Psi4 is an ab initio electronic structure program providing methods such as HartreeFock, density functional theory, configuration interaction, and coupled-cluster theory. The 1.1 release represents a major update meant to automate complex tasks, such as geometry optimization using complete-basis-set extrapolation or focal-point methods.
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Chemists recurrently utilize "fuzzy" chemical concepts (e.g. atomic charges, the chemical bond, strain, aromaticity, branching, etc.), which lack unique quantitative assessments but, nonetheless, are frequently employed as tools for understanding the intricacies of chemical behaviour. This tutorial review provides an overview of the computational schemes specifically developed to quantify four of the most commonly employed, yet debated, chemical concepts: the chemical bond, atomic charges, (hyper)conjugation, and molecular strain. The enhanced knowledge gained from these schemes not only helps in the depiction of molecules with unique properties, but also provides breadth to our fundamental understanding of chemistry. Nevertheless, the numerous existing methodologies often result in different interpretations that culminate in discrepancies. Through recent examples in the literature, guidelines are provided which illustrate the strengths and weaknesses of various schemes for each individual concept.
We develop a simple methodology for the computation of symmetry-adapted perturbation theory (SAPT) interaction energy contributions for intramolecular noncovalent interactions. In this approach, the local occupied orbitals of the total Hartree-Fock (HF) wavefunction are used to partition the fully interacting system into three chemically identifiable units: the noncovalent fragments A and B and a covalent linker C. Once these units are identified, the noninteracting HF wavefunctions of the fragments A and B are separately optimized while embedded in the HF wavefunction of C, providing the dressed zeroth order wavefunctions for A and B in the presence of C. Standard two-body SAPT (particularly SAPT0) is then applied between the relaxed wavefunctions for A and B. This intramolecular SAPT procedure is found to be remarkably straightforward and efficient, as evidenced by example applications ranging from diols to hexaphenyl-ethane derivatives.
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