Ab initio study of the ammonia-ammonia dimer:: BSSE-Free structures and intermolecular harmonic vibrational frequencies

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 Moller-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. (C) 2004 Wiley Periodicals, Inc.