Hard to say. Usually DFT calculations have an accuracy about 0.1eV for specific chemical configurations, BUT:
The quality of your results depends on how good is your basis-set, pseudo potentials and so on ... have you checked this parameters? I´ve seen some papers where people have just used the standard DZP basis and got completely spoiled results ... for exemplo, a guy found the ZnO cohesive energy of 14eV/ZnO pair. With a optimized basis I've got 7,5 ( experimental 7.6). And in the case all these simulation parameters are ok, your results still depend on how good is your final geometry. I've seen you have performed no geometry optimization, so there is almost no chance that your structure is a local minimum. Regards Ney On Fri, Jul 16, 2010 at 10:57 AM, tarang mungole <[email protected]>wrote: > I got the energy differences which are less than 10eV and I guess they are > corrent . But can you please tell me what is the uncertainity in the > energies that i get. This is the output that i got. > > siesta: Final energy (eV): > siesta: Kinetic = 148077.062865 > siesta: Hartree = 1387018.221902 > siesta: Ext. field = 0.000000 > siesta: Exch.-corr. = -20554.562630 > siesta: Ion-electron =-2900798.341991 > siesta: Ion-ion = 1266423.882434 > siesta: Ekinion = 0.000000 > siesta: Total = -119833.737421 > > siesta: Atomic forces (eV/Ang): > siesta: 1 2.254951 1.301912 0.921874 > siesta: 2 -0.073621 1.977399 1.398322 > . > . > . > siesta: 126 0.032933 -2.375644 -1.678582 > siesta: Tot 0.012066 0.008208 -0.123697 > > siesta: Stress tensor (static) (eV/Ang**3): > siesta: 0.002093 0.000205 0.000146 > siesta: 0.000205 0.001856 0.000084 > siesta: 0.000146 0.000084 0.001860 > > > > On Thu, Jul 15, 2010 at 2:00 AM, tarang mungole > <[email protected]>wrote: > >> Sorry for the way I asked the question.... >> >> Thanks for your reply....well I checked the geometry in Xcrysden and it >> showed the correct picture(if I replicate the picture in all directions) . >> Actually apart from this , I calculated the energies of the slabs extending >> upto infinity in 2 directions and restricted in the z direction by giving a >> big vacuum space in the unit cell(I guess this is the cluster you are >> talking about). I used Ni infinite slabs, then i used Ni infinite slabs with >> adatoms and calculated the enery differences and the differences were like >> 10KeV! >> Well in this case, I had put a big vacuum space in all the 3 >> directions and it says that virtual memory is not enough >> >> InitMesh: MESH = 500 x 500 x 500 = 125000000 >> InitMesh: Mesh cutoff (required, used) = 100.000 104.475 Ry >> forrtl: severe (41): insufficient virtual memory >> >> >> On Wed, Jul 14, 2010 at 1:10 PM, N H <[email protected]> wrote: >> >>> Did you checked the output geometry to see whether this is really the >>> geometry you want or not? >>> >>> Another question: Do you like to calculate a Slab or a cluster? It seems >>> that your calculation is set to a Cluster. >>> >>> It is also not clear which energy you would like to calculate. Remember >>> that absolute total energies calculated with SIESTA ( or any other method ) >>> have no meaning in itself - once the energy reference is arbitrary. You have >>> to compare it with the energy of another well-defined system >>> and establish energy differences. >>> >>> Just to give an idea, I calculate ZnO slabs with more or less 40 atoms ( >>> including some molecules adsorbed ) and the total energies are something >>> like -35KeV ... and it is normal. >>> >>> A last tip: The people here in this list use to be very friendly and >>> helpful. So there is no need to urging someone to solve the problem you >>> posted with "It's very urgent!!". Everybody here is open to help each other, >>> but everybody here also have urgent stuff to do besides answering the SIESTA >>> mail-L. If you want help, please ask it nicely. >>> >>> Best Regards >>> >>> >>> NH >>> >>> >>> >>> >>> On Wed, Jul 14, 2010 at 12:01 PM, Juan Beltran < >>> [email protected]> wrote: >>> >>>> I find that your system do not have that many C atoms, how does the >>>> energy compare with a pure Ni system?, still large?. Compare also the >>>> energy >>>> you obtain for that big supercell with the pure FCC Ni system (1 atom may >>>> do). The difference should be due to the presence of vacum along the three >>>> directions and I guess it should be less that 100eV from the 50KeV you >>>> obtain from the large system. >>>> >>>> Juan >>>> >>>> On 07/14/2010 12:45 AM, tarang mungole wrote: >>>> >>>>> I have made a slab with FCC configuration and I want to calculate the >>>>> energy of the slab!! The slab contains 5 layers of Nickel atoms and a >>>>> carbon >>>>> atom on the top of it. I just want to calculate the energy of this >>>>> system!!! >>>>> ....I made a supercell having 5X5X5 lattice constants in each direction ( >>>>> 5a >>>>> 5a 5a 60 60 60) and then gave a BIG vaccum space so that the nearby >>>>> Replicated supercells do not interact.(Is this a correct concept??????) . >>>>> The problem is that the energies that i get are very large!!!! about >>>>> -50KeV!! Can any body tell what is wrong in my fdf input file....it's very >>>>> urgent!! >>>>> >>>>> ---------------------------------------------------FDF >>>>> FILE---------------------------------------------------------------------------------------------------------------------------------------------- >>>>> >>>>> %block ChemicalSpeciesLabel >>>>> 1 28 Ni >>>>> 2 6 C >>>>> %endblock ChemicalSpeciesLabel >>>>> >>>>> SaveRho .true. >>>>> >>>>> LatticeConstant 1.0 Ang >>>>> >>>>> %block LatticeParameters >>>>> 99.6 99.6 99.6 60. 60. 60. >>>>> %endblock LatticeParameters >>>>> >>>>> %block AtomicCoordinatesOrigin >>>>> 0.000 0.000 0.000 >>>>> %endblock AtomicCoordinatesOrigin >>>>> >>>>> >>>>> >>>>> MD.TypeOfRun CG >>>>> MD.NumCGSteps 0 >>>>> >>>>> AtomicCoordinatesFormat Fractional >>>>> >>>>> %block AtomicCoordinatesAndAtomicSpecies >>>>> 0.000 0.000 0.000 1 >>>>> 0.025 0.000 0.000 1 >>>>> 0.050 0.000 0.000 1 >>>>> 0.075 0.000 0.000 1 >>>>> 0.000 0.025 0.000 1 >>>>> 0.025 0.025 0.000 1 >>>>> 0.050 0.025 0.000 1 >>>>> 0.075 0.025 0.000 1 >>>>> 0.000 0.050 0.000 1 >>>>> 0.025 0.050 0.000 1 >>>>> 0.050 0.050 0.000 1 >>>>> 0.075 0.050 0.000 1 >>>>> 0.000 0.075 0.000 1 >>>>> 0.025 0.075 0.000 1 >>>>> 0.050 0.075 0.000 1 >>>>> 0.075 0.075 0.000 1 >>>>> 0.000 0.000 0.025 1 >>>>> 0.025 0.000 0.025 1 >>>>> 0.050 0.000 0.025 1 >>>>> 0.075 0.000 0.025 1 >>>>> 0.000 0.025 0.025 1 >>>>> 0.025 0.025 0.025 1 >>>>> 0.050 0.025 0.025 1 >>>>> 0.075 0.025 0.025 1 >>>>> 0.000 0.050 0.025 1 >>>>> 0.025 0.050 0.025 1 >>>>> 0.050 0.050 0.025 1 >>>>> 0.075 0.050 0.025 1 >>>>> 0.000 0.075 0.025 1 >>>>> 0.025 0.075 0.025 1 >>>>> 0.050 0.075 0.025 1 >>>>> 0.075 0.075 0.025 1 >>>>> 0.000 0.000 0.050 1 >>>>> 0.025 0.000 0.050 1 >>>>> 0.050 0.000 0.050 1 >>>>> 0.075 0.000 0.050 1 >>>>> 0.000 0.025 0.050 1 >>>>> 0.025 0.025 0.050 1 >>>>> 0.050 0.025 0.050 1 >>>>> 0.075 0.025 0.050 1 >>>>> 0.000 0.050 0.050 1 >>>>> 0.025 0.050 0.050 1 >>>>> 0.050 0.050 0.050 1 >>>>> 0.075 0.050 0.050 1 >>>>> 0.000 0.075 0.050 1 >>>>> 0.025 0.075 0.050 1 >>>>> 0.050 0.075 0.050 1 >>>>> 0.075 0.075 0.050 1 >>>>> 0.000 0.000 0.075 1 >>>>> 0.025 0.000 0.075 1 >>>>> 0.050 0.000 0.075 1 >>>>> 0.075 0.000 0.075 1 >>>>> 0.000 0.025 0.075 1 >>>>> 0.025 0.025 0.075 1 >>>>> 0.050 0.025 0.075 1 >>>>> 0.075 0.025 0.075 1 >>>>> 0.000 0.050 0.075 1 >>>>> 0.025 0.050 0.075 1 >>>>> 0.050 0.050 0.075 1 >>>>> 0.075 0.050 0.075 1 >>>>> 0.000 0.075 0.075 1 >>>>> 0.025 0.075 0.075 1 >>>>> 0.050 0.075 0.075 1 >>>>> 0.075 0.075 0.075 1 >>>>> 0.000 0.000 0.100 1 >>>>> 0.025 0.000 0.100 1 >>>>> 0.050 0.000 0.100 1 >>>>> 0.075 0.000 0.100 1 >>>>> 0.000 0.025 0.100 1 >>>>> 0.025 0.025 0.100 1 >>>>> 0.050 0.025 0.100 1 >>>>> 0.075 0.025 0.100 1 >>>>> 0.000 0.050 0.100 1 >>>>> 0.025 0.050 0.100 1 >>>>> 0.050 0.050 0.100 1 >>>>> 0.075 0.050 0.100 1 >>>>> 0.000 0.075 0.100 1 >>>>> 0.025 0.075 0.100 1 >>>>> 0.050 0.075 0.100 1 >>>>> 0.075 0.075 0.100 1 >>>>> 0.033 0.033 0.125 2 >>>>> 0.100 0.000 0.000 1 >>>>> 0.100 0.025 0.000 1 >>>>> 0.100 0.050 0.000 1 >>>>> 0.100 0.075 0.000 1 >>>>> 0.100 0.100 0.000 1 >>>>> 0.100 0.000 0.025 1 >>>>> 0.100 0.025 0.025 1 >>>>> 0.100 0.050 0.025 1 >>>>> 0.100 0.075 0.025 1 >>>>> 0.100 0.100 0.025 1 >>>>> 0.100 0.000 0.050 1 >>>>> 0.100 0.025 0.050 1 >>>>> 0.100 0.050 0.050 1 >>>>> 0.100 0.075 0.050 1 >>>>> 0.100 0.100 0.050 1 >>>>> 0.100 0.000 0.075 1 >>>>> 0.100 0.025 0.075 1 >>>>> 0.100 0.050 0.075 1 >>>>> 0.100 0.075 0.075 1 >>>>> 0.100 0.100 0.075 1 >>>>> 0.100 0.000 0.100 1 >>>>> 0.100 0.025 0.100 1 >>>>> 0.100 0.050 0.100 1 >>>>> 0.100 0.075 0.100 1 >>>>> 0.100 0.100 0.100 1 >>>>> 0.000 0.100 0.000 1 >>>>> 0.025 0.100 0.000 1 >>>>> 0.050 0.100 0.000 1 >>>>> 0.075 0.100 0.000 1 >>>>> 0.000 0.100 0.025 1 >>>>> 0.025 0.100 0.025 1 >>>>> 0.050 0.100 0.025 1 >>>>> 0.075 0.100 0.025 1 >>>>> 0.000 0.100 0.050 1 >>>>> 0.025 0.100 0.050 1 >>>>> 0.050 0.100 0.050 1 >>>>> 0.075 0.100 0.050 1 >>>>> 0.000 0.100 0.075 1 >>>>> 0.025 0.100 0.075 1 >>>>> 0.050 0.100 0.075 1 >>>>> 0.075 0.100 0.075 1 >>>>> 0.000 0.100 0.100 1 >>>>> 0.025 0.100 0.100 1 >>>>> 0.050 0.100 0.100 1 >>>>> 0.075 0.100 0.100 1 >>>>> %endblock AtomicCoordinatesAndAtomicSpecies >>>>> >>>>> SolutionMethod diagon >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> Thnx in advance!! >>>>> >>>>> >>> >> >
