. The same is true for General Relativity. Even the tiniest of particles makes an indentation in the fabric of spacetime.
Here's another analogy: think about a rubber sheet. This is spacetime. Now, imagine you put a bowling ball on that rubber sheet. It'd definitely make a dip, right? Now, let's say you put a baseball on the rubber sheet. It'd still make a dip, though not as big. Now, let's bring it down to a marble. The dip would be small, but it would still be there, right? Now, imagine an atom. It would an insanely small dip, but it would still be there, just like in General Relativity.
Here's yet another analogy. Imagine you are the size of a planet, and there are other planets and a star near you (relatively speaking, of course). Look down at the fabric of spacetime beneath you. Wouldn't you make a dip? Now, imagine you are the size of a molecule of salt, say. Now, imagine other molecules of salt, and electrons, and other atoms around you. Now, imagine the fabric of spacetime beneath you. Wouldn't you still make a dip?
As this website says,
Large objects such as the Sun and planets aren't the only masses that warp the fabric of space-time. Anything with mass—including your body—bends this four-dimensional cosmic grid. The warp, in turn, creates the effect of gravity, redirecting the path of objects that travel into it. The strength of gravity depends on the size of the space-time warp. A large object with little mass creates a smaller distortion than a tiny object with a huge mass.
If our bodies warp spacetime, why wouldn't particles?
Hope this helps!