Temporal QM/MM
|
MOLECULAR DYNAMICS (MD) is
a powerful tool for study simulating the atomic-level evolution of chemical
systems. In an MD simulation, the nuclei are treated at classical particles that
are propagated according to Newton's equations of motion. In MD simulations,
the forces acting on the nuclei are typically obtained through either of two
methods: force-fields (FFs) or quantum chemical (QC) calculations. FFs are empirical
relationships between a molecule's potential energy and geometry. These models
are computationally-inexpensive, but cannot generally describe the changes in
bonding that occur during reactions. Meanwhile, QC methods can describe the changes
in electronic structure that occur when bonds form and break during reactions.
Unfortunately, the computational expense of QC calculations limits QC-based MD
simulations to studying sub-nanosecond time scales, which is too short to observe
most reactions without special sampling techniques, thereby limiting the potential
of using MD as a tool for identifying reactions. |
Temporal QM/MM: Using FFs when a system is close to a potential energy minimum,
and switching to QC methods only when the system approaches the transition state provides
access to longer MD simulations and the ability to model changes in bonding. |
|
TO OVERCOME the limited time
scales accessible in MD simulations of reactions, we are developing a hybrid
technique that employs forces derived from both FF and QC methods. Specifically,
FFs are used to describe the system near minima on the potential energy surface,
and QC methods are used to model the system when it undergoes transitions between
these minima. Specifically, the system is modeled with a FF and its behaviour is
monitored for the onset of reactions. When the onset of a reactive event is detected,
the description of the system is switched smoothly from FF to QC models, and when the
reaction is complete, the description of the system reverts to an FF. Since reactive
events occur very infrequently and complete very quickly, this approach promises to
significantly reduce the use of QC methods in MD simulations of reactions. |
|
IN PRACTICE, this method promises to permit MD
simulations on the microsecond timescales accessible with FFs, while retaining the ability to describe
chemical reactions. This method is analogous to existing technique called QM/MM that spatially divide
a chemical system into a region that is describe with QC methods and another described with FFs as a
way to study large systems. Since the method we are developing divides the use of QC and FF methods in
time, we call it temporal QM/MM. |
