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RWIGS and LORBIT 是两个与局域态密度或投影态密度相关的参数；RWIGS指的是Wigner–Seitz radius，LORBIT前面的LO指的就是Local。
P. Strange, A. Svane, W. M. Temmerman, Z. Szotek & H. Winter，Understanding the valency of rare earths from first-principles theory，Nature 399, 756-758 (24 June 1999) | doi:10.1038/21595。 The rare-earth metals have high magnetic moments and a diverse range of magnetic structures1. Their magnetic properties are determined by the occupancy of the strongly localized 4f electronic shells, while the outer s–d electrons determine the bonding and other electronic properties2. Most of the rare-earth atoms are divalent, but generally become trivalent in the metallic state. In some materials, the energy difference between these valence states is small and, by changing some external parameter (such as pressure), a transition from one to the other occurs. But the mechanism underlying this transition and the reason for the differing valence states are not well understood. Here we report first-principles electronic-structure calculations that enable us to determine both the valency and the lattice size as a function of atomic number, and hence understand the valence transitions. We find that there are two types of f electrons: localized core-like f electrons that determine the valency, and delocalized band-like f electrons that are formed through hybridization with the s–d bands and which participate in bonding. The latter are found only in the trivalent systems; if their number exceeds a certain threshold, it becomes energetically favourable for these electrons to localize, causing a transition to a divalent ground state.
RWIGS = 2.330; RWIGS = 1.233 wigner-seitz radius (au A)
For mono-atomic system RWIGS can be defined unambiguously. The sum of the volume of the spheres around each atom should be the same as the total volume of the cell (assuming that you do not have a vacuum region within your cell). This is in the spirit of atomic sphere calculations. VASP writes a line Volume of Typ 1: 98.5 % to the OUTCAR file. You should use a RWIGS value which yields a volume of approximately 100%。
网上有个比较好的转帖讲述了RWIGS的取法，原帖出自于ChinaUnix的作者 beyondstar[4] ：
For binary systems there is no unambiguous way to define RWIGS and several choices are possible. In all cases, the sum of the volume of the spheres should be close to the total volume of the cell (i.e the sum of the values given by VASP should be around 100%。 ① One possible choice is to set RWIGS such that the overlap between the spheres is minimized. ② However in most cases, it is simpler to choose the radius of each sphere such that they are close to the covalent radius as tabulated in most periodic tables. This simple criterion can be used in most cases, and it relies at least on some “physical intuition”.
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原文地址：http://www.cnblogs.com/panscience/p/4694296.html
