## Important general tips## [edit]Initial Input GeometryTo know how to input your crystal of choice into PWSCF, you'll need to know its atomic coordinates. Almost always, these can be found in a paper in the literature or by using software such as Vesta to apply spacegroup symmetry operations to a few known atomic positions (or you can do that by hand!) The important thing is that generally, you will have fractional atomic coordinates available, which are given in terms of the lattice vectors a, b, and c. ATOMIC_POSITIONS (crystal) (crystal) tells PWSCF to look for fractional coordinates. Otherwise, it defaults to Cartesian! Here is a very clear and helpful guide to inputting crystal geometries into PWSCF for calculations, from Stefano de Gironcoli: http://encina.northwestern.edu/510/tutorial_wyckoff.pdf ## [edit]PseudopotentialsFor all elements in your calculations, you will need to obtain pseudopotentials from quantum-espresso.org. Go there and click on "PSEUDO." You should see a periodic table that shows all the elements that have been parametrized for use with PWSCF. General tips: - For the most robust calculations, select PBE or PW91 pseudos (these are both GGA); use LDA only if you have no other choice
- Try to make the pseudos you select as similar as possible. That is, if you select one Vanderbilt potential ("van" in the name), try to get Vanderbilt potentials for your other elements. If one of your pseudos is norm-conserving, they all should be.
- You will, of course,
**not**be able to mix PBE with PW91 or with LDA, etc. You need to use the same style exchange-correlation in all your pseudos.
## [edit]Transition Metal Oxides and Rare-Earth CompoundsIf you are interested in modeling these materials, congratulations! Vanilla DFT is not enough for you, and will often predict insulators to be metals (check the band structure!) You will need to implement the DFT+U formalism, which adds a correction term to the Hamiltonian for cases in which electronic behavior is highly correlated. To enable DFT+U, add lines like below to &SYSTEM: lda_plus_u = .true., Hubbard_U(1) = X Hubbard_U(2) = Y ... where each Hubbard_U line corresponds to one element in your input. The values of X and Y are the U parameters in eV. Please come see me if you are modeling these materials; they are the focus of my research as well, so I have more tips I can share. ## [edit]MagnetismIf your calculation involves a magnetic element (Mn, Fe, Co, Ni...), you must explicitly enable spin polarization if you want magnetism to be reflected in your results! Here's a look at how to do that in your .in file under &SYSTEM: nspin = 2, starting_magnetization(i) = 0.3, nspin=2 turns on magnetism, but you need to give PWSCF a guess for the initial spins of magnetic elements. So, for each magnetic element i, you must enter a starting_magnetization(i) between -1 and 1. ## [edit]DiagonalizationYou have the choice between conjugate gradient (cg) and Davidson (david) algorithms in PWSCF. Each has advantages and disadvantages; typically, the choice won't matter, but I would default to Davidson. ## [edit]How to Know if You're RightThere are some basic things you should check in the output of any simulation. First of all, look at the energy. If it's positive, something is |