Basis sets atomic polar tensors /dipole

With quantum-mechanical methods, the second derivatives of the energy could be used directly for the FF and atomic polar tensors (APT) for the dipole derivatives. Both are standardly computed in most quantum-chemistry programs but for accurate results, moderately large basis sets and/or some accommodation for correlation interaction is needed. Until recently, this has restricted most ab initio studies to modest-sized molecules. 

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. 

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