Ab Initio Study of the Ammonia-Ammonia Dimer:BSSE-Free Structures and Intermolecular Harmonic Vibrational Frequencies

摘要

The influence of basis set superposition error (BSSE) in optimized geometries,force constants and intermolecular harmonic vibrational frequencies of the ammonia-ammonia dimer have been studied both at the Hartree-Fock and correlated (second-order M011er-Plesset perturbation theory) levels of theory using several different basis sets as (6-31G,6-311G,6-31 + +G,6-311 + +G,6-31G(d,p),6-311G(d,p),6-31 + +G(d,p) 6-311+ +G(d,p) and 6-311+ +G(2d,2p)).The widely used a posteriori Boys-Bernardi "counterpoise" (CP) correction scheme has been compared with the a priori method utilizing the chemical Hamiltonian approach (CHA).The results show that practically there is no difference between these two methods,so the a priori CHA scheme can be considered as an ultimate solution of the BSSE problem.It is also concluded that the BSSE influence is very significant,so removing this effect is very important.