# Release66:QMMM Transition States

### From NWChem

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

task qmmm <qmtheory> saddle

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).

Here is an example of direct transition state calculation on the chloroform and OH- solvated species.

memory total 2000 Mb start clfoh permanent_dir ./perm scratch_dir ./data charge -1 basis "ao basis" * library "6-31+G*" end dft XC b3lyp iterations 5000 end md system clfoh_ts cutoff 1.5 end qmmm bqzone 15.0 region qm solvent xyz ts maxiter 10 1000 ncycles 2 density espfit end task qmmm dft saddle

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