When we build a pseudopotential (psp), we froze the inner core states and treat only the valence electrons due to the fact that the properties of materials are largely determined by the outer-layer valence electrons. In order to obtain a psp with good transferability, there should be very small overlap between core and valence electron distributions which is true for most light elements. For most the elements, the overlap between core and valence electron can be so large that the psp built in the usual way cannot lead to the correct properties of materials composed of that element (e.g., Na, K). There are two methods to solve the above problem. 1) Nonlinear core correction, we use an artificial core charge distribution to replace the real core charge distribution. When building psp, we substract the contribution to Vxc and VHartree, which comes from the artificial core, and when using the psp, add the corresponding ones (see PRB 26, 1378). When doing nonlinear core correction, we don't increase the number of valence electrons but only increase the cutoff energy for plane-wave calculations due to the added core. 2) Semicore states, which means including some states that you usually treat as core states when you build a pseudopotential. For example, when building a psp for Ca, we not only include 4s, 4p, 3d but also include 3s, 3p in the valence states. For this method, we include a lot more electrons (e.g. 8 more e/atom for Ca), and also need a much large cutoff energy because we have included much more localized 3s, 3p electrons. So including semicore states will result in much harder calculations.
In fact, for most cases, nonlinear core correction is already enough for a good psp and including semicore states is kind of overkill. So my suggestion is: use the nonlinear core correction when you need it, use semicore states unless you have to. Hui ----- Original Message ----- From: daijianhong001 To: SIESTA-L@listserv.uam.es Sent: Monday, July 13, 2009 2:42 AM Subject: [SIESTA-L] semicore state meaning Dear all, Some pseudopotential includes semicore state.I wonder what is the meaning of semicore state and which element includes semicore state? Best wishes! Dai ------------------------------------------------------------------------------ 200万种商品,最低价格,疯狂诱惑你
