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posts 280 Best regards, /Dan

Question 6:

Author againsmile Post Sat Apr 21 2007, 04:08AM Hi all, Registered Member #202 I'm a new user of VASP. Joined Tue Jun 14 These days, I try to calculate zero point energy of H2 molecule. 2005, 04:15PM posts 6 I made an INCAR file as followings, SYSTEM = H2 ISMEAR = 0 SIGMA = 0.01 ISIF = 1 ISPIN = 2 PREC = Accurate NSW = 5 IBRION = 5 ENCUT = 875 VOSKOWN =1 NFREE = 2 POTIM = 0.02 IALGO =48 GGA = PE EDIFF = 1E-5 Is there any problem in this? And where I can find the zero-point energy? Thanks, Back to top lcyin Mon Apr 23 2007, 01:48PM Registered Seemly, your INCAR settings are right, and you can find the ZPE in the OUTCAR file, which looks like this: Member #109 1 f = 130.237331 THz 818.305288 2PiTHz 4344.249631 cm-1 538.618501 Joined: Fri Mar meV 18 2005, 08:07AM posts 30 Back to top admin Wed May 02 2007, 02:07PM please note that H is treated as a classical particle by VASP, though it is so light that it should be treated as a quantum particle. [ Edited Wed May 02 2007, 02:08PM ] [ Edited Wed May 02 2007, 02:09PM ] posts 1833

Question 7: Author pavel Post Tue Sep 25 2007, 04:45PM Registered Member #923 Joined Tue Oct 17 2006, 04:08PM Location: Karlsruhe, Germany posts 16 I tried to calculate the dissociation energy of H2 using PAW_GGA potential for H (version 07Jul1998) using the following INCAR files: SYSTEM = isolated H2 molecule ISMEAR = 2 SIGMA = 0.2 ENCUT = 450 NPAR = 4 GGA = 91 ISPIN = 1 IALGO = 48 LREAL = .FALSE. NSIM = 4 NSW = 20 IBRION = 2 POTIM = 0.3 NELMIN = 4 EDIFF = 1E-2 EDIFFG = 1E-4 and SYSTEM = isolated H atom ISMEAR = 2 SIGMA = 0.001 ENCUT = 450 NPAR = 4 GGA = 91 ISPIN = 1 IALGO = 48 LREAL = .FALSE. NSIM = 4 NSW = 20 IBRION = 2 POTIM = 0.3 NELMIN = 4 EDIFF = 1E-2 EDIFFG = 1E-4 I have got a dissociation energy about 6.7 eV, while experimental value is about 4.52 eV. Then I realized that I have forgotten spin. I recalculated both values with spin, but the dissociation energy changed only in the forth digit after comma. What could be the reason for this discrepancy? Back to top ojwang Tue Sep 25 2007, 05:19PM Are you sure you know what you're doing? If I understand you correctly, the Registered process you're trying to calculate is Member #19 Joined: Mon Sep H2 --> H + H 20 09:54AM posts 7 2004, 6.7 ev --> 1.1188ev + 1.1188 eV Comes to about 4.5 eV or close enough. In you case you are trying to compute the free energy of H2 and not the dissociation energy! Back to top pavel Wed Sep 26 2007, 11:56AM Dear Ojwang, Certainly, I must be wrong and I am trying to understand where. Registered Member #923 Joined: Tue Oct The value of 6.7 eV I have got as a difference between the energy of two free 17 04:08PM Location: Karlsruhe, Germany posts 16 2006, hydrogen atoms and the energy of a molecule. You are right, it should be called formation energy, not dissociation energy. But then dissociation energy defined as the energy necessary to separate H2 molecule should be higher as the height of the activation barrier should be added to the formation energy. However it is lower (4.5 eV). Something is still wrong with my understanding. Could you also explain what is 1.1188 eV? Back to top radny Wed Oct 17 2007, 07:54AM There are two ways of doing the calculations. Registered Member #230 Joined: Wed Jun 15 2005, 01:34AM posts 2 1. Calculate the energies of H2 and H+H in the same box seperately and then calculate the B. energy as E(H2) - E(H+H), or 2. Calculate the energies of H2 and H in the same box seperately and calculate the energy from E(H2) - 2E(H). For large systems these two ways of calculating the b. energetics are not the same. Back to top Franky Wed Oct 17 2007, 01:08PM Dear Pavel, Registered Member #629 please switch on spinpolarization for the isolated atom using: Joined: Mon Apr ISPIN = 2 10 03:05PM posts 33 2006, If you say you did so and the binding energy didnt change, something is very wrong ! The total energy of the H-Atom changes by 1 eV after turning on spinpol., so the binding energy changes by 2 eV ! { E_total(H2) - 2*E_total(H; Spin) = E_binding ~ - 4.6eV} @radny: I would be very worried if the calculation using on H-atom per cell results in a different total energy than 0.5*E_total(H+H) using a cell that is twice as big as the first on containing two seperated H-atoms! If you use the same setup as for one H-atom per cell, you might want to think about changing the occupation numbers manually for the 2 H-atoms. Otherwise, you are not calculation the total energy of two individual H-atoms! Back to top yzt102 Wed Nov 21 2007, 12:41AM I calculate the energy for a sigle H and it comes out to about -0.9eV. How did you guys got 1.1188eV, Here is my INCAR Registered Member #1937 Joined: Wed Nov SYSTEM = Hydrogen absoption 21 12:17AM posts 3 2007, LWAVE = .F. LCHARG = .FALSE. ISMEAR = 0 SIGMA = 0.1 IALGO = 48 LPLANE=.TRUE.