Comparing QM/MM and QM/MD: What's the Difference?

[greisen]

Hi all,

I have a general questions on which is the difference between these two methods QM/MM and QM/MD?
Any hints appreciated - thanks in advance
Best

 

[chemisttree]

QM = quantum mechanical?
MM = molecular mechanics?
MD = molecular dynamics?

Is this what you are talking about?

 

[greisen]

Yes - that is my question.

I would presume that in a QM/MM one would converge to structure when the QM part would converge due to some gradient tolerance but I am not quite sure about the QM/MD.

Any advise appreciated - thanks

 

[chemisttree]

Perhaps in the MD system, the structure is allowed to deform to an energy minimized trasition state (by MM) rather than to a ground state energy minimized structure. The path to that transition state will contain multiple energy minimized geometries. Those structures leading to the transition state could be calculated with MM resulting in the eventual geometry of the transition state. Perhaps the overall process using MM calculations for ground state, intermediate and transition state structures would collectively be referred to as molecular dynamics? A quote from "Computational Chemistry" by Errol Lewars:

Programs like those in AMBER are used not only for calculating geometries and energies, but also for simulating molecular motion, i.e. for molecular dynamics, and for calculating the relative populations of various conformations or other geometric arrrangements (e.g. solvent molecule distribution around a macromolecule in Monte Carlo simulations. In molecular dynamics Newton's laws of motion are applied to molecules moving in a MM forcefield, although relatively small parts of the system (system: with biological molecules in particular modelling is often done not on an isolated molecule but on a molecule and its environment of solvent and ions) may be simulated with quantum mechanical methods. In Monte Carlo methods random numbers decide how atoms or molecules are moved to generate new conformations or geometric arrangements (states) which are then accepted or rejected according to some filter. Tens of thousands (or more) of states are generated, and the energy of each is calculated by MM, generating a Boltzmann distrubution.