OpenMP loop parallel for loop with function calls and STL vector
The code seems thread-safe to me. All operations are either done with local variables or with data addressed via the loop index, so data races are seemingly absent. Thus, no additional protection like #pragma omp atomic is necessary.

I do not think you can make it "more parallel". Though technically parts of the loop iteration are independent and can be computed in parallel, the overall amount of computation seems rather small, so exploiting that "inner" parallelism is unlikely to bring more performance. If you want to try it though, I would recommend you to use #pragma omp task, not sections.