The basis set superposition error (BSSE) influence in the geometry structure, interaction energies, and intermolecular harmonic and anharmonic vibrational frequencies of cyclic formamide-formamide and formamide-water dimers have been studied using different basis sets (6-31G, 6-31G**, 6-31

G**, D95V, D95V**, and D95V

**). The a posteriori "counterpoise" (CP) correction scheme has been compared with the a priori "chemical Hamiltonian approach" (CHA)

The equilibrium structures, the binding energies, and the second-order energy components of a series of hydrogen-bonded complexes involving acetylene are studied. The strength of the binding energy of the selected systems (HF . . . HCCH, HCl . . . HCCH, HCN . . . HCCH, and HCCH . . . HCCH) was different, ranging from a very weak interaction to a strong interaction. Calculations have been carried out at both the Hartree-Fock and correlated (second-order

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

Theoretical studies have been performed to calculate the force constants and harmonic vibrational frequencies in the hydrogen fluoride and water dimers. The calculations have been undertaken both at the Hartree-Fock and correlated (second-order Moller-Plesset perturbation theory) levels of theory using several different basis sets ranging from the weak to the intermediate. The basis set superposition error (BSSE) has been excluded by using the chemical

The different configurations (linear, zig-zag, and cyclic) of formamide dimers have been studied at the level of both Hartree-Fock (HF) and second order Moller-Plesset perturbation theory (MP2). The widely used a posteriori Boys-Bernardi "counterpoise" (CP) correction scheme has been compared with our a priori methods utilizing the "chemical Hamiltonian approach" (CHA). The appropriate interaction energies have been calculated in six different basis