APOST-3D: Chemical concepts from wavefunction analysis.

Open-source APOST-3D software features a large number of wavefunction analysis tools developed over the past 20 years, aiming at connecting classical chemical concepts with the electronic structure of molecules. APOST-3D relies on the identification of the atom in the molecule (AIM), and several analysis tools are implemented in the most general way so that they can be used in combination with any chosen AIM. Several Hilbert-space and real-space (fuzzy atom) AIM definitions are implemented.

Spurious Oscillations Caused by Density Functional Approximations: Who is to Blame? Exchange or Correlation?

We analyze the varying susceptibilities of different density functional approximations (DFAs) to present spurious oscillations on the profiles of several vibrational properties. Among other problems, these spurious oscillations cause significant errors in harmonic and anharmonic IR and Raman frequencies and intensities. This work hinges on a judicious strategy to dissect the exchange and correlation components of DFAs and pinpoint the origins of these oscillations.

All-Purpose Measure of Electron Correlation for Multireference Diagnostics.

We present an analytical relationship between two natural orbital occupancy-based indices, and , and two established electron correlation metrics: the leading term of a configuration interaction expansion, , and the diagnostic. Numerical validation revealed that and can effectively substitute for and , respectively. These indices offer three distinct advantages: (i) they are universally applicable across all electronic structure methods, (ii) their interpretation is more intuitive, and (iii) they can be readily incorporated into the development of hybrid electronic structure methods.

Reply to the Correspondence on "How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring".

A recent article by Anderson and co-workers challenges our conclusions on the aromaticity of the four oxidation states of a butadyine-linked six-porphyrin nanoring, based on the experimental H-NMR data and some recent calculations they have performed using the BLYP35 functional. Here, we show that BLYP35 should be taken with caution and demonstrate that the indirect evidence of a ring current from experimental H-NMR data is not a definite proof of aromaticity.

How Reliable Are Modern Density Functional Approximations to Simulate Vibrational Spectroscopies?

We show that properties of molecules with low-frequency modes calculated with density functional approximations (DFAs) suffer from spurious oscillations along the nuclear displacement coordinate due to numerical integration errors. Occasionally, the problem can be alleviated using extensive integration grids that compromise the favorable cost-accuracy ratio of DFAs. Since spurious oscillations are difficult to predict or identify, DFAs are exposed to severe performance errors in IR and Raman intensities and frequencies or vibrational contributions to any molecular property.