uneven masses and slightly distorted triangles. All three molecules were of planar, trape-zoidal geometry with constraints applied to the four sides and the longer diagonal. As a test, we "flipped" the constraint along the diagonal of the molecules to constrain the other, shorter diagonal which resulted in a decrease of λ max in all three cases (BZTA and BZTH : 0.76 to 0.71, MINDA: 0.76 to 0.65).