Simulation of the vibrational spectra of organic liquids with combination of molecular dynamics and normal coordinate analysis

The aim of this work is the computer simulation of the infrared and Raman spectra of neat organic liquids with the help of molecular dynamics. Molecular geometry, vibrational force field, vibrational frequencies, band intensities, depolarization ratios and atomic net charges were calculated quantum-chemically for the isolated molecule. The box was constructed based on the crystal structure of the frozen crystalline liquid. Periodic boundary conditions were used. Initial velocities were given according to the temperature. Canonical restrictions were applied. The intermolecular interactions were calculated using Buckingham potentials. The linear and rotational motions were simulated using the modified Verlet 'leap-frog' algorithm. After a series of time steps, the required equilibration was arrived. During the following time steps, the distortions of the molecules were allowed under the action of the intermolecular forces. For each molecule and at each time step, the vibrational frequencies were calculated assuming a harmonic intramolecular force field. Band intensities were calculated with Lorentzian profiles taking into account the quantum-chemically calculated infrared and Raman intensities. All the calculated individual spectra were summarized to the ensemble spectra. Effects of the strength of the intermolecular forces on the resulted vibrational spectra were studied. (C) 2002 Elsevier Science B.V. All rights reserved.