Hi, so we have not explored parallelism with OpenMP very much. But please do try it and let us know if you run into any issues. The kind of parallelism you are describing would sit outside of ITensor, which is why we have not documented it (it's not a feature of ITensor, but just a way of using ITensor inside of a larger code).
In the past, I have used ITensor within a multi-threaded code where I created a parallel for loop using the C++ threading library and it worked extremely well. So hopefully you can use an OpenMP parallel for loop in the same way. The reason I say "let us know if you run into any issues" is that there may be some special handling of certain parts of ITensor we need to put in to make things thread safe within OpenMP. We can do this and add a compiler flag to turn it on, but please just try it first.