NWChem Frequently Asked Questions

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# NWChem FAQ ## SCF/DFT How do I reproduce the XC numerical grid used in G9X? Which Columb fitting basis set should I use? Are the DFT unoccupied orbital energies shifted? I got the error message: "`ao_replicated: insufficient memory XXXXXXXX`"; what can I do? How can I do a X3LYP calculation in NWChem? How do you address Bq's to get an integration grid? How do I reproduce the XC numerical grid used in G9X? G9X and NWChem default grids are different. To get a grid close to the default G9X grid you need the following input lines.
    dft
     grid ssf euler lebedev 75 11
    end
Which Coulomb fitting basis set should I use? We strongly recommend use of the Ahlrichs Auxilliary basis sets for fitting the Coulomb potentatial in DFT calculations. The following is an example of an in input file using this basis set.
    basis "cd basis"
     C library "Ahlrichs Coulomb Fitting"
    end
For analysis of the accuracy of this basis set, see Skylaris, C.-K.; Gagliardi, L.; Handy, N.C.; Ioannou, A.G.; Spencer, S.; Willetts, A., Journal of Molecular Structure: THEOCHEM 501-502, 229 (2000). Are the DFT unoccupied orbital energies shifted? Yes, in all versions of NWChem including 4.0 and below, the DFT unoccupied orbital energies are shifted by the amount of level-shifting used to converge the wavefunction. If you use these energies for any reason, you need to subtract out the level-shifting value. Check out the DFT Convergence section of the User Manual for more information about level-shifting. I got the error message: "`ao_replicated: insufficient memory XXXXXXXX`"; what can I do? You can fix this problem by having the code adopting the distributed data Fock build (that is less memory demanding compared to the default replicated data build). This is accomplished by adding the following line anywhere in you input file
     set fock:replicated logical .false.
How can I do an X3LYP calculation in NWChem? The X3LYP functional is not available as a keyword. However, using section 11.3 (specifically 11.3.1 and 11.3.2) of the NWChem user manual, and applying the parameters of the X3LYP paper on http://www.pnas.org/cgi/content/full/101/9/2673 by Xu and Goddard you can define this exchange-correlation functional using the following line in the DFT input block:
    XC vwn_1_rpa 0.129 lyp 0.871 Hfexch 0.218 slater 0.782 becke88 nonlocal 0.542 xperdew91 nonlocal 0.167
How do you address Bq's to get an integration grid? Particular care is required to compute BSSE by the counter-poise method for the DFT module. In order to include terms deriving from the numerical grid used in the XC integration, the user must label the ghost atoms not just bq, but bq followed by the given atomic symbol. For example, the first component needed to compute the BSSE for the water dimer, should be written as follows:
    geometry h2o autosym units au
     O        0.00000000     0.00000000     0.22143139
     H        1.43042868     0.00000000    -0.88572555
     H       -1.43042868     0.00000000    -0.88572555
     bqH      0.71521434     0.00000000    -0.33214708
     bqH     -0.71521434     0.00000000    -0.33214708
     bqO      0.00000000     0.00000000    -0.88572555
    end

    basis
     H library aug-cc-pvdz
     O library aug-cc-pvdz
     bqH library H aug-cc-pvdz
     bqO library O aug-cc-pvdz
    end
Please note that the ``ghost'' oxygen atom has been labeled bqO, and not just bq. NWChem | Capabilities | Platforms | Download | User's Manual | Programmer's Manual | Release Notes | FAQ Known Bugs | Support | Tutorial | Contributors | Benchmarks | Search | Mol Sci. Soft. Group | Citation Contact: NWChem Support Updated: February 22, 2005