QMMM Reaction Pathway Calculations with NEB
Experimental implementation of Nudged Elastic Band (NEB) method is available for reaction pathway calculations with QM/MM. The actual pathway/beads construction involves (by default) only the region containing QM and link atoms (referred to as qmlink). The rest of the system plays a passive role and is quenched/optimized each time a gradient on a bead is calculated.
The initial guess for NEB pathway can be generated using geometries of the starting and ending point provided by the .rst files. These are set in the input using the following directive
set qmmm:neb_path_limits xxx_start.rst xxx_end.rst
where xxx_start.rst xxx_end.rst refers to starting and ending point of the NEB pathway. Both rst files have to be present at the top level directory. It should be noted that only coordinates of qmlink region will be used from these two files. The initial coordinates for the rest of the system come from reference rst file provided in the MD block
md
system xxx_ref
...
end
Typically this reference restart file ( xxx_ref.rst ) would be a copy of a restart file for starting or ending point.
The number of beads in the NEB pathway, initial optimization step size, and number of optimization steps are set using the following directives
set neb:nbeads 10
set neb:stepsize 10
set neb:steps 20
The calculation starts by constructing initial guess for the pathway (consisting of a sequence of numbered rst files) by combining linearly interpolated coordinates of the qmlink regions from starting and ending rst files and classical coordinates from the reference file. Next phase involves calculation of the gradients on qmlink region atoms for each of the beads. This involves two steps. First classical region around the qmlink region is relaxed following standard QM/MM optimization protocol. Aside the fact that optimization region cannot be qmlink, all other optimization directives apply and should be set in the QM/MM block following standard convention, e.g.
qmmm
region solvent
maxiter 1000
ncycles 1
density espfit
end
or
qmmm
region mm_solute solvent
maxiter 300 1000
ncycles 3
density espfit
end
In both examples presented above we utilized espfit option for density to speed up calculations. Note that optimization region cannot be qmlink!
After the optimization has been performed the gradient on qmlink region is calculated. The procedure is repeated for all the beads. After that the bead coordinates will be advanced following NEB protocol and the entire cycle will be repeated again.
In addition to interpolated initial guess, one can also specify custom
initial path represented by numbered sequence of restart files stored in
the perm directory. This behavior will be triggered automatically in
the absence of qmmm:neb_path_limits directive. The default naming of
the custom initial path is of the form
set qmmm:neb_path
The progress of NEB calculation can be monitored by
grep gnorm <output file>
Experience shows that the value of gnorm less or around O(10^-2) indicates converged pathway. The current pathway in the XYZ format can be found in the output file (look for XYZ FILE string) and viewed as animation in some of the molecular viewers (e.g. JMOL)