Error estimates for Q(1) isoparametric elements satisfying a weak angle condition

We prove optimal order error estimates in the H-1 norm for the Q(1) isopara metric interpolation on convex quadrilateral elements under rather weak hyp otheses, improving previously known results. Choose one diagonal and divide the element into two triangles. We show that, if the chosen diagonal is th e longest one, then the constant in the error estimate depends only on the maximum angle of the two triangles. Otherwise, the constant depends on that maximum angle and on the ratio between the two diagonals. In particular, w e obtain the optimal order error estimate under the maximum angle condition as in the case of triangular elements. Consequently, the error estimate is uniformly valid for a rather general cl ass of degenerate quadrilaterals.