C

Chemical Hardness and Softness

Brayden Banks

Chemical Hardness and Softness is a measure of the HOMO-LUMO gap of atoms, molecules, and ions, providing a numerical measure to affinities that appeared to be based on polarizability. The operational definition of hardness is half of the energy difference between the ionization energy and electron affinity, which by Koopmann’s theorem provides a definition of half of the HOMO-LUMO gap.

Learn More →

Coherence

Gabriel Levine

That multiple states can overlap at once leads to the notion of superposition and entanglement. The maintenance of a phase relationship between such states over time is a coherence. Coherences can exist between quantum systems of the same type of object, such as two molecules, or for different kinds of objects, such as light-matter or electron-phonon interactions.

Learn More →

Complete Active Space (CAS) Methods

Serina Tressler

A complete active space (CAS) calculation is a type of multi-configurational self-consistent-field (MCSCF) method, designed to address the lack of treatment of electron correlation by Hartree-Fock (HF) theory. MCSCF methods create a linear combination of the possible electronic configurations for a molecule and perform a configuration interaction calculation to minimize its energy.

Learn More →

Configuration Interaction Singles (CIS)

Joshua Edwards

Configuration interaction is a method to approximate the non-relativistic Schrodinger equation for a multielectron system. Configuration interaction (CI) builds on the framework the Hartree-Fock method of using a wedge product or Slater determinant. The premise…

Learn More →

Continuum Solvent Model

Sarah Moe

A continuum solvent model provides a computational approach for approximating solute-solvent interractions. The aim of a continuum solvent model is to properly capture the electrostatic potential and charge distribution contained in a solute cavity that is surrounded by solvent.

Learn More →

Correlation Energy

Brian Wang

In quantum chemistry, the correlation energy quantifies the gap between the exact non-relativistic energy of a many-electron system and its estimation by the Hartree-Fock (HF) method. HF simplifies the electron interactions, assuming they move independently within an averaged electron field, which neglects the dynamic electron-electron repulsion or correlation.

Learn More →

Coupled Cluster Theory

Jaemin Kim

The coupled cluster (CC) theory is used to construct many-electron Schrödinger equations by considering electron correlations. What differentiates the CC theory from earlier computational methods to solve for the molecular Schrödinger equation is that the CC theory (and its truncated version) is size-extensive. The size-extensivity problem of truncated configuration interaction (FCI) methods can be linked to the linear parametrization of the CI wavefunction. The CC wavefunction, on the other hand, relies on an exponential parametrization.

Learn More →