Good luck: it does not appear to be open source, and it is a product of the US NIST, so I imagine the source is very hard to come by.  My suggestion would be to contact NIST directly and see if they'd be willing to share the source for that module.

Here it is...

Thanks Dudley J Benton Sir for your response but there is a little problem while opening this Zip file; perhaps it is not compressed properly. So pl share it once again.

You can get the fluud propeetues from NIST chemistry webbook.

Check this link

https://github.com/jowr/librefprop.so

Hi Sir ,

Can anyone provide me NIST REFPROP 9