In many cases it is difficult to guess a good starting point for the local
gradient-based transition state optimizer while the simple scanning approach
doesn't work (e.g. due to the complex reaction mechanism). For these cases Gaussian
offers two variants of the transit-guided quasi-Newton (STQN) method which first
interpolate between reactant and product structures in a non-linear way and then
initiate a local transition state optimization from the point closest to the
approximate transition state structure. The two variants available in Gaussian
are used with the keywords:
opt=QST2
or opt=QST3
The first version QST2 only requires the
reactant and product structures as input, while the QST3
version needs structures for the reactant, the product, and an approximate transition state
(in that order). In both cases the reactant and product structures must be described with
all atoms in the same order. Regardless of the actual coordinates used in the input file,
the actual interpolation and transition state searches will be performed in redundant internal
coordinates. Using the isomerization from HCN to CNH again as an example,
the QST2 input file would be:
#P HF/6-31G(d) opt=(QST2,Z-Matrix)
HCN to CNH isomerization pathway, HF/6-31G(d), QST2 method, reactant structure
0 1
N1
C2 1 r2
H3 2 r3 1 a3
r2=1.13248
r3=1.05913
a3=180.0
HCN to CNH isomerization pathway, HF/6-31G(d), QST2 method, product structure
0 1
N1
C2 1 r2
H3 2 r3 1 a3
r2=1.154199
r3=2.139352
a3=0.0
Please observe that a full geometry specification with comment line and charge and multiplicity
must be provided for each of the structures.
In this particular case the QST2 method fails due to the very unfavorable choice of redundant
internal coordinates made by Gaussian. Instead of moving the hydrogen atom around
the C-N moiety, the QST2 coordinates try to move the hydrogen atom through the two
central atoms. This will, of course, never lead to a meaningfull starting point for the
transition state search. The QST3 method is more successful in this instance, the input file
being:
#P HF/6-31G(d) opt=(QST3,Z-Matrix)
HCN to CNH isomerization pathway, HF/6-31G(d)
QST3 method, reactant structure
0 1
N1
C2 1 r2
H3 2 r3 1 a3
r2=1.132528
r3=1.059009
a3=180.0
HCN to CNH isomerization pathway, HF/6-31G(d)
QST3 method, product structure
0 1
N1
C2 1 r2
H3 2 r3 1 a3
r2=1.154199
r3=2.139352
a3=0.0
HCN to CNH isomerization pathway, HF/6-31G(d)
QST3 method, ts structure
0 1
N1
C2 1 r2
H3 2 r3 1 a3
r2=1.16868
r3=1.14549
a3=80.0
The third structure in the input file corresponds to what has been used as the starting
point for the ts optimization before. The transition state is located in this case within
9 optimization cycles. In contrast to transition state optimizations using the local
methods discussed before, a full calculation of the Hessian is not required in
this case.
last changes: 16.10.2004, HZ questions & comments to: zipse@cup.uni-muenchen.de