Quadratic configuration interaction with double and single excitations

There is also a hierarchy of electron correlation procedures. The Hartree-Fock (HF) approximation neglects correlation of electrons with antiparallel spins. Increasing levels of accuracy of electron correlation treatment are achieved by Mpller-Plesset perturbation theory truncated at the second (MP2), third (MP3), or fourth (MP4) order. Further inclusion of electron correlation is achieved by methods such as quadratic configuration interaction with single, double, and (perturbatively calculated) triple excitations [QCISD(T)], and by the analogous coupled cluster theory [CCSD(T)] [8],... 

QCISD(T) quadratic configuration interaction with single and double excitation and... 

QCISD(T) Quadratic configuration interaction with single double and perturbative triple excitations... 

From an all-electron (quadratic configuration interaction with all single and double excitations) potential [22]. — For gaseous ND2 derived [23] from LMR [24] and lODR frequencies [25]. For NHD in a nitrogen matrix from an IR spectrum [20]. — For gaseous ND2 from an LMR spectrum [24]. 

Two approaches to electron correlation that are widely used today for the studies of organic radical cations are Coupled Cluster (CC) calculations or the similar, but not identical, Quadratic Configuration Interaction (QCI) method with single and double excitations, often followed by CCSD(T) or QCISD(T) single point calculations with a larger basis set. These methods suffer to a much lesser extent from... 

We have used a new seven-dimensional ab initio PES in which the CH3 group retains Csv symmetry and fixed CH bond lengths in our time-dependent calculations. [52] The energy near the minimum energy path (MEP) is described by a basis set corrected calculation using the quadratic configuration interaction treatment with all single and double excitations (QCISD) ... 

CCSD = coupled-cluster with singles and doubles CCSD(T) = CCSD with noniterative triple excitations Cl = configuration interaction CISD = Cl singles and doubles CP = counterpoise correction QCISD = quadratic Cl with singles and doubles. 

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