Transition State Optimizations with Opt=QST2

Technical Note:

The Synchronous Transit-Guided Quasi-Newton (STQN) Method, developed by H. B. Schlegel and coworkers [Peng93], uses a linear synchronous transit or quadratic synchronous transit approach to get closer to the quadratic region around the transition state and then uses a quasi-Newton or eigenvector-following algorithm to complete the optimization. As for minimizations, it performs optimizations by default using redundant internal coordinates. This method will converge efficiently to the actual transition structure using an empirical estimate of the Hessian and suitable starting structures. Unlike other methods, STQN does not require a guess for the transition structure; instead, the reactant and product structures are input.

This method is requested with the QST2 and QST3 options to the Opt keyword. QST2 requires two molecule specifications, for the reactant and product, as its input, while QST3 requires three molecule specifications: the reactant, the product, and an initial structure for the transition state, in that order. The order of the atoms must be identical within all molecule specifications. Note that the TS option should not be specified with QST2 or QST3.

For example, at the left is an input file which may be used to locate the transition structure for the reaction SiH4 → SiH2 + H2. The title section and molecule specification for the product follows those of the reactant.

#T RHF/6-31G(d) Opt=(QST2,AddRedundant)
SiH2+H2>SiH4 Reactants    1st title section.
0,1                       1st molecule spec.
X 1 1.0
H 1 1.48 2 55.0
H 1 1.48 2 55.0 3 180.0
H 1 R 2 A1 3 90.0
H 1 R 5 A2 2 180.0
R=2.0                      Note long bond length.
4 5
SiH2+H2>SiH4 Products      2nd title section.
0,1                        2nd molecule spec.
X 1 1.0
H 1 1.48 2 55.0
4  5                        Add Redundant input.

In this case, because we happen to be interested in the H-H bond length, we specify the internal coordinate which bonds those two atoms to the AddRedundant option so that its value will be included in the printout of the optimized structure (the Si-H bond lengths will be included by default).

Input files for Opt=QST3 will similarly include three title and molecule specification sections: the reactants, the products, and an initial guess for the transition structure. The optimized structure found by QST2 or QST3 appears in the output in a format similar to that for other types of geometry optimizations:

                !     Optimized Parameters      !
                !   (Angstroms and Degrees)     !
! Name  Definition    Value    Reactant  Product  Deriv Info.   !
! R1    R(2,1)        1.0836    1.083     1.084   -DE/DX =  0.  !
! R2    R(3,1)        1.4233    1.4047    1.4426  -DE/DX = -0.  !
! R3    R(4,1)        1.4154    1.4347    1.3952  -DE/DX = -0.  !
! ...                                                           !

In addition to listing the optimized values, the table includes those for the reactants and products.

Last updated on: 21 June 2017.