The space-energy synthesis approach has been improved by employing reaction rate weighting, by the use of realistic trial functions, and by deriving a more general analytical solution for the synthesis equations which includes the necessary case of complex B2. The use of reaction rates as weight functions and physically realistic trial functions made it possible to reduce the error of the space-energy synthesis method to such small values that its application in routine calculations of neutron spectra in fast reactors may be considered. The error reduction as compared to previous versions was typically a factor of 100 in δk and a factor of 20 in quantities which are sensitive to the nonseparability of space and energy. All cases with accurate results required a complex B2 in the blanket region as compared to real B2 for results with larger inaccuracies.