Increasing the time step with mass scaling in Born-Oppenheimer ab initio QM/MM molecular dynamics simulations.

Born-Oppenheimer ab initio QM/MM molecular dynamics simulation with umbrella sampling is a state-of-the-art approach to calculate free energy profiles of chemical reactions in complex systems. To further improve its computational efficiency, a mass-scaling method with the increased time step in MD simulations has been explored and tested. It is found that by increasing the hydrogen mass to 10 amu, a time step of 3 fs can be employed in ab initio QM/MM MD simulations. In all our three test cases, including two solution reactions and one enzyme reaction, the resulted reaction free energy profiles with 3 fs time step and mass scaling are found to be in excellent agreement with the corresponding simulation results using 1 fs time step and the normal mass. These results indicate that for Born-Oppenheimer ab initio QM/MM molecular dynamics simulations with umbrella sampling, the mass-scaling method can significantly reduce its computational cost while has little effect on the calculated free energy profiles.