xu yuehua wrote: > and is that theory used in PWSCF code?
http://www.springer.com/physics/book/978-1-4020-3287-5 go to 'Sample pages', download the file. It contains a brief introduction to density-fgunctional perturbation theory, including some essential info on how to calculate IR cross sections P. -- Paolo Giannozzi, Democritos and University of Udine, Italy
