Dear PW users, I have found that any code capable of phonon calculation (including PWscf-phonon)) would calculate the curve of F as a function of T under the framework of qausi-harmonic approximation (QHA).
My question is: Is this calculation (F as a function of T)of any real significance? Since under QHA, free energy F is a funciton of temperature T and volume V. In a real situation, the system volume would change with temperature T, so does the phonon spectrum. Thus, in calculation of F(T) curve, we need phonon spectrum at each specific T which is sure different under QHA. This is for sure not the practice done by any codes. F is simply calculated with only a specific set of spectra (usually the spectra at static equilibrium geometry). It is understandable if this sort of calculation neglects the thermal expansion, but we will end up with harmonic approxiation then. Yes, from my own experience, the changes of phonon spectrum are small w.r.t. volume, but at least in principle it should be accounted for. Anybody interested? plase give some comments,thanks! W. YU ____________________________________________________________________________________ Need Mail bonding? Go to the Yahoo! Mail Q&A for great tips from Yahoo! Answers users. http://answers.yahoo.com/dir/?link=list&sid=396546091
