From owner-chemistry@ccl.net Sat Jan 26 22:13:00 2013 From: "Georg Lefkidis lefkidis*|*physik.uni-kl.de" To: CCL Subject: CCL: A physical interpretation of RPA Message-Id: <-48136-130126025125-5910-zxPkld6sLH5WPTvgKCowpA ~ server.ccl.net> X-Original-From: "Georg Lefkidis" Content-Transfer-Encoding: 8bit Content-Type: text/plain;charset=iso-8859-1 Date: Sat, 26 Jan 2013 08:51:14 +0100 (CET) MIME-Version: 1.0 Sent to CCL by: "Georg Lefkidis" [lefkidis-#-physik.uni-kl.de] Dear John, are we talking RPA like Random Phase Approximation? If yes, then the physical meaning is that the interaction between particles (molecules or atomic orbitals, or crystals, or matter and light, etc) is approximated by the average over all possible phase differences. Although the phase of a single wavefunction is not important, since only the observables matter, when it comes to interactions, the phase difference \phi gives a prefactor e^{i\phi} between the matrix elements. This would mean to calculate the interactions for *all* possible prefactors and, only at the end, average over all molecules (ensemble) to get the mean value. In RPA we average *before* calculating the property (usually by taking all wavefunctions real for convenience), which, strictly speaking, is correct only for linear properties. For two-photon processes, for example, this neglects one interference term (see also Phys. Rev. B, 87, 014404 for a more extended example of two-photon processes). Best regards George > > Sent to CCL by: John McKelvey [jmmckel,,gmail.com] > I hate to ask this... > > My wife is a biochem major and she asked me to give her a definition > of "RPA."... I know what the SCF-MO based formulas look like, but tat > won't do here... Can someone provide a physical definition of RPA? > > Regrets, > > John > > -- > John McKelvey > 10819 Middleford Pl > Ft Wayne, IN 46818 > 260-489-2160 > jmmckel{=}gmail.com> > ------------------------------------------------------------------- Dr. Georg Lefkidis Dept. of Physics University of Kaiserslautern PO Box 3049 67653 Kaiserslautern e-mail: lefkidis(at)physik(dot)uni(dash)kl(dot)de Tel.: +49 631 205 3207 -------------------------------------------------------------------