Because of your question without any details, I can only answer it generally. For getting convergency lattice parameter by energy-volume method, you can read the paper,Acta Materialia 54 (2006) 1641-1657, in which the author discussed that issue by Wien2k code. Or, if you want to use "vc-relax", you'd better decrease the convergence threshold to very small number with convergence cutoff energy, k-mesh and so on. One more to be noted is that the minimum energy configuration corresponds to zero force, but minimum enthalpy corresponds to zero force and stress. In "vc-relax" case, we try to find zero both for force and stress instead of only for total energy.
-- GAO Zhe CMC Lab, MSE, SNU, Seoul, S.Korea At 2011-09-14 17:14:08,"kondaiah samudrala" <konda.physics at gmail.com> wrote: Dear all, I am doing test calculations for orthorhombic layered compounds having van der waals forces. i calculated the volume by varying c/a, b/a and a with energy and taken the minimum values of c/a, b/a and a. But my volume change is surprisingly 2% with experimental ( even van der waals forces having). I tried with the "vc-relax" for same system i found 12% volume change with experimental.I tried with psedopotentials also and the results are same. I am very thankful any to solve this problem. Thanks in advance S.Appalakondaiah Research Scholar University of hyderabad Hyderabad India -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20110916/3bc17b0d/attachment.htm
