ORCA Molecular Dynamics Module

Overview  •   List of Changes  •   Manual  •   Input Example  •   Input Library

— Overview —


Figure 1: ORCA AIMD Simulation showing how NH3
abstracts a proton from an [Al(H2O)6]3+ complex.
(download input file)

I am the developer of the molecular dynamics (MD) module in the ORCA program package. This module enables to run ab initio molecular dynamics (AIMD) simulations of small to medium-sized (non-periodic) systems, using all the different electron structure methods which ORCA includes (Hartree–Fock, Semiempirics, MP2, DFT with LDA, GGA, Meta-GGA, hybrid, and double-hybrid functionals, excited state dynamics, continuum solvation models, multi-reference calculations, etc.) There exists rarely any other ab initio molecular dynamics package which offers such a wide variety of methods, which underlines the synergistic effect of including an MD module into ORCA.

Apart from the underlying electron structure methods, the ORCA MD module also offers some useful features for running different kinds of molecular dynamics simulations. This is a work in progress, and new features are introduced with every release. The current program version is ORCA 4.2, which has been released in August 2019.

The language of the input for the ORCA MD module is SANscript, which is a scripting language for scientific purposes that I am currently developing. A full documentation of the language was not yet published. For a first glimpse, please see the SANscript page.

If you have suggestions on useful features which could be implemented in future releases of the ORCA MD module, feel free to contact me via email.

— List of Changes —

List of changes in the molecular dynamics module. Click on the tabs to see the changes in previous ORCA releases.

Released in August 2019.

  • Added a Cartesian minimization command to the MD module, based on L-BFGS and simulated annealing. Works for large systems (> 10'000 atoms) and also with constraints. Offers a flag to only optimize hydrogen atom positions (for crystal structure refinement).
  • The MD module can now write trajectories in DCD file format (in addition to the already implemented XYZ and PDB formats).
  • The thermostat is now able to apply temperature ramps during simulation runs.
  • Added more flexibility to region definition (can now add/remove atoms to/from existing regions).
  • Added two new constraint types which keep centers of mass fixed or keep complete molecules rigid.
  • Ability to store the GBW file every n-th step during MD runs (e.g. for plotting orbitals along the trajectory).
  • Can now set limit for maximum displacement of any atom in a MD step, which can stabilize dynamics with poor initial structures.
  • Runs can be cleanly aborted by "touch EXIT".
  • Better handling/reporting of non-converged SCF during MD runs.
  • Fixed an issue which slowed down molecular dynamics after many steps.
  • Stefan Grimme's xTB method can now be used in the MD module, allowing fast simulations of large systems.

Released in December 2018.

  • Molecular dynamics simulations can employ Cartesian, distance, angle, and dihedral angle constraints, which are enforced by the RATTLE algorithm.
  • The MD module features cells of several geometries (cube, orthorhombic, parallelepiped, sphere, ellipsoid), which can help to keep the system inside of a well-defined volume. The cells have repulsive harmonic walls.
  • The cells can be defined as elastic, such that their size adapts to the system. This enables to run simulations under constant pressure.
  • Trajectories can be written in XYZ and PDB file format.
  • A restart file is written in each simulation step. With this file, simulations can be restarted to seamlessly continue (useful for batch runs or if the job crashed).
  • Ability to individually define regions (i. e., subsets of atoms). Regions can be used to thermostat different parts of the system to different temperatures (e. g., cold solute in hot solvent), or to write subset trajectories of selected atoms.
  • The energy drift of the simulation is now displayed in every step (in units of Kelvin per atom). Large energy drift can be caused by poor SCF convergence, or by a time step length chosen too large.

— Manual —

The manual of the MD module is included in the ORCA manual (section 9.34). Here, you can download a PDF file which only contains the manual of the MD module:

FileType / SizeLast Changed
ORCA 4.2 MD Manual .pdf, 364 kiB Aug 09 2019

— Input Example —

For a simple AIMD simulation of a water dimer at 300 K with BLYP–D3, please consider the following input example:

! MD BLYP D3 def2-SVP %md initvel 300_K timestep 0.5_fs thermostat berendsen 300_K timecon 10.0_fs dump position stride 1 filename "trajectory.xyz" run 2000 end * xyz 0 1 O -2.03740 -1.21799 -0.08342 H -1.06493 -1.04408 -0.02285 H -2.37327 -1.07034 0.83692 O -1.65042 1.84243 0.07893 H -0.72656 1.49786 -0.01029 H -2.07086 1.65422 -0.79801 *

— Input Library —

A library with many input files for ORCA AIMD runs will be established here at a later time.

Please also have a look at the ORCA input library, where many input samples for ORCA can be found.