Basis set corrections

The dissociation problem is solved in the case of a full Cl wave function. As seen from eq. (4.19), the ionic term can be made to disappear by setting ai = —no- The full Cl wave function generates the lowest possible energy (within the limitations of the chosen basis set) at all distances, with the optimum weights of the HF and doubly excited determinants determined by the variational principle. In the general case of a polyatomic molecule and a large basis set, correct dissociation of all bonds can be achieved if the Cl wave function contains all determinants generated by a full Cl in the valence orbital space. The latter corresponds to a full Cl if a minimum basis is employed, but is much smaller than a full Cl if an extended basis is used. 

G.P. Francis and M.C. Payne, Finite basis set corrections to total energy pseudopotential calculations, J. 

Since analytic second derivatives are available for MP2 calculations, numerical difference calculations of CCSD(T) energies are only required for a relatively small basis set. This type of basis set correction approximation is also available in Grow. It is not possible to use some composite methods which, like the G2 and G3 schemes,66 involve adding non-differentiable corrections to the estimated electronic energy. However, there are other recently developed composite methods which might be effectively employed to construct this type of interpolated PES.67... 

Substitution of one hydrogen of HOH with an aromatic group leads to a phenol molecule. When paired with methanol, phenol acts as the proton donor molecule in a structure very much akin to the water dimer itself At the SCF/6-31G level, the interoxygen distance is 2.89 A. The electronic contribution to the binding energy is computed to be 6.0 kcal/mol, after removal of BSSE, and 7.1 kcal/mol at the MP2 level with the same basis set. Correction of the correlated result by ZPVE yields a of 5.8 kcaPmol, leading to the conclusion that phenol is a more potent proton donor than is water. 

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) ... 

We have also noted that the current state-of-the-art methods (DFT based BeckeSLYP method with an extended basis set) correctly reproduce the geometries of the [Ru(H—H)(C5Mej)(dppm)] complex except for the H-H distance where the experimental data (1.10 A) was far from the value at... 

Comparison of values calculated with the same basis set correctly shows the increasing charge on each H atom as the electronegativity increases from C to N to O to F, but comparison of values calculated with different basis sets could erroneously lead one to say that the C—H bond in CH4 is more polar than the O—H bond in H2O. 

In order to provide some insights into the intensity enhancement undergone by the H—Y stretch of the proton donor molecule in a H-bond [162, 163], Zilles and Person made use of atomic polar tensors in their study of the water dimer [164]. These quantities are defined in terms of the effect on the dipole moment of a small displacement of each atomic center of the system under examination. Further information arises from a partitioning into charge, charge flux, and overlap contributions (CCFO). Even though their work was limited to a rather small basis set (4-3IG), they were able to extract some very useful information. Comparison of certain elements of their calculated spectra with experimental data led them to-believe that the 4-3IG basis set correctly reproduces the essential ingredients. 

The next step was to determine the proportional factor Q in Eq. 3, and test the chosen form of the basis set correction factor . The C, s were determined from a number of MP2/cc-pVQZ runs selected from a modified experimental... 

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