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I'm using Critic2 software to perform post-processing of the charge density of metallic alloys. With the charge density obtained from the Self-Consistent-Field calculation (for that I'm using Quantum ESPRESSO), Critic uses both the Yu and Trinkle and Bader methods to perform the charge analysis.

The results are the attractors (namely the atoms) within the structure, the atomic volumes, the all-electron density integrated into its basis (POP), the Laplacian of the all-electron basis (LAP), and the pseudo-density (since we use pseudo-potentials) ($2).

Bellow an example of its output using the Yu and Trinkle method:

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My question is: how to analyze such results? Or in a more general fashion, how to analyze charge results? All we got are numbers, what are their meanings?

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  • $\begingroup$ Hi @Anibal, could you please write the detailed steps for the critic2 charge analysis (for interstitial charge determination)? Thanks in advance. $\endgroup$
    – Sak
    Dec 9, 2022 at 4:21

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I would make the argument that for a single, fixed configuration of some system, charge analysis doesn't tell you much. Since different methods for computing charges can vary widely in the value they produce and atomic charge isn't an observable, the absolute value of charge/population on an atom isn't necessarily meaningful.

However, like many properties in computational chemistry, useful information can be derived from differences in these populations between related structures. This could mean, for example, looking at the population for different conformers of a molecule, how the population varies over the course of a reaction, or a family of molecules with varying substitutions/dopants.

Taking the reaction example, you might be interested in figuring out why, intuitively, a given reaction occurs, beyond just determining that the products are more stable than the reactants. By monitoring the charge of key atoms over the reaction pathway, you may be able to determine a reason why the reaction proceeds by that mechanism (e.g. the reaction may reduce the charge on a metal center).

So I would say there is quite a lot you can do with charges, but it is very context dependent what information you can or want to derive from them.

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