This page gives hints on how to to perform a Δ-SCF calculation of neutral excitations with the ABINIT package.
Although formally not justified, difference in total energy using constrained occupation numbers sometimes results in surprisingly good agreement with experimental results, for neutral excitations, in molecules or doped solids. See e.g. [Jia2017].
The manual specification of the occupation numbers is needed in this case. This is accomplished with the input variable occ, coupled with occopt=0 for the homogeneous occupation of a band throughout the Brillouin Zone, or with occopt=2 for the specific occupation of a state for a selected k-wavevector. For very big cells, both should be equivalent, and occopt=0 is easier to use.
The “linear combination of images” algorithm allows one to work with a set of images of the same geometry, but with different electronic states, and even to optimize this geometry, according to the linear combination of forces, see imgmov=6 and mixesimgf.
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