four node quad element with one quadrature point

Hello all,



Could anyone help me with the four node quadrilateral element with one quadrature point for nonlinear implicit solid mechanics problem? To be more precise, I would like to know

  (1) the form of the internal force vector and

  (2) the form of the tangent matrix



of this special element.



I read the book of Prof. Belytschko already but unfortunately it didn't help much. Literature on this element is not clear to me. For example in some papers I could find (1) but not (2) and vice versa in other papers :).



Thanks a lot for your time.

Phu