A comparative DFT study on the differences between normal modes of polyethylene and polyethylene glycol via B3LYP Hamiltonian and the Hartree–Fock method in multiple bases

The vibrations of polyethylene and polyethylene glycol molecule using Gaussian 03W software via density functional theory and Hartree–Fock method have been studied numerically. Infrared spectra of polyethylene in room temperature is used, reported by Kholodkov et al. [Vacuum, 70, 505 (2003)] and comparing obtained numerical results with this spectra. The aim of this paper is to investigate various numerical methods in calculating frequency of vibrations of polyethylene. This manuscript represents a technical study where well established methodologies are used to calculate the vibrational properties of a molecular system. Crystal vibrations of polyethylene are calculated via density functional theory by Srinivasan et al. [Canadian J. Chem., 47, 3877 (1969)] and Hartree–Fock method by Kholodkov et al. [Vacuum, 70, 505 (2003)]. The effects of applying ether's groups on the vibrations of the obtained results for polyethylene glycol are studied in different bases. The calculations are done on basis sets of 6-31G(2d), 6-311G(d,p), 6-311G(2d), 6-311G(2d,2p). Various functionals were evaluated considering the agreement of the calculated modes with the corresponding experimental data. From these evaluations we obtained that the B3LYP functional gives the closest agreement between calculated and experimental values.