__ Listed Alphabetically__

**AIM** – (Atoms In Molecule) An analysis method based upon the shape of the total electron density; used to define bonds, atoms, etc. Atomic charges computed using this theory are probably the most justifiable theoretically, but are often quite different from those from older analyses, such as Mulliken populations.

**AO** – (Atomic Orbital) An orbital described by wavefunction for a single electron centered on a single atom.

**AOM** – (Angular Overlap Model)

**AM1** – (Austin Model 1) Dewar's NDDO semiempirical parameterization.

**ASE** – (Aromatic Stabilization Energy)

**BAC-MP4** – (Bond Additive Corrections to energies from Møller-Plesset 4th order perturbation theory) empirical corrections to calculated energy based (exponentially) on bond lengths.

**Bader's analysis** – see AIM

**Basis Set** – finite set of functions used to approximately express the Molecular Orbital wavefunction(s) of system, normally atom centered, consisting of AOs differing in local angular momentum for each atom.

**BCUT** – (Burden CAS University of Texas) topological molecular similarity index of Burden.

**BDE** – (Bond Dissociation Energy)

**BSSE** – (Basis Set Superposition Error) error introduced when the energy of two molecules modeled together is lower than the sum of the energies when modeled separately, because more basis sets are available for each fragment (more degrees of freedom) during the calculation. Minimal basis sets can have less BSSE because only diffuse functions can span a to b. A particular problem for binding energies of weakly bonded molecular complexes, less with more complete basis sets.

**BLYP** – (Becke-Lee-Yang-Parr)

**bohr** - One atomic unit of distance, equal to 0.5292 Å.

**CADPAC** – (Cambridge Analytical Derivatives PACkage)

**CAS** – (Complete Active Space)

**CASPT** – (Complete Active Space Perturbation Theory)

**CASSCF** – (Complete Active Space Self-Consistent Field) popular variant of the MCSCF method, using a specific choice of configurations. One selects a set of active orbitals and active electrons, then forms all of the configurations possible by placing the active electrons in the active orbitals, consistent with the proper spin and space symmetry requirements. Essentially equivalent to the FORS method.

**CBS-QCI** – (Complete Basis Set Quadratic Configuration Interaction) alternative extrapolation algorithm to complete basis set.

**CC** – (Coupled Cluster) A perturbation theory of electron correlation with an excited configuration that is "coupled" to the reference configuration. Complete to infinite order, but only for a subset of possible excitations (doubles, for CCD). Newer than CI.

**CCD** – (Coupled Cluster, Doubles only.) Complete to infinite order for doubles excitations.

**CCSD** – (Coupled Cluster, Singles and Doubles only.) Complete to ƒ order for singles & doubles excitations.

**CCSD(T)** – (Coupled Cluster, Singles and Doubles with Triples treated approximately.) Size consistent. ΔH_{diss} ± 1.0 kcal/mol

**CCSDT** – (Coupled Cluster, Singles, Doubles and Triples)

**CCSS** – (Correlated Capped Small Systems) Special case of IMOMO. See Truhlar's and Hass's work.

**CEPA** – (Coupled Electron Pair Approximation)

**CFF** – (Consistent Force Field) Class II force field based on ab initio data developed by Biosym Consortium/MSI.

**CFMM** – (Continuous Fast Multipole Method) linear scaling method for matrix formation for DFT.

**CFSE** – (Crystal Field Stabilization Energy)

**CHA** – (Chemical Hamiltonian Approach) excludes BSSE effects from wavefunction w/ BSSE free Hamiltonian.

**CHF** – (Coupled Hartree-Fock method)

**CHELP** – (CHarges from Electrostatic Potential) an attempt to obtain atomic charges by fitting the electrostatic potential to a set of atomic point charges. The key component to the CHELP method is that it is non-iterative, rather using a Lagrangian multiplier method.

**chemical shift tensor** – representation of the chemical shift. 3x3 matrix field felt in a direction induced by current due to applied field in b direction.

**CI** – (Configuration Interaction) The simplest variational approach to incorporate dynamic electron correlation. Combination of the Hartree-Fock configuration (Slater determinant) and a large set of other configurations is used as a many e^{-} basis set. The expansion coefficients are determined (in principal) by diagonalizing the Hamiltonian matrix and variationally minimizing the total energy of the CI wavefunction. Not size consistent.

**CID** – (Configuration Interaction, Doubles substitution only) post-Hartree-Fock method that involves configuration interaction where the included configurations are the Hartree-Fock configuration and all configurations derived from doubles substitution.

**CIDEP** – (Chemically Induced Dynamic Electron Polarization)

**CIDNP** – (Chemically Induced Dynamic Nuclear Polarization)

**CIS** – (Configuration Interaction with Singles excitations) simplest method for calculating electronically excited states; limited to singly-excited states. Contains no e^{-} correlation and has no effect on the ground state (Hartree-Fock) energy.

**CISD** – (Configuration Interaction, Singles and Doubles substitution only) post-Hartree-Fock method that involves configuration interaction where the included configurations are the Hartree-Fock configuration and all configuration derived from singles and doubles substitution. Comparable to MP2.

**CKFF** – (Cotton-Kraihanzel Force Field)

**CNDO** – (Complete Neglect of Differential Overlap) The simplest of the semi-empirical methods. The principle feature is the total neglect of overlap between different orbitals. In other words, the overlap matrix S is the unit matrix. The only two-electron integrals kept are those where electron 1 is in just one orbital and electron 2 is in just one orbital. Like all semiempirical methods the integrals are evaluated empirically.

**Combinatorial Chemistry** – testing a large number of related compounds (as a mixture?) to find a compound that is active.

**CoMFA** – (Comparative Molecular Field Analysis) technique used to establish 3-D similarity of molecular structure in relation to a target property.

**
COMPASS **– (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) Molecular Mechanics force field derived from Class II force field, but optimized for condensed phase properties.

Correspondence Principle

COSMO

Coulomb integral

CPHF

CVFF

GIAO

(Grain of Salt)

This dictionary of Computational Chemistry terms has been assembled over several years from a variety of sources, including my own interpretation of published useage, as well as a compilation and/or paraphrasing of definitions from other sources. Some of these come from Dictionaries published on the Internet, which are no longer available. Two similar, extensive dictionaries/glossaries which are still available on the Internet are:

Karl Irikura's Glossary of Common Terms and Abbreviations in Quantum Chemistry.

and a, Dictionary of Computational Chemistry by Martin Grayson.

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7/25/02 Ernie Chamot / Chamot Labs / / echamot@chamotlabs.com