Molecular dynamics

Molecular dynamics (MD) requires in principle only energies and gradients from the electronic structure calculation, so it is possible to have any flexible external MD program call ORCA and request an energy+gradient evaluation (! EnGrad) using any available electronic structure method in ORCA (in practice only methods with analytical gradients). The external program then solves Newton's equations of motion and handles all other aspects of the simulation (maintaining temperature etc.). MD programs like Chemshell can be used with ORCA in this way.

Molecular dynamics can also be performed in ORCA directly nowadays.

Since ORCA 4 there is a new molecular dynamics code, more flexible than the previous version (thermostats are now available for example). Any electronic structure method in ORCA with available gradients (ideally analytic) can be used to perform Born-Oppenheimer MD simulations. The Velocity Verlet algorithm is used for solving Newton's equations of motion. See chapter 9.26 in the ORCA 4 manual for much more documentation and also see the developer's website.

Important notes:

  • A molecular dynamics simulation using a QM method is expensive and will NOT enable you to access the nanosecond/microsecond timescales that are possible with classical molecular mechanics MD. A few picoseconds for a small molecule is achievable at high cost and it's up to the user to carefully set up such a simulation and determine whether a a few ps MD simulation will result in useful data or not. Chemical reactions are also rarely accessible at the picosecond timescales.

  • MD trajectories are sensitive to numerical noise. SCF convergence criteria, DFT numerical integration etc. needs to be monitored for stable trajectories. Doing an MD simulation is not as straightforward as doing a geometry optimization.


Simple Example MD input

Note: since the new MD code in ORCA uses a very different syntax it will not be described here in any detail here (at least for now) and the interested user should read the MD Manual Chapter in detail before using.

This input below will perform a simple MD simulation at the PBE/def2-SVP level of theory, using the default Velocity Verlet algorithm in the NVT ensemble. Timestep is 0.5 femtoseconds and initial velocities according to a temperature of 350 K . Temperature is maintained at 350 K using a Berendsen thermostat.

! PBE def2-SVP def2/J MD

%md
timestep 0.5_fs
initvel 350_k
thermostat berendsen 350_k timecon 10_fs
dump position stride 1 filename "trajectory.xyz"
 run 200
end