# CrossingBarriers

This page gives hints on how to calculate barriers for crossings with the ABINIT package.

## Introduction¶

The knowledge of geometries at which crossings between two electronic states happen, with minimal energy, or geometries at which the energy difference between the ground state and the excited state is small, and the energy is still low, plays an important role in the study of non-radiative transitions.

It is possible to formulate the search for such geometries in terms of minimisation of a functional that is the linear combination of the energy of the two states at the same geometry, with Lagrange multipliers (Ref. to be given). This is also related with a simple approach to Ensemble DFT: just make a linear combination of the DFT energies, the XC correlation energy being not computed with a single common density, but from each density separately.

In ABINIT, with imgmov==6, it is possible to deal with such linear combination of systems with the same geometry, but differing occupation factors occ. It is possible to find the geometry at which the resulting energy is minimal, for a given value of the mixing factors mixesimgf. Set nimage=2, and set the occupation numbers for image 1 to the ground-state occupations, and for image 2 to the excited-state occupations.

useful:

## Selected Input Files¶

No input file associated to this topic.
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