Hi! In my work I use the WE theorem to evaluate matrix elements. F being the total spin and m the projection onto the z-axis, I am using: <JIFm|r_(-q)|J'IF'm'>=(-1)^(F'+m'-m)<Fm1q|F'm'>sqrt(2F+1)sixj(F, F', 1:J',J,I)<J'||r||J> I have a problem with the (-1)^ part which I suspect to be wrong, but have not been able to find a formula to compare a check. Am quite sure the rest is right. Could somebody help and tell what should be in the exponent?