Ab initio molecular dynamics: Propagating the density matrix with Gaussian orbitals. III. Comparison with Born–Oppenheimer dynamics

In a recently developed approach to ab initio molecular dynamics (ADMP), we used an extended Lagrangian to propagate the density matrix in a basis of atom centered Gaussian functions. Results of trajectory calculations obtained by this method are compared with the Born–Oppenheimer approach (BO), in which the density is converged at each step rather than propagated. For NaCl, the vibrational frequency with ADMP is found to be independent of the fictitious electronic mass and to be equal to the BO trajectory result. For the photodissociation of formaldehyde, H2CO→H2+CO, and the three body dissociation of glyoxal, C2H2O2→H2+2CO, very good agreement is found between the Born–Oppenheimer trajectories and the extended Lagrangian approach in terms of the rotational and vibrational energy distributions of the products. A 1.2 ps simulation of the dynamics of chloride ion in a cluster of 25 water molecules was used as a third test case. The Fourier transform of the velocity–velocity autocorrelation function showed the expected features in the vibrational spectrum corresponding to strong hydrogen bonding in the cluster. A redshift of approximately 200 cm−1 was observed in the hydroxyl stretch due to the presence of the chloride ion. Energy conservation and adiabaticity were maintained very well in all of the test cases.