In response to the following questions I received today:
(1), For transition metal compound, how can we choose active space? There is a double shell effect for 3d transition metal, this concept has no physical meaning.
(2), I am also confused of the principle of CASSCF. Now I want to write the CASSCF code, then I can understand it better, but I do not know how to do.
Q1. It is not entirely obvious which orbitals should be in the active space. When you know the exact solution, in order to perform meaningful perturbation theory, it is often recommended to include all natural orbitals with occupation between 0.02 and 1.98. Of course, you cannot solve the entire problem. So there are three papers which I can tell you about:
http://dx.doi.org/10.1063/1.4885815 In this paper a large window around the Fermi level is solved very approximately with DMRG to yield approximate natural orbitals and NO occupation numbers.
It turns out that the second d-shell for transition metals also has significant partial occupation (far away from NOON 0 or 2), so it should be included in the CAS.
In response to the following questions I received today:
(1), For transition metal compound, how can we choose active space? There is a double shell effect for 3d transition metal, this concept has no physical meaning.
(2), I am also confused of the principle of CASSCF. Now I want to write the CASSCF code, then I can understand it better, but I do not know how to do.
Q1. It is not entirely obvious which orbitals should be in the active space. When you know the exact solution, in order to perform meaningful perturbation theory, it is often recommended to include all natural orbitals with occupation between 0.02 and 1.98. Of course, you cannot solve the entire problem. So there are three papers which I can tell you about:
It turns out that the second d-shell for transition metals also has significant partial occupation (far away from NOON 0 or 2), so it should be included in the CAS.
Q2. I implemented the following paper: http://scitation.aip.org/content/aip/journal/jcp/74/4/10.1063/1.441359