How can I create a descriptor state space model (dss(A,B,C,D,E)) with a nonlinear term, f?

NOTE: This question may stem from insufficient knowledge of state space and nonlinear dynamics on my part. Therefore, it may not necessarily be a MATLAB question. However, I'd still appreciate assistance if someone is willing to help.

Hello. I want to create a dss(A,B,C,D,E) model, but with an f function as per:

https://morwiki.mpi-magdeburg.mpg.de/morwiki/index.php/Mass-Spring-Damper

However, I do not know how to include the f. Does anyone here know how to?

I have included a picture of all of the system information. This may add context. It is a mass-spring-damper system:

• Thank you