Re: [Ifeffit] Recommendations for DFT/computational chemistry software?

Hi Matthew, You are absolutely correct in that the old adage still applies, "garbage in equals garbage out". There is no substitute for knowing what you are doing. On the other hand, the great increases in computer power have made what was a nearly intractable problem much more manageable and it is a distinct advantage to be able to use ab-initio software to get insight into the chemistry. Of course, the scaling problem with plane-wave DFT is still there (memory = cube of the number of atoms), but even on a Mac Pro with 32 GB of memory you can simulate the ground state of hundreds of atoms in reasonable times whereas ten years ago, you would have had to use a supercomputer for the same problem. The codes themselves are much easier to use and their associated mailing lists make it a great way to learn the details of a given calculation. It certainly beats writing your own code from scratch (which I did back in 1990 when I was at the Univ. of Illinois on the NCSA machines)! On the other hand, I realize that I should have recommended a couple of books for background reading. A relatively easy read on solid state physics including some details of DFT Efthimios Kaxiras: "Atomic and Electronic Structure of Solids" My favourite book is a more comprehensive work. Richard M. Martin: "Electronic Structure: Basic Theory and Practical Methods" I should also add that abinit (http://www.abinit.org) is a free (as in gnu copyleft) code that has a set of very nice tutorials online starting with bonding two hydrogen atoms with references to appropriate papers to read for deeper information. To summarize, my point is that the hardware/software barriers to using DFT codes have gotten much smaller over the years and then investing the time in learning them can potentially offer greater insight into hard to understand problems. It is certainly useful for EXAFS in that one can obtain the relaxed structures in a 0 K calculation to compare for instance different possible XANES structures while at the same time potentially offering calculated material properties (optical reflectivity, Raman, etc.) that can offer additional results that can be compared to experiment. Certainly the ASE environment allows one to program a series of calculations using python and helps shorten the time necessary for writing input files and keeping track of the voluminous output. I give all of my postdocs extensive training in using DFT to get insight into material science problems. While it is not always useful, it can provide useful information in many cases. Paul On Aug 17, 2013, at 1:45 AM, Matthew Marcus wrote:

Please correct me if I'm wrong, but I get the impression that if you don't know exactly what you're doing, these programs will cheerfully return wrong answers. There seem to be many parameters and choices to be made. I once looked at Quantum Espresso but gave up when I saw that the script for doing MD on a single water molecule ran for over a page of incomprehensible code. I didn't see anything that looked like a step-by-step tutorial or manual. Gaussian with the Gaussview UI is simple enough for an experimentalist like me to use; are there better packages out there which are as well?

I also get the impression that you need some pretty hefty compute power. A Linux system is probably to be preferred over Windows.

One of these days I should learn Python, not just for this stuff. mam

On 8/16/2013 7:27 AM, Paul Fons wrote:

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Hi Scott, I have been dabbling in DFT for a while. There are many free packages around, but if you would like to model XAFS as well, I would suggest an all electron code for accuracy (such as Wien2K). For general purposes, I am also using VASP and CASTEP. The former uses projector augmented waves and the other ultrasoft pseudopotentials. VASP is fast and scalable to the largest machines and is designed from the ground up for quantum molecular dynamics. Both VASP and CASTEP use pseudopotentials whereas Wien2K uses a linearized augmented plane wave basis (read as radial wavefunctions and Ylms in a sphere about each atom and plane waves between spheres with boundary condition matching at the surface of the spheres. This way it is possible to model even the 1s electrons for heavier atoms and yes it does affect valence electron wavefunctions via orthogonality. All of these approaches are single particle DFT approaches, but it should be good enough for a start, if you want to go beyond these, with solutions of the Bethe-Saltpeter equation (electron and a hole) things get complicated and expensive (in terms of computer time) very quickly.

If you are interested in free software I would suggest gpaw, quantum espresso, and abinit. I would also suggest learning the atomic simulation environment in which you can program multiple codes in python (and even solve for maximally localized Wannier functions in a few lines of code!) (Atomic Simulation Environment — ASE 3.8.0.3329 documentation https://www.google.co.jp/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CC8QFjAA&url=https://wiki.fysik.dtu.dk/ase/&ei=WDYOUs2vBsaHkwX3h4H4Cg&usg=AFQjCNGJtTBmzvPVE0wgaKpLivT6K59Whw&bvm=bv.50768961,d.dGI).

What exactly are you trying to do? If you are looking for a non-spherical approximation for EXAFS beyond feff8's spheres, you might try FNDMES by Joly as well.

Cheers, Paul

On Aug 12, 2013, at 10:30 PM, Scott Calvin mailto:scalvin@sarahlawrence.edu> wrote:

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Hi all,

I know many of you use DFT calculations to help model EXAFS data for molecular compounds. Do you have recommendations for good computational chemistry packages, commercial or otherwise, to use for that purpose?

--Scott Calvin Sarah Lawrence College _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov mailto:Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

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