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<TITLE>NWChem Frequently Asked Questions</TITLE>
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<td align="left" width="325"><img src="../../images/nwchem_logo_dark.gif" border=0 width=200 alt="NWChem - computational chemistry on parallel computers"></td>
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<td width="300" align="right"><a href="gapss.html">Next FAQ Catagory</a> | <a href="NWChem_FAQ.html">Return to Main FAQ</a></td>
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<br><br>
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<h1>NWChem FAQ</h1>
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<p><h2>SCF/DFT</h2>
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<!--DO NOT REMOVE Begin Question and End Question Comments.-->
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<ul>
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<!--Begin Question-->
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<li><a href="#0">How do I reproduce the XC numerical grid used in G9X? </a></li>
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<li><a href="#1">Which Columb fitting basis set should I use?</a></li>
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<li><a href="#2">Are the DFT unoccupied orbital energies shifted?</a></li>
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<li><a href="#3">I got the error message: "<tt>ao_replicated: insufficient memory XXXXXXXX</tt>"; what can I do?</a></li>
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<li><a href="#4">How can I do a X3LYP calculation in NWChem?</a></li>
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<li><a href="#5">How do you address Bq's to get an integration grid?</a></li>
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<!--End Question-->
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</ul>
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<hr>
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<p>
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<a name="0"></a>
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<font color="purple">How do I reproduce the XC numerical grid used in G9X?
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</font>
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<p>
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G9X and NWChem default grids are different. To get a grid close to the default
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G9X grid you need the following input lines.
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<pre>
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dft
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grid ssf euler lebedev 75 11
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end
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</pre>
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</p>
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<hr>
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<p>
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<a name="1"></a>
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<font color="purple">Which Coulomb fitting basis set should I use?
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</font>
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<p>
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We strongly recommend use of the Ahlrichs Auxilliary basis sets for fitting
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the Coulomb potentatial in DFT calculations. The following is an example
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of an in input file using this basis set.
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<pre>
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basis "cd basis"
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C library "Ahlrichs Coulomb Fitting"
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end
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</pre>
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For analysis of the accuracy of this basis set, see <br>
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Skylaris, C.-K.; Gagliardi, L.; Handy, N.C.; Ioannou, A.G.; Spencer, S.; Willetts, A.,
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Journal of Molecular Structure: THEOCHEM <b>501-502</b>, 229 (2000).
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</p>
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<hr>
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<p>
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<a name="2"></a>
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<font color="purple"> Are the DFT unoccupied orbital energies shifted?</font>
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<p>
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Yes, in all versions of NWChem including 4.0 and below, the DFT unoccupied
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orbital energies are shifted by the amount of level-shifting used to
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converge the wavefunction. If you use these energies for any reason, you
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need to subtract out the level-shifting value. Check out the
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<a href="http://www.emsl.pnl.gov/docs/nwchem/doc/user/node13.html#SECTION001350000000000000000">DFT Convergence</a> section of the User Manual for more
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information about level-shifting.
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</p>
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<hr>
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<p>
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<a name="3"></a>
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<font color="purple">I got the error message: "<tt>ao_replicated: insufficient memory XXXXXXXX</tt>"; what can I do?</font>
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<p>
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<!There are two possible ways to fix this problem>
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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
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data build). This is accomplished by adding the following line
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anywhere in you input file
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<pre>
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set fock:replicated logical .false.
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</pre>
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<! 2) try to increase the memory allocation to make the replicated data
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algorithm go. This can be done by introducing the memory line
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as described in the Users Manual>
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<! Option 1) is safer, but the replicated data algorithm (option 2) is
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faster.>
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</p>
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<hr>
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<p>
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<a name="4"></a>
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<font color="purple">How can I do an X3LYP calculation in NWChem?</font>
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<p>The X3LYP functional is not available as a keyword. However, using section 11.3 (specifically 11.3.1 and 11.3.2)
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of the NWChem user manual, and applying the parameters of the X3LYP paper on
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<a href="http://www.pnas.org/cgi/content/full/101/9/2673" target="_blank">
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http://www.pnas.org/cgi/content/full/101/9/2673</a> by Xu and Goddard you can define this exchange-correlation
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functional using the following line in the DFT input block:</p>
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<pre>XC vwn_1_rpa 0.129 lyp 0.871 Hfexch 0.218 slater 0.782 becke88 nonlocal 0.542 xperdew91 nonlocal 0.167</pre>
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<hr>
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<p>
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<a name="5"></a>
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<font color="purple">How do you address Bq's to get an integration grid?</font>
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<p>Particular care is required to compute BSSE by the counter-poise method for the DFT module.
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In order to include terms deriving from the numerical grid used in the XC integration, the user must label the
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ghost atoms not just bq, but bq followed by the given atomic symbol. For example, the first component
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needed to compute the BSSE for the water dimer, should be written as follows:</p>
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<pre>
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geometry h2o autosym units au
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O 0.00000000 0.00000000 0.22143139
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H 1.43042868 0.00000000 -0.88572555
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H -1.43042868 0.00000000 -0.88572555
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bqH 0.71521434 0.00000000 -0.33214708
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bqH -0.71521434 0.00000000 -0.33214708
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bqO 0.00000000 0.00000000 -0.88572555
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end
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basis
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H library aug-cc-pvdz
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O library aug-cc-pvdz
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bqH library H aug-cc-pvdz
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bqO library O aug-cc-pvdz
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end
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</pre>
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<p>Please note that the ``ghost'' oxygen atom has been labeled bqO, and not just bq.</p>
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<p>
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</font>
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<hr>Contact: <a href="mailto:ms3distribution@emsl.pnl.gov">NWChem Support</a><br>
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Updated: February 22, 2005
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