QMMM Transition States

QM/MM transition states calculations for qm or qmlink regions can be performed using

task qmmmsaddle

The overall algorithm is very similar to QM/MM optimization calculations, but instead of optimization, transition state search will be performed for qm or qmlink region for specified number of steps ( as defined by maxiter keyword). The remaining classical regions (if any) will be optimized following the standard optimization protocol, which may involve, if specified, ESP charge representation of the QM atoms (a recommended option).

The success transition state calculations is strongly dependent on the initial guess. User may consider generation of the latter using QM/MM reaction pathway calculation. Another useful strategy involves precalculation of the Hessian. Following the example presented above one could have precalculated numerical Hessian for the qm region

...
qmmm
region qm
end

freq
animate
end

task qmmm dft freq

and then used this information in the TS calculation

...
driver
clear
inhess 2  #read in hessian from perm directory
moddir 1  #follow 1st mode
end

qmmm
bqzone 15.0
region  qm solvent
xyz  ts
maxiter 10 1000
ncycles 2
density espfit
end

task qmmm dft saddle