Mini-Symposium, Paper M4
Janez Mavri
National Institute of Chemistry
Hajdrihova 19, Ljubljana, Slovenia
A density matrix evolution (DME) method (Berendsen and Mavri, J. Phys. Chem., 97, 13464, 1993) to simulate the dynamics of quantum system embedded in a classical environment will be presented. The method is applicable when the quantum-dynamical degrees of freedom can be described in a Hilbert space of limited dimensionality. The DME method in combination with classical molecular dynamics simulation was applied to calculate the rate constant for proton tunneling in the intramolecular, double well hydrogen bond of hydrogen malonate in aqueous solution. Moreover the rate of proton transfer between Asp25 and Asp125 at the active site of apoenzyme HIV-1 protease has been determined. The proton hops between the minima on a nanosecond time scale.
First principle calculations of the vibrational spectra of strong hydrogen bonded systems present a special challenge due to high anharmonicity of the OH stretching and strong coupling to the other degrees of freedom. The proton motion is treated quantum dynamically i.e. excited vibrational states are included while the remaining intramolecular degrees of freedom and solvent degrees of freedom are treated by classical mechanics. The IR spectrum is calculated numerically from the time dependent dipole moment using the Fourier Transform techniques and by assuming electrical harmonicity of the OH bond. The asymmetric OH stretching in acetylacetone in the gas phase and in the polar solution is studied. The effects of deuteration are considered. Simulated and experimental IR spectra are in decent agreement.