一般来说，时间步长应该设为分子运动的最小振动周期的1/10 左右为宜。在化学分子中振动周期最短的是碳氢键的伸缩振动，其特征振动频率大约是3000cm-1，其振动周期大约是10fs。所以通常的分子动力学时间步长最高不能高于1fs。
http://blogold.chinaunix.net/u2/85704/showart_1884250.html
The choice of timestep
A key parameter in the integration algorithms is the integration timestep. To make the best use of the computer time, a large timestep should be used. However, too large a timestep causes instability and inaccuracy in the integration process.
Relation of timestep to molecular vibration
The timestep used depends on the model as well as the integrators. The main limitation imposed by the model is the highest-frequency motion that must be considered. A vibrational period must be split into at least 8-10 segments for models to satisfy the Verlet assumption that the velocities and accelerations are constant over the timestep used.
In most organic models, the highest vibrational frequency is that of C-H bond stretching, whose period is on the order of 10-14 s (10 fs). The integration timestep should therefore be about 0.5-1 fs. If you use the SHAKE or RATTLE constraint algorithm (Constraints during dynamics simulations), a longer timestep is possible.
If you are studying simple model liquids or solids and are not interested in internal modes, much longer timesteps may be used, e.g., up to 20 fs. A timestep of about 5 fs should be adequate for ionic material models.
Appropriate for the integrator
The timestep must also be appropriate to the integrator. For the ABM4 method, the timestep should be about half that needed for the Verlet algorithm. The Runge-Kutta-4 method seems to require a much smaller timestep than the other methods.
http://www.chem.cmu.edu/courses/09-560/docs/msi/ffbsim/5_Dynamics.html#348259
Choosing the time step
All the dynamics objects need a time step. Choosing it too small will waste computer time, choosing it too large will make the dynamics unstable, typically the energy increases dramatically (the system “blows up”). If the time step is only a little to large, the lack of energy conservation is most obvious in Velocity Verlet dynamics, where energy should otherwise be conserved.
Experience has shown that 5 femtoseconds is a good choice for most metallic systems. Systems with light atoms (e.g. hydrogen) and/or with strong bonds (carbon) will need a smaller time step.
All the dynamics objects documented here are sufficiently related to have the same optimal time step.
https://wiki.fysik.dtu.dk/ase/ase/md.html