Hi Michael, "Channel" refers to angular momentum channel (I can't quite remember why it's called so, but you'll see it sometimes). So (n=2, l=0) is the 2s orbital or 2s "channel"; (n=2, l=1) is the 2p "channel" and so on.
So actually, when you refer to basis sets, you should specify them, strictly, for each channel (angular momentum l). Therefore, in your example, in an article, you should say "2s orbitals were represented with a triple-zeta (TZ) basis set, and 2p orbitals were represented with a triple-zeta basis set plus one polarization function (TZP)", or something equivalent. However, I suppose Siesta experts will immediately assume that, if you say "we used a DZP basis set for Carbon", it means "we used the standard basis set that Siesta creates when you just write PAO.BasisSize DZP in the fdf file". Returning to the example you gave, I suppose then that if you wanted to describe your basis set for Carbon as a "Siesta TZP", the number of zetas for each angular momentum channel should be just those you would get if you wrote PAO.BasisSize TZP. If ths is the case, it's possible that you are just being a little too descriptive in your fdf file, writing the basis set explicitly. Please bear in mind that this is just my opinion, not a truth written in stone ;) Cheers, Marcos On Thu, Feb 10, 2011 at 12:41 AM, Michael R. C. Williams < [email protected]> wrote: > Hi everyone, > > I have a (hopefully) quick question about polarization functions. The topic > has been well covered in this list (as well as in the tutorials), but I > wasn't able to find the answer to a minor point. One can add polarization > functions as well as adjust the number of zeta components for said > polarization functions through the PAO.Basis block. (A good description of > this that I found on the archives is pasted below). In the explanation > below, another zeta is added to the 2p orbitals of carbon. My question is > this: if I, for example, wanted to make a "TZDP" basis set for carbon, do I > have to change the polarization setting on the line defining the 's' > orbitals, or just the zeta? Basically, I want to make sure that when I am > making basis sets and referring to them with a name like 'DZDP', 'TZP', > etc., that they correspond to the types of basis sets used in publications. > I am mainly confused by the use of the term 'channel' in referring to > orbital types (see below). If I had something like: > > C > n=2 0 3 > 0.000 0.000 0.000 > 1.000 1.000 1.000 > n=2 1 3 P 1 > 0.000 0.000 0.000 > 1.000 1.000 1.000 > > Is this describing a TZP basis set for carbon, or just a TZP basis for the > (n=2, l=1) 'channel'? > > > Thanks very much for your time, > > > Mike > > > -- > *Michael R. C. Williams* | | Ph.D. Candidate > Yale University | | Department of Chemistry > > > It's difficult to give you more specific advice without knowing exactly > > which is your atomic species, but here goes a short example for TZP, TZ2P, > > QZP and higher. > > > > If you specify your basis as DZP for Carbon (as an example) with 2s and 2p > > electrons, here is the output of the basis set siesta determines from a > > certain EnergyShift: > > > > %block PAO.Basis # Define Basis set > > C 2 # Species label, number of l-shells > > n=2 0 2 # n, l, Nzeta > > 4.994 3.563 > > 1.000 1.000 > > n=2 1 2 P 1 # n, l, Nzeta, Polarization, NzetaPol > > 6.254 3.889 > > 1.000 1.000 > > %endblock PAO.Basis > > > > Suppose now that you want to make TZP for the 2p channel. You should, > > then, modify the line saying " n=2 1 2 P 1" and the other two lines > > following it to: > > > > n=2 1 3 P 1 > > 0.000 0.000 0.000 > > 1.000 1.000 1.000 > > > > That means that you will have, for n=2, l=1 (the 2p channel), three zetas > > + 1 polarization function. The 0.000's appearing in the second line tell > > siesta to determine the rc's from the EnergyShift, whether user-specified > > or default. If you want to go to TZ2p (TZ + 2 polarization functions), you > > should write > > > > n=2 1 3 P 2 > > > > instead. The other channels can be changed in the same way (but sometimes > > siesta may complain about setting rc's with energy shift and then you will > > have to set them by hand). By doing so, you can not only increase the > > number of zetas, but include also unoccupied orbitals in the basis set, > > which may give you better results, but will render your calculation > > heavier. I have tested the effect of TZP in bulk moduli of diamond, and it > > is definitely better, but the calculation gets definitely heavier, as > > well. So, for large systems, this might be a computational effort not > > really worthy. > > > > >
