Hello Everyone,Abaqus 

I am performing a coupled structural acoustic analysis in Abaqus Standard 3D. The first step is frequency extraction (Lanczos), followed by a steady-state dynamics analysis. I have used surface-based *TIE constraints between the structural and acoustic nodes with the following command:

 
*TIE, NAME = group1, POSITION TOLERANCE = 5.0, ADJUST=NO     
structAcou,AcouStruct

Although I don't have any coupling elements in my model, I encountered the following error: 

THREE FACTORIZATIONS IN A ROW FAILED. PLEASE CHECK THE MODEL. IT IS POSSIBLE THAT THE MODEL CONTAINS THE KINEMATIC COUPLING DEFINITION SET UP IN A WAY THAT A DEGREE OF FREEDOM HAS NEITHER MASS NOR STIFFNESS.

Could someone please help me understand this error and how to resolve it?

Thank you!

Edit: I used AMS instead of Lanczos. The run completed successfully. But still, can anybody tell me what the issue was with Lanczos?