Use external hard drive to install applications to it I have a 120 GB SSD on my Mac and it is almost 90% full. I have another 500 GB external hard drive and I want to install some big applications to that hard drive so it does not occupy space of SSD.  

I tried to move apps directly from Application to the external drive but I think some files remain on the SSD.
You can use an external HDD to install and run applications there. Make sure they support USB 3.0 or Thunderbolt. I use Mathematica on mine and it works fine. The preferences files will remain on your internal disk but they are very small.