Basis set and polarization function effects on optimized geometries and harmonic frequencies at the second-order Mo/ller–Plesset perturbation level

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Abstract
Basis set and polarization function effects on the ground state optimized geometries and harmonic frequencies at the second‐order Mo/ller–Plesset perturbation (MP2) level have been studied for 11 small molecules (one or two heavy atoms) containing one or two carbon atoms, as well as for propene, propane, isobutene, acetaldehyde, methyl ether, c i s‐ and t r a n s‐1,2‐difuoroethylene. A series of basis sets ranging in quality from 4‐21G to 6‐311G** have been used for the small systems; for the larger systems 6‐31G and 6‐31G* basis sets were compared. In addition, three modified 6‐31G basiss sets in which d basis functions are added to certain (but not all) heavy atoms were introduced to study the effect of polarization functions in systems containing heteroatoms. It was found that the inclusion of d functions in basis sets is important for calculating the equilibrium geometries, especially for CC and CX (X=N, O, and F) bonds. For vibrational frequencies, however, addition of d functions to basis sets often does not produce a significant improvement; for many alkenes and alkanes MP2/6‐31G (MP2 with a 6‐31G basis set) and MP2/4‐21G calculations give good results for the frequencies that are comparable to those obtained with MP2/6‐31G* and MP2/4‐21G*, respectively. For molecules containing heteroatoms, the MP2/6‐31G (MP2/4‐21G) frequencies are generally rather close to the MP2/6‐31G* (MP2/4‐21G*) results except for the vibrations involving CX or XH stretching, for which the MP2/6‐31G(MP2/4‐21G) values are usually too low. Such deficiencies can be removed by addition of d basis functions to one of the atoms involved in CX or XH bonding. It is suggested that such basis sets with limited polarization functions can be usefully applied to larger molecules. Some experimental frequencies which are not consistent with the a b i n i t i o values are discussed and reassignments are proposed.