So, I'm trying to get a grasp on how exactly ABAQUS shape functions are formulated for its four node quad element, CPE4. The theory guide in “Solid isoparametric quadrilaterals and hexahedra,” Section 3.2.4" explains the shape functions associated with this element. However, when I'm using these shape functions to produce a simple stiffness matrix, my stiffness matrix does not match ABAQUS's. This makes me wonder if ABAQUS does something beyond these shape functions in their formulations? Previously my calculated stiffness matrix for CPE3 did match ABAQUS's stiffness matrix. Taking a look at the "two-dimensional solid element library", I notice that there is a superscripted (S) next to CPE4 that is not next to CPE3. Does anyone know what this (S) means? Thanks for your help!