Basis-set convergence dipole moments

Dipole Moment Convergence As examples of molecular properties we will look at how the dipole moment and harmonic vibrational frequencies converge as a function of level of theory. functions. This illustrates that care must be taken when calculating properties other than the total energy, as standard basis sets may not be able to describe important aspects of the wave function. The HF dipole moment is too large, which is quite general, as the HF wave function... 

The HF level as usual overestimates the polarity, in this case leading to an incorrect direction of the dipole moment. The MP perturbation series oscillates, and it is clear that the MP4 result is far from converged. The CCSD(T) method apparently recovers the most important part of the electron correlation, as compared to the full CCSDT result. However, even with the aug-cc-pV5Z basis sets, there is still a discrepancy of 0.01 D relative to the experimental value. 

Some molecular properties which are of primary importance in calculations of intermolecular forces, like electric dipole moments, polarizabilities etc., are poorly reproduced by calculations with small basis sets. Evidently, these errors introduce errors into calculations of intermolecular forces, e.g. too large dipole moments yield too strong interactions. Table 3 shows convergence of the results on the (HF)2 complex when a near Hartree-Fock quality of the calculation is reached. 

We have recently completed a detailed study (27) of the dipole moment, electron affinity, and polarizability of LIF and the Isoelectronic BeO molecule. A wide range of basis sets were investigated to assure convergence of each of the properties and to serve as a guide for basis set selection for the heavier alkali halides. The results at the equilibrium geometries of LIF and BeO are compared in Table 1 together with those for HF, for which experimental polarizabilities are known. The columns... 

Recently, the convergence of the many-body sapt expansion for the interaction-induced dipole moment of He-H2 (24) and the polarizability of He2 (24, 32). has been checked by comparison with fci results in the same basis set. The numerical results for the helium dimer are summarized in Table 1 where we consider the anisotropy 7 and the trace a 7 = Aa — Aasx and a = (Act, + 2Aazz)/3, in which the z-axis is the molecular axis. An inspection of Table 1 shows that for all distances considered in Ref. (32) the many-body SAPT expansion reproduces the fci results to... 

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