# Δ-SCF methodology

### This page gives hints on how to to perform a Δ-SCF calculation of neutral excitations with the ABINIT package.

##### Copyright (C) 2016-2017 ABINIT group (FJ)

Mentioned in help_features#7.

### Table of content:

- 1. Introduction.
- 2. References.

### 1. Introduction.

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.
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### 2. References.

[Jia2017] Yongchao Jia, Samuel Poncé, Anna Miglio, Masayoshi Mikami and Xavier Gonze, "Assessment of First-Principles and Semiempirical Methodologies for Absorption and Emission Energies of Ce3+-Doped Luminescent Materials", Adv. Optical Mater. 5, 1600997 (2017).

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