Off-diagonal elements in the mass matrix

[madanyrami1999]

I want to perform an eige analysis in STKO for the MDOF mass-spring system shown in the attached figure. I'm using Node-to-Node links and Zero-length elements. However, the mass matrix shown in the figure contains Off-diagonal elements. How can these elements be inserted?
Thank you.

[STKO Team]

zero length elements in opensees compute a lumped mass matrix (only with diagonal terms).
However, you can simulate such a simple model directly in python or matlab