I think you would simply run it as follows, by specifying input and output files and running it in the background. e.g.
nohup hspice -i {inputFilename} -o {outputFilename} &
This should allow hspice to continue running after you log out. You can use jobs to list running jobs and fg to put the process back into the foreground.
You could also consider redirecting stdin/stdout/stderr if you find that you need to do this:
nohup hspice -i {inputFilename} -o {outputFilename} > stdout.txt 2> stderr.txt </dev/null &
You may also consider the more flexible command, "screen" if your system has it which will also allow you to put the job in the background and reattach to it later.