2 added 275 characters in body edited Apr 14 '14 at 3:27 fabio.hipolito 9655 bronze badges The simple rule I to -I is not guaranteed to work, e.g. Exp[3 I] /. I -> -I  and ComplexConjugate might be too slow (for lengthy expressions). Therefore, I rather define an alternative function to conjugate ClearAll[AltConjugate] AltConjugate[x_] := ReplaceAll[xReplaceAll[FullSimplify[x], Complex[a_, b_] -> Complex[a, -b]];  This functions looks for the paternpattern Complex[a_, b_] and replaces it by Complex[a, -b]. In-so @celtschk -far this roots might be problematic, simple functions like f[x_]=Sqrt[-x^2] can be handle by simplifying the input function returns, i.e. adding FullSimplify in the correct resultdefinition of AltConjugate. Nevertheless, ifthis will fail for functions including more general roots, such as f[x]=Sqrt[-x^2 +I b] where both x and if only all variablesb are realreals. Nevertheless Use this carefully and always test it. Cheers. The simple rule I to -I is not guaranteed to work, e.g. Exp[3 I] /. I -> -I  and ComplexConjugate might be too slow (for lengthy expressions). Therefore, I rather define an alternative function to conjugate ClearAll[AltConjugate] AltConjugate[x_] := ReplaceAll[x, Complex[a_, b_] -> Complex[a, -b]];  This functions looks for the patern Complex[a_, b_] and replaces it by Complex[a, -b]. In-so-far this function returns the correct result, if and if only all variables are real. Nevertheless test it. Cheers. The simple rule I to -I is not guaranteed to work, e.g. Exp[3 I] /. I -> -I  and ComplexConjugate might be too slow (for lengthy expressions). Therefore, I rather define an alternative function to conjugate ClearAll[AltConjugate] AltConjugate[x_] := ReplaceAll[FullSimplify[x], Complex[a_, b_] -> Complex[a, -b]];  This functions looks for the pattern Complex[a_, b_] and replaces it by Complex[a, -b]. @celtschk - roots might be problematic, simple functions like f[x_]=Sqrt[-x^2] can be handle by simplifying the input function, i.e. adding FullSimplify in the definition of AltConjugate. Nevertheless, this will fail for functions including more general roots, such as f[x]=Sqrt[-x^2 +I b] where both x and b are reals. Use this carefully and always test it. Cheers. 1 answered Apr 13 '14 at 11:27 fabio.hipolito 9655 bronze badges The simple rule I to -I is not guaranteed to work, e.g. Exp[3 I] /. I -> -I  and ComplexConjugate might be too slow (for lengthy expressions). Therefore, I rather define an alternative function to conjugate ClearAll[AltConjugate] AltConjugate[x_] := ReplaceAll[x, Complex[a_, b_] -> Complex[a, -b]];  This functions looks for the patern Complex[a_, b_] and replaces it by Complex[a, -b]. In-so-far this function returns the correct result, if and if only all variables are real. Nevertheless test it. Cheers.